NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 6 n.1, 2001

NOTIZIARIO Neutroni e Luce di Sincrotrone Rivista del Consiglio Nazionale delle Ricerche

SOMMARIO

Cover photo: The high temperature proton conducting phase of CsDSO4 [5].

The individual atom positions in a 5376 atom (8x8x3 unit cell) model have been translated into a single unit cell - S are green, O are red and Cs are not shown. The D density distribution is represented as a contour plot in two planes, red indicating high density and blue low density.

EDITORIALE

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C. Andreani

RASSEGNA SCIENTIFICA Electron-volt Spectroscopy at a Pulsed Neutron Source Using a Resonant Detector Technique . . . . . . . . . . . . . . 3 A. Pietropaolo and M. Tardocchi

Structural and Magnetic Disorder in Crystalline Materials - A New View . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 R. McGreevy, A. Mellergard and P. Zetterström Il

NOTIZIARIO Neutroni e Luce di Sincrotrone

The Gas Phase Photoemission Beamline at ELETTRA: a Facility for Atomic Molecular and Chemical Physics Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

è pubblicato a

cura del C.N.R. in collaborazione con il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

Vol. 6 n. 1 Giugno 2001 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96 ISSN 1592-7822

M. Alagia et al.

PROGETTO E.S.S. Research and Development for the Target System of the ESS Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

DIRETTORE RESPONSABILE:

C. Andreani COMITATO DI DIREZIONE:

G. S. Bauer

M. Apice, P.Bosi COMITATO DI REDAZIONE:

L. Avaldi, F. Carsughi, G. Ruocco, U. Wanderingh

VARIE

SEGRETERIA DI REDAZIONE:

SCUOLE E CONVEGNI

D. Catena GRAFICA E STAMPA:

om grafica via Fabrizio Luscino 73 00174Roma Finito di stampare nel mese di Giugno 2001

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CALENDARIO

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FACILITIES

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PER NUMERI ARRETRATI:

Paola Bosi, Tel: +39 6 49932468 Fax: +39 6 49932456 E-mail: p.bosi@dcas.cnr.it.

PER INFORMAZIONI EDITORIALI:

Desy Catena, Università degli Studi di Roma “Tor Vergata”, Dip. di Fisica via della Ricerca Scientifica, 1 00133 Roma Tel: +39 6 72594364 Fax: +39 6 2023507 E-mail: catenadesy@roma2.infn.it

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EDITORIALE

n vista della scadenza dell’accordo internazionale CNR-CCLRC che regola l’accesso dei ricercatori italiani alla sorgente di neutroni pulsati ISIS, previsto nel marzo dell’anno 2002, la Commissione di Studio per il Coordinamento delle Attività di Spettroscopia Neutronica del CNR ed il delegato CNR nell’ESS R&D Council, Prof. Marcello Fontanesi, hanno predisposto un consuntivo dell’attività di ricerca e di sviluppo di strumentazione per la spettroscopia neutronica e di muoni, svolte dalla comunità italiana ad ISIS ed in ambito ESS. I documenti, pubblicati in versione integrale in questo numero, sono stati inviati sia al Presidente sia alla Commissione per le Relazioni Internazionali del CNR. Come risulta da queste relazioni le attività di ricerca svolte ad ISIS in questo settore vedono l’attiva partecipazione di ricercatori CNR ed universitari. Inoltre, grazie a questo accordo, nel corso degli anni, si è consolidata una efficace collaborazione scientifica tra ricercatori italiani ed inglesi. Questa ha permesso ai ricercatori italiani l’acquisizione di risorse finanziarie aggiuntive della Comunità Europea per progetti di sviluppo di strumentazione di neutroni presso Large Scale Facilities Europee. Si ricorda che per l’elegibilità a tali finanziamenti è richiesta l’esistenza di accordi di collaborazione, quali l’accordo CNR-CCLRC, tra i gruppi di ricerca proponenti ed ISIS, Laboratorio Large Scale Facility. Anche per questo, a riconoscimento dell’efficace politica svolta dal CNR in questo settore e che ha visto il crescente coinvolgimento di ricercatori universitari, mi auguro che il nuovo accordo veda una diretta partecipazione delle Università e di altri Enti, quali l’INFM, che si affianchi a quella del CNR in una collaborazione che riguardi sia il finanziamento del personale sia della strumentazione. Le attività di ricerca ad ISIS nel prossimo decennio si prospettano interessanti sia nel campo della ricerca scientifica sia nel campo delle applicazioni industriali ed è quindi necessario realizzare, anche in armonia con gli indirizzi nazionali ed europei per la ricerca presso le Large Scale Facilites, un maggiore ed efficace coordinamento tra enti ed istituzioni universitarie.

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n view of the expiring of the international agreement between CNR and CCLRC, due in March of the year 2002, the Neutron Scattering Committee of the CNR and the CNR delegate within the ESS R&D Council, Prof. Marcello Fontanesi, have prepared two documents illustrating the research activities of the Italian community at ISIS and within the ESS. These reports, published in this issue, have been addressed to both the President and the International Affairs Commission of the CNR. From these documents it appears that the research activities of the Italian community have seen the active involvement of both the CNR and University researchers. In addition, thanks to this agreement during the years an effective scientific collaboration has developed between Italian and British scientists.

The latter has allowed the Italian researchers the acquisition of additional financial resources for projects on neutron instrumentation from the Large Scale facilities program of the European Community.

I recall that the existence of an international agreement between the ISIS Large Scale Facility and the Italian proponents represents an essential prerequisite for the eligibility of such European projects.

For this reason, in the recognition of the effective policy of the CNR for financing neutron scattering activities which has seen the growing participation of university researchers, I do hope in the new agreement a direct participation of universities and other Italian research institutions, such as the INFM, to support personnel and neutron instrumentation in collaboration with the CNR. Indeed it is foreseen that the research which will be addressable at ISIS in the next decade will open interesting perspectives in both the academic and industrial research.

For this reason it is compulsory, also in view of the national and European issues in research matter at the Large Scale Facility, to realize a grater and effective coordination among national research institutions.

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Carla Andreani

RASSEGNA SCIENTIFICA Articolo ricevuto in redazione nel mese di Maggio 2001

ELECTRON-VOLT SPECTROSCOPY AT A PULSED NEUTRON SOURCE USING A RESONANT DETECTOR TECHNIQUE A. Pietropaolo Università degli Studi di Roma “Tor Vergata” and INFM

M. Tardocchi Università degli Studi di “Milano-Bicocca” and INFM

Abstract We present measurements on the energy resolution of a neutron Resonance Detector Spectrometer (RDS) using Pb and 4He/H2 mixture scattering samples.

The final neutron energy is selectively measured by means of an absorbing 238U foil which emits a prompt gamma ray cascade generated by (n,γ) reactions, revealed employing a conventional NaI(Tl) scintillator, properly shielded.

Despite a low signal to background ratio, it is possible to identify four recoil peaks corresponding, in energy, to the four resonances of the 238U in the epithermal region and compare their position with the prediction. Moreover, the double difference technique, which imply the use of two foils of different thickness, shows to enhance the energy resolution, as indicated by the reduction of the intrinsic width of the recoil peak at 6.6 eV. Introduction Resonance Detector Spectrometer have been long recognised as a potentially attractive technique for epithermal neutron spectroscopy at pulsed neutron sources [1,2,3], allowing the experimental investigation of neutron scattering experiments in energy and momentum transfer regions (hω>1eV, Q>30 Å-1) normally not accessible to conventional reactors and detectors, due to the low incoming neutron flux and to a low detection efficiency in the epithermal region, respectively.

The allowance of high energy and momentum transfers, makes it possible to access a kinematical region of the deep inelastic neutron scattering (DINS) where the dynamical structure factor S(Q,ω), which contains the physical information about the probed system, takes a simple form. In the region defined by Q>>2/R (where R is the main distance to the nearest-neighbours of the target atom in the condensed matter system) it is possible to calculate S(Q,ω) in an inchoerent approximation, which regards each atom as scattering independently of other atoms. In the limit of infinite Q, one can use the Impulse Approximation (IA), obtaining the result [4]: S(Q,ω) = ∫n(p) δ(hω - hωr - Q·p/M) dp

(1)

Where hωr is the recoil energy and the Dirac’s δ-function describes the energy conservation.

If the momentum distribution is normalised, the n(p) gives the fraction of atoms in the initial state with momentum p in the volume element dp.

Within the framework of the IA [5,6,7], the final state effects can be neglected and the single particle dynamical properties, such as momentum distribution n(p) along the Q direction and mean kinetic energy Ek, can be derived in a simple way.

The measurements, which are discussed later, have been performed at the pulsed spallation neutron source ISIS at Rutherford Appleton Laboratory (UK) on the electron volt spectrometer (eVS) [8]. eVS is a Filter Resonance Spectrometer (FRS) employing 32 6Li-glass detectors positioned at an average distance L1 = (0.50 ± 0.01) m from the sample position, while the primary path length, L0, from moderator to sample is (11.055 ± 0.025) m.

The energy spectrum of the incident neutron is sketched in fig.1 and is typical of an under moderated spallation neutron source: It has a peak at 0.03 eV and falls off as 1/En in the epithermal neutron energy, where En is the neutron energy.

In an inverse geometry configuration, as the one employed on eVS, the scattered neutrons are analysed in a selective mode by a resonant detector, composed of a resonant 238U foil, which has isolated neutron resonances in the energy range of interest (1÷100 eV), and a gamma detector as described below.

In these conditions the resonance fixes the final neutron energy, the initial one being measured through the time of flight (tof) technique.

The neutron time of flight is defined as the time taken, starting from the instant the neutron leaves the moderator, to travel along the beam tube, scatter from the sample and then reach the detector. The five parameters defining the time of flight τ are: L0, L1, 2θ (scattering angle), E0 and Ef (initial and final neutron energies).

The mathematical relation linking τ to the parameters mentioned above is [9]:

τ–t0 = L0/v0 –L1/v1

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Fig.1. Energy spectrum of the moderated neutrons produced by spallation reactions in the proton beam-target interaction.

Where t0 is a timing constant that determines which channel, in the time of flight spectrum, correspond to neutrons with τ = 0 and it is determined by the electronic delay times in the detector-discriminator-electronicscomputer chain, v0 and v1 are the initial and final velocities.

From the knowledge of these parameters one can measure the momentum and energy transfer.

The neutron absorption resonance is defined by its energy (Er) and cross section (σ0), the latter characterised by a certain width (∆Er, usually expressed as FWHM) given by the intrinsic (Breit and Wigner) and the thermal doppler contributions. The resulting broadening (∆ET) represents a lower limit to the energy resolution of a RDS. The resulting Voigtian shape of the response function, with infinite lorentzian tails on either sides of the cross section peak, means an infinite second moment and so a divergent estimation of the mean kinetic energy. This unfortunate characteristic is removable, as discussed later, by the application of the double difference technique.

A neutron with energy Er±∆Er, arriving onto the resonant foil is absorbed with a certain probability; the resulting excited nucleus decays in different channels whose probability is measured by the branching ratio for the various processes [10].

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One of such possible processes is the production of a prompt gamma ray cascade, i.e. (n,γ) reaction, revealed through a gamma detector.

Experiment Two different samples have been used for these measurements : a polycrystalline lead slab, 1 mm thickness and 100 cm2 area, and a 4He/H2 mixture. A 3” x 3” NaI(Tl) scintillator connected to a photomultiplier tube (PMT) and the 6Li-glass detectors has been positioned at a 2θ = 38°± 3° and 2θ = 37°± 2° respectively, in forward direction.

The gamma detector resolution shows an energy dependence [11,12,13] as can be seen from fig. 2, and its yield is about Κ x 104 γ/MeV, where Κ=3 ÷ 4.

The PMT has been fed through a HV-NIM module, with a constant positive biasing voltage of 800 V, while the output signals, provided by the anode and preamplifier outs, has been sampled by the standard Data Acquisition Electronics (DAE) of eVS and by a multichannel analyser, obtaining time of flight and pulse height spectra, as discussed below in this section.

To avoid direct incoming high energy scattered neutrons into the detection system, this latter has been positioned not in front of the resonant foil which has been oriented so that its axis formed an angle φ ≅18° about to the

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Fig.2. Plot of the detector resolution (FWHM) as a function of gamma-ray energy.

neutrons whose energy doesn’t lye in the right energy range, fixed by the resonance energy, pass through the 238 U foil and enter in the detection apparatus-shielding system, which contains impurities like Sb isotopes, concerning the shielding section, and Tl plus U isotopes, even if with different concentrations, concerning the gamma detector. All these elements, including the iodine (127I) of the scintillator, have resonance energies in the range 5 ÷ 200 eV. Most time of flight spectra have been collected with a discrimination threshold set to accept events with energy above 800 keV, while no discrimination has been applied to pulse high spectra and, due to high background sensitivity of the scintillator, no strong features have been found.

To study the effect of thickness on the energy resolution, two different thickness (40 µm and 120 µm) have been used. The thickness of the absorbing foil leads to self

Fig.3. Experimental set-up for the deep inelastic neutron scattering experiments on eVS with the NaI(Tl) and 6Li-glass scintillators.

scattered neutron direction. This means that the effective thickness and area seen by a neutron were 5% higher and lower, respectively, with respect to the case φ=0. The experimental set up is sketched in fig. 3.

In order to shield the gamma detector from environmental gamma and neutron background, an inner lead and an external borated-wax shielding has been built. A schematic of the shielding is given in fig. 4.

Blocks of lead, 5 cm thick, are enough to stop gamma rays produced by thermal neutron capture from Boron (Eγ=478 keV with a mean free path of 4 mm in lead). The use of a pure paraffin slab (5 cm thick) just in front of the NaI(Tl), employed as neutron moderator especially effective in suppressing unwanted features in the background spectrum, is justified by considering two time of flight spectra shown in fig. 5: the scattered

shielding effects : the probability for neutron absorption is given by [14] : PA(E) = 1 – exp [Ndσ0ψ(ξ,x)]

(3)

where Nd and σ0 are the number density of the resonant absorbing atoms per unit area perpendicular to the neutron beam and the cross section of the process respectively.

The term ψ(ξ,x), in the exponential, is given by the relation: (4) where ξ= 2(E-E0-R) ΓI-1 , x=Γi/∆ and ∆=2(R KB Teff)1/2 .

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Fig. 4. Sketch, not in scale, of the experimental setting for the shielding apparatus. Only a part of the 4π has been drawn.

where KB is the Boltzmann constant, T the thermodynamic temperature and f(E) the phonon density of states of the solid. The probability approaches the unitary value for very thick absorbers but this situation is correlated with a decrease of the energy resolution. The resulting values of Nσ has been 0.9 for the thin foil (30 µm) and 3.78 for the thick one (120 µm).

Fig. 5. Time of flight spectra of background obtained with and without paraffin slab in front of the NaI(Tl) window.

Γi is the line width of the Breit-Wigner, R is the recoil energy of the absorbing atom and E0 is the energy a neutron would have at the resonance in the case of a recoilfree absorption.

KBTeff is given, using the “weak binding” approximation [15], by the relation: -1 KBTeff = ∫ ∞ o ∫ dE E f(E) cotg[E(2KBTeff ) ]

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Data reduction and analysis Table 1, summarises the DINS measurements performed on the lead sample, using NaI and 6Li-glass detectors.

The measured scattering spectra have been normalised with respect to the incident monitor intensity and the integrated proton current and, subsequently, they have been corrected for the sample-dependent gamma background, measured without the resonant foil in front of the gamma detector.

Fig. 6a shows the lead recoil spectrum using the 238U foil at T=300 K in the time of flight region between 50 and 350 µsec.: four resonances are well observed in correspondence to final neutron energies determined by 238U resonances in the epithermal region, namely E1= 6.66 eV, E2=20.9 eV, E3=36.7 eV, E4=66.02 eV. In fig. 6b, for comparison, a lead recoil spectrum acquired using the standard 6 Li-glass detectors is also shown.

It is important to stress that the relative intensity of these

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experimental condition [14] (that is by placing the resonant foil in the sample position) that the energy resolution obtained subtracting two spectra measured with foils of different thickness is enhanced with respect to the one relative to one foil only.

The difference to be considered is : Id=IT(t) - Γ It(t)

(6)

where IT(t) and It(t) are the thick and thin tof spectra, properly normalised and corrected for background, respectively: Γ is given by the ratio: Γ=

Fig. 6. Scattering spectrum of 1 mm lead sample: the five resonances are indicated in the top of the recoil peaks.

peaks are comparable and this is an important feature as it allows to observe scattered neutrons in the high energy region where the 6Li-glass detectors efficiency is low, due to the 1/E functional behaviour of the absorption cross section.

3.1 Improvement of energy resolution through the application of the double difference technique As it has been mentioned, the resolution of the spectrometer shows a dependence on the thickness of the resonant foil. It has been shown, even if in a different

Technique

Filter

N Td N td

with Nd previously defined and, as we have used 238U foil with the same area, the Γ ratio is simply given by the ratio of the thickness of the foils (Γ=4).

It has to be stressed that in principle, using gamma detectors, the double difference technique only requires a single difference between the thick and thin foil, whereas in the standard single difference technique [17] employed with the 6Li-glass detectors an additional difference with the foil-in and foil-out spectra has to be performed.

Referring to the 6.67 eV resonance, we analysed the lead recoil spectrum to evaluate the instrument-induced broadening. In fig. 7a, the recoil spectra for the 30 µm, 120 µm foils and derived from the application of the DDT are presented. In fig. 7b and 7c, has been performed a curve fitting. The smaller value of the FWHM obtained with gamma detectors using the double difference technique is expected due to the suppression of the lorentzian wings in Voigt instrumental response function.

The dropping out of the lorentzian wings in the difference spectrum is an essential goal, as using this spectrum one can derive the single particle kinetic energy by calculating the second moment of the spectral distribution plotted using the West scaling variable, y= (ω−ωr) h M/Q) instead of time of flight. Reducing the

Detector

IC (µAh)

2θ (degrees)

WL (Å-1)

WG (Å-1)

NaI

900

38ű10

85ű5

182ű18

Li

900

37ű2

85ű1

173ű7

64ű5

182ű18

RDT

238

U, t= 30 µm

SDT

238

U, t= 30 µm

DDT

238

U, t= 30 µm

NaI

900

38ű10

DDT

238

U, t= 30 µm

NaI

190

38ű10

6

Table 1. Summary of DINS Measurements performed on the lead sample, using NaI and 6Li-glass detectors. IC represents the Integrated Proton Current, 2θ represents the scattering angle and WG and WL the HWHM of gaussian and lorentzian components obtained from the fitting procedure.

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Fig. 7. a) Time of flight spectra in the 6.67 eV peak region for the three cases of thin foil, thick foil and DDT spectrum using lead sample, uranium foil and NaI detector. The spectra are normalised so that their gaussian fits has unit intensity (height). The bin width is 1.5 µsec.

∆E/E ratio, by means of the DDT, is very important as the energy contribution, in the energy range considered for this analysis, to the total recoil spectrum broadening is much higher than the geometric one [18] and so the lower the ∆E/E, the better the mean kinetic energy estimation. Neutron time of flight spectra from 4He/H2 mixtures were recorded using both a thin and thick 238U foil, as a test of the performance of the present RDT apparatus for samples of low atomic mass. An independent measurement of the background signal, with foil out, was also performed. In Fig. 8a and 8b the foil in signal and the background-corrected spectra are shown. From Fig. 8(b) we observe that the relevant scattering from hydrogen and sample container (aluminium) occur in the time of flight region t< 325 µs.

In Fig. 8c the ratio between the difference (Fig. 8(b)) and signal (Fig. 8a) spectra is also shown.

Several observations can be made: the narrow peaks in the difference spectrum due to neutrons scattered from the Al sample holder, coming from four distinct values of the final energies, defined from the 238U resonances are

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Fig. 8. Neutron time of flight spectra from 50% 4He/H2 mixture, measured with 120 µm thick 238U foil and NaI γ detector (fig. 6a). The signal peaks can be identified on top of a large background component. The bin width is 0.25 µsec for t < 400 µsec. In fig.6b same spectrum as in fig.6a but after background subtraction. The vertical lines mark the expected time of flight values for neutron scattering on Al (dashed) and H (full) atoms, assuming neutron final energies of 6,67, 20.9, 36.7 and 66.02 eV. A bin width of 1 µsec is used to reduce the statistical noise. In 6c ratio between spectrum b and the foil-in spectrum 6a is plotted. A bin of 1 µsec is used to reduce statistical noise.

marked in the figure (dashed lines). The signals from hydrogen, coming from the 6.6 eV resonance energy, appear to be shifted to longer times with respect to their normal position expected by the IA (full lines). This is an indication of the presence of Final State Effect (FSE) occurring at this low scattering angle from H mass. The other H peaks coming from the higher U resonances are very poorly defined and, due to the poor statistics and smaller scattering cross section, the helium recoiling peak, expected at an intermediate time of flight, is not observed.

As a general comment from Fig. 8c we note that as fax the signal/background ratio is concerned, the scattering signal events represent a small fraction of the total: the signal/total ratio, which should approach unity under low

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Fig. 9. Neutron time of flight spectra from lead sample recorded using a 30 µm thick 238U foil in the experimental set-up of fig. 3. The NaI spectrum (top) has a peak corresponding to the 6.67 eV 238U resonance. The same resonance gives rise to the absorption peak in the 6Li-glass spectrum (bottom)

background conditions, is ≈ 4% for the 6.67 eV resonance, and decreases to values below 2% for the high energy resonances. For these reasons the analysis in y space, performed for lead, was not attempted for this sample.

Conclusions The results obtained in this paper should be regarded as a first step towards the routinely use of the Resonant Detector Technique for spectroscopy of epithermal neutrons in the 1 eV range. A first result we stress is the successful use of a Nal detector for this application.

Previous experiments [19] had discarded Nal in favour of BGO scintillators mainly because of their lower background. Yet the present results indicate that the use of Nal scintillators, in combination with a U converter foil, using the Resonant Detector approach, allows neutron spectroscopy measurements in the energy range 1÷10 eV. This finding is supported from Fig. 9 where we note that the achieved signal/ background ratio for the Nal detector is comparable with that obtained from a standard 6Li scintillator. Moreover in the latter case (fig.

9b) we stress that the foil out measurement represents an effective background level which is inherent in the SDT approach and for this reason cannot be avoided in the

experimental technique. On the other hand the intrinsic advantage of the RDT approach, is that low background γ detection, which can be achieved by the use of more sensitive detectors or more suitable shielding, can provide a remarkable increase in the signal/background ratio. In this case the RDT signal will provide the direct measurement of the scattering signal, rather than a difference between two large count-rates as in the SDT, so that bigger statistical accuracy can be achieved.

A second result which has been assessed by the use of NaI detector is a neutron energy window up to 90 eV, including four distinct resonance peaks, as shown in Fig. 6.

This experiment has also allowed for the first time to demonstrate experimentally that a net decrease of the intrinsic width of the 6.6 eV resonance peak can be achieved by employing the double difference spectrum technique, with two uranium foils of different thickness, as suggested in previous papers.

In conclusion the present results provide a useful empirical basis for ongoing investigations of the Resonant Detector Technique using different combinations of resonant foils and γ-detectors. We nevertheless believe that much better results should be expected by further improvements in the shielding design and, especially, by the use of more advanced γ-detectors. It is therefore our intention to further investigate the use of different γ detectors, or combinations of detectors. In order to exploit the RDT in the neutron energy range 10÷100 eV several improvements are envisaged.

Most important is the signal/background ratio, which needs to be increased by over an order of magnitude.

Since some background component scale with detector volume, it is mandatory to consider small detectors which, however, can only detect low energy γ with adequate efficiency. A promising approach is to look for resonant foils that have a large probability (say, >50%) of emitting low energy γ‘s following resonant neutron capture. Several examples of suitable foils exist and will be tested in the near future. A second requirement concerns the time resolution of the measurements. As the neutron energy approaches 100 eV, the peaks in the time of flight spectrum become very narrow and an estimated time resolution of 50 ns appears to be required and can be met by scintillator and solid state detectors.

Another comment regards the count rate. Due to the large background, count rates exceeding 100 kHz were recorded with the present Nal detector. This was more than a regular spectroscopy amplifier could handle, so that time amplifiers had to be used instead.

By going to low volume and low area detectors, the rate should be kept to suitable levels allowing full energy analysis of the events. This is necessary in order to set tight γ energy windows for enhanced background rejection. The ultimate solution in background

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suppression is the use of coincidence techniques.

These have the drawback of decreasing the overall system efficiency by typically an order of magnitude: depending on the intensity of the scattered neutron flux, coincidence technique may turn out to be too costly in term of beam time requirements. The latter should nevertheless be investigated for applications where measurements of highest quality needed.

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3.

4.

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7. G. Watson : J. Phys. Cond. Matter 8, 5955 (1996) 8. J. Mayers, A.C. Evans: Technical Report RAL-TR-96-044 (1996) 9. A.L. Fielding, J. Mayers: Technical Report RAL-TR-2000-013 (2000) 10. L.Landau, E. Lifsitz: Fisica Teorica MIR ed. vol. 3 (1982) 11. R.L. Heath: “Scintillation spectrometry” IDO-16880 Vol 1&2 (1964) 12. W.W. Morgan: IRE Trans. On Nucl. Sc. Vol NS-9 n^3 (1962) 13. J.R. Prescott, P. Ireadal : Nucl. Instr, Meth. 11, 340 (1961) 14. H. Rauh, N. Watanabe: Nucl. Instr. Meth. in Phys. Res. A 222, 507 (1984) 15. M.S. Nelkin, D.E. Parks: Phts. Rev. 119, 1060 (1960) 16. F. Mughabghab, N.E. Holden: “Neutron cross sections”, Academic Press, NY 1981, vol.1 17. P.A. Seeger et al: Nucl. Instr. Meth. In Phys. Res. A 240 (1985) 98 18. C. Andreani, G. Baciocco, R.S. Holt, J. Mayers: Nucl. Instr. Meth.

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Articolo ricevuto in redazione nel mese di Maggio 2001

STRUCTURAL AND MAGNETIC DISORDER IN CRYSTALLINE MATERIALS - A NEW VIEW Robert McGreevy, Anders Mellergård and Per Zetterström Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nyköping, Sweden “When I use a word,” Humpty Dumpty said in a rather scornful tone, “it means just what I choose it to mean - neither more nor less.”

‘Alice in Wonderland’ and ‘Through the Looking Glass’ were written in the 1860’s by Lewis Carroll (real name Charles Dodgson), a professor of mathematics at Oxford University.

‘Crystal structure’ - what does it mean?

What is meant by the ‘structure’ of a crystalline material?

We are all used to seeing pictures of ‘crystal structures’, such as that in fig. 1, but what do they actually show? Is this really ‘where the atoms are’?

Fig. 1. The ‘crystal structure’ of NaCl.

This article has a specific scientific purpose, which is to describe how the technique of ‘total scattering’, in combination with reverse Monte Carlo (RMC) modelling, can provide a different but complementary view of the structure of crystalline materials, including magnetic materials, which can be a valuable aid in understanding their properties. It also has a more general purpose,

which is to suggest that we all have to be careful that our view of nature is not determined too strongly by the particular methods that we choose to study it. Following the theme of ‘Through the Looking Glass’, the article is written ‘back-to-front’. First you can read about some interesting science and then, if still interested, find out how it was done. But before we start a short summary will be helpful.

‘Conventional’ crystallography measures elastic (Bragg) scattering which gives a ‘time average picture’ of the structure of a crystalline material. In the total scattering method energy integrated scattering is measured, which gives an instantaneous ‘snap-shot picture’ of the structure. These ‘pictures’ can be very different even though there is only one ‘structure’!

A simple example - rotational disorder As a simple first example we can look at NH4Cl, or in fact at its deuterated analogue ND4Cl. The ‘conventional’ view of the crystal structure is shown in figure 2(a). Cl– ions form a simple cubic structure within which the tetrahedral ND4+ ions can have one of two orientations, and between which they can rotate at higher temperatures. Two questions arise: how do they rotate and are there correlations between the orientations of neighbouring ND4+?

The model shown in figures 2(b)-(e) can be used to answer these questions very easily. Note that the 2 × 4 = 8 D positions in 2(a) form the corners of a cube. If you look carefully in 2(b) you can see the orientations of individual ND4+. No particular correlations are found between neighbouring ions. 2(c) shows that there are a variety of orientations, not just those shown in 2(a). This is of course obvious - if the ions rotate then they must spend at least some time at ‘intermediate’ orientations and in addition there are ‘normal’ thermal vibrations. 2(d) shows this more clearly, but also shows that the D distribution is not completely spherical so the ‘intermediate’ orientations are less common than the ‘equilibrium’ orientations of 2(a). 2(e) then shows that the most likely path between the ‘equilibrium’ orientations is approximately along the ‘edges’ of the D ‘cube’. D positions on the ‘faces’ of the ‘cube’ are rare, which is why

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(a)

(d)

(b)

(e)

Fig. 2. The ‘structure’ of ND4Cl [1]. (a) ‘Conventional’ crystal structure showing the two ND4+ orientations. (b) 10 Å thick slice from a ‘snap-shot’ model of 10368 atoms (12x12x12 unit cells) - N are red, D are green and Cl are blue. (c) A projection of the whole model. (d) As (c) but individual atoms are now translated into a single unit cell. (e) An isodensity surface of the distribution shown in (d). The D density is 0.27 of its maximum value.

(c)

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the red N can still clearly be seen through the green D in 2(d). Translated into more scientific language this means that ND4+ rotate preferentially about two-fold axes, with less than 5% rotating about three-fold axes.

Paddle wheels and protons Our second example also concerns rotational disorder, but now coupled with diffusion. The ‘paddle wheel’ model was developed many years ago to explain ion conduction in Li2SO4 [2]. The basic idea is that as SO42– ions rotate they carry with them the small Li+ ions, i.e. rotation and diffusion are highly correlated. This led to arguments stretching over nearly two decades, since others believed that uncorrelated diffusion (percolation) could also explain the experimental results [3]. The ‘paddle wheel’ model was invoked to explain many other systems, sometimes under other names such as ‘turnstile mechanism’. For example the model was used to explain proton conduction in CsHSO4 on the basis of quasi-elastic neutron scattering (QENS) data [4].

Figure 3 shows that although the SO42– ions are very disordered due to their rotations and vibrations, a preferential orientation can still be identified - clusters of O positions can be seen forming the vertices of tetrahedra. The D distribution is also highly disordered and follows a curved path very similar to that proposed on basis of QENS data [4]. One thing that is immediately obvious is that the D pathway intersects strongly with the O positions occupied when the tetrahedra are rotating.

Since D and O cannot be in the same place at the same

Fig. 3. The high temperature proton conducting phase of CsDSO4 [5]. The individual atom positions in a 5376 atom (8x8x3 unit cell) model have been translated into a single unit cell - S are green, O are red and Cs are not shown. The D density distribution is represented as a contour plot in two planes, red indicating high density and blue low density.

time, rotation and diffusion must be correlated.

Fig. 4 shows the partial radial distribution function for O and D, gOD(r), as determined from the model in figure 3; this function gives the probability that a D atom can be found at a distance r from an O atom. The first peak in gOD(r) occurs at the same distance at high and low T, which is the same as the shortest O-D distance in the time

Fig. 4. gOD(r) for CsDSO4 in the non-conducting phase at T < 412 K (bottom) and in the conducting phase at T > 412 K (top). Bars indicate DO distances determined from the time-average crystal structure.

average low T structure, but not in the time average high T structure. Actually from figure 3 it is clear that the high proton diffusion rate at high T makes it completely meaningless to try to define a time average D position - D will always spend more time away from such a position than at it. The other thing to note is that the peak in gOD(r) is still sharp at high T (in fact even sharper than at low T), despite the high level of disorder in both O and D positions shown in figure 3. This means that not only are SO42– rotations and D+ diffusion correlated - they are highly correlated! For the majority of the time they must effectively exist as DSO4– ions, with individual D being exchanged between chains of neighbouring ions in a way that is also highly correlated - a ‘multi-paddle wheel’ mechanism. It is interesting to note that a similar study of Li2SO4 concluded that the rotation/diffusion correlation was much weaker - effectively both ‘paddle wheel’ and ‘percolation’ mechanisms took place with approximately equal probability [6]. This explains the long running controversy - both were right!

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Colossal magnetoresistance (CMR) The third example is related to magnetic disorder. CMR materials have been known about for almost 40 years, but in the last decade there has been a revival of interest due to the realisation that thin films could be made with the potential for applications in devices such as magnetic recording heads. Various experimental and theoretical studies have invoked a variety or combination of mechanisms to explain their extreme properties - double exchange, lattice polarons, magnetic polarons, spinlattice coupling etc.

Some results from a structural model for the CMR material La0.8Sr0.2MnO3 are shown in fig. 5 [7]. It is worth stressing that the average structure of this model is highly consistent with that determined from Rietveld refinement, and with other studies in the literature. The radial distribution function for the magnetic Mn atoms shows the separations of magnetic moments.

The average cosine between moments as a function of their separation has a high value at low T where there is long range ferromagnetic order (0.6 is close to the predicted quantum mechanical expectation value).

As T increases above the magnetic ordering temperature the cosine decreases to zero at long distances but remains finite at short distances, indicating that short range magnetic ordering remains (magnetic polarons).

At the same time there is the onset of short range lattice disorder (lattice polarons) which can be monitored

through the width of the first peak in gMnO(r).

These results show that lattice and magnetic polarons coexist at high temperatures. However the model can reveal even more subtlety. If you look carefully you can see that at high T the average cosine actually rises steeply as r decreases for distances within the first peak of gMnMn (r). The full cosine distribution function, shown in fig. 6, explains this. When two Mn moments are slightly closer together they have a higher probability of being ferromagnetically aligned (cos θ = 1), giving the peak at the back right in the distribution. If they are slightly further apart they have a higher probability of being antiferromagnetically aligned (cos θ = –1), giving the peak on the front left. Lattice and magnetic polarons are therefore coupled - indeed they are one and the same.

Since La0.8Sr0.2MnO3 has long range ferromagnetic order at low T the ordered magnetic structure is rather uninteresting to look at! Just for a comparison with the pictures of atomic structure shown in figs. 2 and 3, we show in fig. 7 the magnetic structure of the ‘classical’ antiferromagnet MnO, above and below the ordering temperature [8].

How is it done?

The structural models described above were all obtained using the combination of total neutron scattering and RMC modelling. In the next section we describe the theoretical basis behind the total scattering method. This

Fig. 5. Various correlations relating to magnetic ordering in La0.8Sr0.2MnO3.

Top: Radial distribution function for Mn atoms, gMnMn(r).

Centre: Average cosine between magnetic moments on Mn atoms as a function of their separation. Red - 15 K; Green - 200 K; Blue - 300 K; Magenta - 900 K. Bottom: Variation of short range lattice distortion (red), short range magnetic order (green) and long range magnetic order (blue) as a function of temperature. The arrow indicates the macroscopic magnetic ordering temperature.

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picture’. We then discuss some particular aspects of the experimental technique and outline the RMC method.

Finally we look to the future …

Fig. 6. Spin-spin cosine distribution function, p(cosθ(r), r), for La0.8Sr0.2MnO3 at 325 K. Only distances corresponding to Mn-Mn near neighbours, 3.4 < r < 4.4 Å, are shown.

Elastic diffraction and total scattering.

What is the difference?

To understand the differences between ‘conventional’ diffraction and total scattering we must first go through some basic theory. In a powder diffraction experiment one usually measures the number of neutrons scattered from a sample as a function of scattering angle relative to the incident beam. Here we only consider a monochromatic incident beam, though the principles are no different for time of flight techniques, and we restrict ourselves to a monatomic sample for simplicity. At each point in the diffraction pattern the detector actually measures the total number of neutrons scattered as a function of angle (θ ) relative to the incident beam. The intensity, I(θ ), is therefore related to an energy integral of the dynamical structure factor, S(Q,ω), with the limits of the integral depending on the incident energy, E0. The momentum transfer Q is a function of both θ and ω.

I(θ)∝

∞

∫ S(Q,ω)dω|θ

(1)

–E0

S(Q,ω) is related to the van Hove correlation function G(r,t), which contains information on the positions of atoms as a function of time.

S(Q,t) = ∫eiQ,r G(r,t)dr = = 1 ∑ <e–iQ,ri(0) eiQ,ri(t)> N S(Q,ω) = ∫S(Q,t) e–iωt dt

(2)

(3)

where the sample contains N atoms at positions Rj(t).

S(Q,t) is known as the intermediate scattering function. If the incident energy is much higher than any excitation energy in the sample then a constant angle integration is equivalent to a constant Q integration, and the lower limit of the integral in eq. (1) goes to infinity, so we obtain the total structure factor, F(Q), I(θ)∝ F(Q) = Fig. 7. The magnetic structure of MnO above (top) and below (bottom) the antiferromagnetic ordering temperature. Magnetic moments are colour coded on a scale from red to blue depending on the cosine of the angle they make relative to the <110> (pseudocubic) direction.

∫ S(Q,ω)dω

–∞

= S(Q,t = 0) = ∫eiQ,r G(r,t = 0)dr

(4)

= 1 ∑ <e–iQ,(Ri(0) – Ri(0))> N i,i

can be skipped if you are willing to accept the idea that elastic scattering gives a ‘time average picture’ of the structure while total scattering gives a ‘snap-shot

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∞

|∑

=1 N

•

i

<e–iQ,Ri(0)

|

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Fig. 8. Total scattering data for Mn0.6Ta0.4O1.65 at 300K [9]. Red - data; green - RMC model fit; Magenta - difference between data and fit; Blue - lattice diffuse scattering; Olive - magnetic diffuse scattering.

Fig. 10. Single crystal X-ray diffraction pattern calculated from a RMC model for Mn0.6Ta0.4O1.65 fitted to powder diffraction data as shown in fig. 8.

(c)

(a)

(d) Fig. 9. (a) Time average crystal structure of the double perovskite Ba2FeWO6 [16]. Magnetic (Fe) ions occupy alternate octahedra. Possible magnetic structures obtained are (b) colinear and (c) non-colinear. (d) The fits to the data are almost identical. Red - data; Blue - colinear fit and magnetic contribution only (dash); Green - non-colinear fit and magnetic contribution only (dash); Olive - difference of magnetic contributions x10.

(b)

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For an isotropic sample (e.g. a powder) F(Q) is related to the radial distribution function, g(r) = G(r,t=0), by F(Q) = 1 + ρ ∫ 4πr2 sin Qr (g(r)–1)dr Qr

(5)

In ‘conventional’ crystallography it is normally assumed that all sharp peaks in the scattering pattern represent elastic scattering, that is Bragg scattering. All other scattering in between or ‘under’ the sharp peaks may be either elastic or inelastic and will here be referred to as diffuse scattering - we make no distinction as to the source of that scattering. This diffuse scattering is simply subtracted and not used in the structural analysis. The elastic scattering only is then related to the elastic structure factor, S(Q), S(Q) = S(Q,ω = 0) = ∫S(Q,t)dt =1 ∑ N i,i

(6)

∫ <e–iQiRi(0) eiQiRi(t)> dt

To a first approximation the integral can be performed and the sum over the N atoms in the sample replaced by a sum over the Nc atoms in the unit cell, giving the more familiar expression

|∑

S(Q) = 1 e–WQ2 N

k

|

<e–iQi<Ri(t)>

2

(7)

where <Rk(t)> are now the time average positions of atoms in the unit cell and W is the Debye-Waller factor (DWF), which is related to the mean square deviation (msd) of atoms from their average positions.

The elastic structure factor is therefore determined by the correlations between the time average atomic positions (eq. 7), while the total structure factor is determined by the correlations between the instantaneous positions (eq.

4). More simply put, the elastic structure factor gives us a ‘time average picture’ of the structure of the material, while the total structure factor gives us an instantaneous ‘snap-shot picture’. Both pictures are space averages (<…> indicating that functions are averaged over all atoms taken as the origin), but only at T = 0 are they the same. At other T the structure is not static but dynamic and so neither picture is ‘correct’.

Which picture is more useful depends on the problem being investigated. For most crystallographic studies the time average structure is the subject of interest and so elastic scattering is sufficient. In a liquid on the other hand all atoms are diffusing, so there is no time average structure and hence no strictly elastic scattering - only the instantaneous structure is meaningful. For crystalline structures with a high degree of dynamical disorder, for

example fast ion conductors, both time average and instantaneous structures may be of interest.

Experimental technique In principle a total scattering measurement should integrate over all energies at constant Q, but in practice this is not possible for neutrons. It may then be considered that the ‘picture’ obtained is an average over a short time interval, which is related to the incident neutron energy. Motions that are significantly faster than this interval will not be seen. It should also be remembered that there will be a finite distance resolution.

In most cases a reasonable approximation to F(Q) is obtained by using wavelengths of order 1 Å or less.

In a total scattering study one needs to measure both the Bragg scattering, i.e. sharp peaks, and the diffuse scattering, which is typically a broad ‘background’ scattering. This means that the true background, including scattering from any container, sample environment etc., must be separately measured and properly corrected for, including the effects of absorption. In addition it is valuable to normalise the data to an absolute scale, which can be done using a vanadium standard. Fortunately the techniques for data correction have been well developed over many years for studying the structures of liquids and glasses. It is worth noting that in the end the amount of information that can be extracted is strongly dependent on the quality of the data and the corrections, so their importance should not be underestimated.

The choice of instrument is not straightforward - there are no instruments that have been specifically built and optimised for total scattering studies. High resolution powder diffractometers give sharp Bragg peaks but usually poor statistics in the diffuse scattering, and such instruments are not normally designed for low background. Diffractometers designed for studying liquids and glasses usually have good control over the background, and high intensity for measuring weak diffuse scattering, but the resolution is too low.

Another relevant parameter is the Q range - there is no ‘redundancy of information’ in the diffuse scattering so in principle all Q are required. In practice this is not possible so the choice of Q range depends on the method of analysis used (see the next section) and the particular system under study. For example, for magnetic structures the relevant information occurs predominantly at low Q. Pulsed source powder diffractometers, such as GEM at ISIS, now offer excellent resolution, high count rates and a wide Q range. However they are not really optimised for reduced background or low Q measurements and the high count rate is achieved partly by the use of longer wavelengths. If we consider reactor source instruments

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then D20 at ILL gives a high count rate at a suitably short wavelength (< 1 Å), but a reduced background and an extension to lower Q are required. All of the results presented in this article were obtained using the SLAD diffractometer at NFL Studsvik. Here the count rate is not as high as would be liked, and the angular calibration is limited by the use of linear position sensitive detectors, but the background is low, particularly at low Q.

Reverse Monte Carlo (RMC) modelling There are two main methods used for the analysis of total scattering data. Here we will concentrate on RMC modelling. The alternative is called the pair distribution function (PDF) method [10]: F(Q) is Fourier transformed directly to give to g(r) (the inverse of eq. 5), which is then fitted by a parametric model, rather like a real space version of Rietveld refinement. The main disadvantages of the PDF method are that wide Q range data are needed to enable a direct transform, and magnetic scattering cannot be dealt with.

RMC modelling has been described in detail elsewhere [e.g. 11] so we will only give a very brief description here.

An initial three dimensional structural model of a system is created, typically consisting of many thousands of atoms (i.e. many tens to hundreds of unit cells for crystalline materials). Periodic boundary conditions are applied to make the model effectively infinite. From the model we calculate a function which can be compared to experiment, e.g. g(r) or F(Q). Atoms are then moved randomly, or magnetic moments rotated, with moves being accepted or rejected in such a way that eventually the model agrees with experiment within the errors. Any set of data can be modelled, provided that the relationship between the model and the data can be written down, and different sets of data can be modelled simultaneously. In addition constraints can be applied, for example chemical bond length limits and coordination numbers.

Initial models can be created by a variety of different methods. In all of the examples given here they have been created by Rietveld refinement of the same data (Bragg scattering only), i.e. RMC has been used as a ‘secondary’ refinement and has not been used for ab-initio structure solution.

There are two basic variants of RMC. The first is to obtain g(r) from F(Q), usually by an inverse method (e.g. program MCGR [12]) and then to fit the RMC model in r space (program RMCA [13]). An approximate equivalent to this is a convolution of F(Q) followed by fitting in Q space [14]. These methods, orginally developed for studies of liquids and glasses, have the disadvantages that the experimental Q space resolution cannot be taken into account and magnetic scattering cannot be dealt

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with. A more recent development (program RMCPOW [15]) allows direct modelling in Q space, including magnetic scattering and treatment of the experimental Q space resolution. A hybrid version which fits in both Q and r space has also been developed [16], but this does not deal with magnetic scattering.

One criticism that is commonly used against RMC modelling is that the results are not unique - many different models can be created that agree with the same data. This is actually an advantage - not a disadvantage!

If there is any aspect of a model which you ‘know’ is wrong then you must have additional information which was not used in making the model. You should then obviously use this information, either as an additional set of data or in the form of a constraint, so that the model does agree with it. Alternatively constraints can be applied to test how well different ideas fit the data.

A sideline: ab-initio determination of magnetic structures It turns out that the RMCPOW program is also a useful tool for aiding ab-initio determination of (time average) magnetic structures from powder diffraction data. In this case only the Bragg peaks are fitted and a small model is used, chosen to be consistent with possible magnetic supercells. The method has two main advantages. Firstly it can start from a random initial structure so no prior idea or knowledge is required. Secondly it may be possible to find more than one spin structure which is consistent with the data. These can either be distinguished on the basis of other available information, or used to suggest further experiments that might distinguish them, e.g. single crystal or higher resolution studies.

Where do we go from here?

X-ray diffraction is also an excellent technique for total scattering studies. It has the advantages that both high resolution and good statistics can be achieved, particularly at 3rd generation synchrotrons, and because of the high photon energy F(Q) is always measured. With the use of hard X-rays the Q range can also be wide. The disadvantages are that the data cannot be absolutely normalised and magnetic structures cannot be studied.

However there have been few X-ray studies, largely for historical rather than scientific or technical reasons.

EXAFS provides local atomic specific information and is a useful complement to X-ray and neutron diffraction.

One future development is therefore the combination of all of these data types (this has already been done for amorphous materials [18]). Further refinements would be the use of isotopic substitution in neutron diffraction or anomalous scattering in X-ray diffraction to provide even

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more atom specific information, or polarisation analysis in neutron scattering to distinguish atomic and magnetic scattering.

It might also be asked why all of the models described here have been based on power diffraction data, whereas diffuse scattering from single crystals has been the ‘traditional’ method for studying disorder in crystalline materials.

Single crystal data provides three dimensional information, whereas powder data is a only one dimensional average. However single crystal data can in practice usually be measured only at a restricted number of Q points in a limited number of planes and is much harder to correct and normalise. In addition many new materials cannot be easily prepared as suitable single crystals, particularly if one is interested in studying for example, many different compositions.

However in general terms one can see that it would actually be advantageous to be able to combine powder and single crystal data. While this has not yet been done, we show in figure 10 a calculated single crystal pattern based on RMC modelling of powder diffraction data.

The first thing to note is that the pattern is far from isotropic, even though it is based on ‘isotropic’ data. In fact the positions and directions of the diffuse scattering streaks agree qualitatively with experimental results. The challenge will be to deal with the data corrections, normalisation, resolution etc, so that a quantitative comparison can be made.

Conclusion Total scattering, in combination with RMC modelling, can provide a new and often valuable insight into the ‘structures’ of many crystalline materials. The method is still relatively young and the results can be surprising and controversial. What should be remembered above all is that the purpose is not to provide a ‘correct’ model of a crystal structure in which every atom sits in exactly the right place. Indeed we have tried to explain that there is no such thing as a ‘correct’ model. Even if there was, no set or sets of data would ever contain enough information to tell us it uniquely. The purpose is to try to help us understand the relationships between ‘structure’ and other properties of the material. Our experience is that the limitations are either in the data, i.e. its intrinsic information content or its quality, or the way in which the method is used, and not with the method itself.

References 1. A.V. Belushkin, D.P. Kozlenko, R.L. McGreevy, B.N. Savenko and P. Zetterström Physica B 1999 269 297 2. N.H. Andersen, P.W.S.K. Bandaranayake, M.A. Careem, M.A.K.L. Dissanayake, C.N. Wijayasekera, R. Kaber, A. Lundén, B-E. Mellander, L. Nilsson and J.O. Thomas. Solid State Ionics 1992 57 203 3. E.A. Secco. Solid State Ionics 1993 60 233 4. A.V. Belushkin, C.J. Carlile and L.A. Shuvalov J. Phys.: Cond.

Matter 1992 4 389 5. P. Zetterström, A.V. Belushkin, R.L. McGreevy and L.A. Shuvalov.

Solid State Ionics 1999 116 321 6. L. Karlsson and R.L. McGreevy. Solid State Ionics 1995 76 301 7. A. Mellergård, R.L. McGreevy and S.G. Eriksson. J. Phys.: Cond.

Matter 12 2000 4975 8. A. Mellergård and R.L. McGreevy. J. Phys. Cond. Matter 1998 10 9401 9. S. Esmaeilzadeh. ‘Crystal chemistry of Manganese Tantalum Oxides’ Doctoral thesis. 2000 Stockholm University, Stockholm, Sweden 10. Th. Proffen and S.J.L. Billinge. J. Appl. Cryst. 1999 32 572 11. R L McGreevy Computer modelling in Inorganic Crystallography ed. C.R.A. Catlow. Academic Press (London) 1997 151 12. L. Pusztai and R.L. McGreevy. J. Neutron Res. 1999 8 17 13. R.L. McGreevy, M.A. Howe and J.D. Wicks. NFL Studsvik Internal Report 1994, or see http://www.studsvik.uu.se/ software/rmc/rmc.htm 14. V M Nield, D A Keen, W Hayes and R L McGreevy J. Phys. Cond.

Matter 1992 4 6703 15. A. Mellergård and R.L. McGreevy. Acta Cryst. A 1999 55 783 16. M.G. Tucker, M.P. Squires, M.T. Dove and D.A. Keen. J. Phys.: Cond. Matter 13 2001 403 17. A.K. Azad, S.G. Eriksson, J. Eriksen, S.A. Ivanov, A. Mellergård and H. Rundlöf. Private communication 2001 18. J.D. Wicks, L. Börjesson, G. Bushnell-Wye, W.S. Howells and R.L. McGreevy. Phys. Rev. Lett. 1995 74 726

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Articolo ricevuto in redazione nel mese di Maggio 2001

THE GAS PHASE PHOTOEMISSION BEAMLINE AT ELETTRA: A FACILITY FOR ATOMIC, MOLECULAR AND CHEMICAL PHYSICS RESEARCH M. Alagia1, L. Avaldi2, G. Battera2, R. Camilloni2, M. Coreno1 C. Furlani3, K.C. Prince4, R. Richter4, M. de Simone3, G. Stefani3, S. Stranges5 and G. Turri6 1

INFM, Gas Phase Beamline at Elettra, Laboratorio TASC, I-34012 Trieste, Italy 2 CNR-Istituto di Metodologie Avanzate Inorganiche, Montelibretti (Rome), I-00016 Italy, Abstract The Gas Phase Photoemission beamline at Elettra is devoted to the study of free atoms and molecules. The monochromator is a variable angle spherical grating instrument (plane mirror and spherical grating between entrance and exit slits), with an undulator as the source. The broad energy range (16-1000 eV), the high resolving power and flux together with the purpose built end-stations makes this facility ideal for investigating the spectroscopy and dynamics of basic processes like inner-shell and multiple excitations and ionization, as well as for characterising key processes in the atmosphere, in chemical reactions and in the preparation of new materials. The beamline and the end-stations are described and a selection of the results obtained in the first period of operation is presented.

1. Introduction The Gas Phase Photoemission beamline at Elettra has been proposed, designed and built by a research team from different institutions (Consiglio Nazionale delle Ricerche-CNR, Istituto Nazionale di Fisica della MateriaINFM, Dipartimento di Chimica-Università di Roma and Sincrotrone Trieste) with the goal to develop a research program focused on the study of free atoms and molecules for a better understanding of key processes in the atmosphere, in chemical reactions and in the preparation of new materials. The beamline has been commissioned and open to users since three years. Instrumentation for sample preparation and handling as well as charged particle transport, analysis and detection has been designed and built and is available to the users.

In the first period of operation, experiments devoted to investigating the spectroscopy and to characterising many-body interactions among the elementary constituents of matter in “reference targets”, like rare gases and simple molecules, have been performed, Furthermore studies of specific targets (radicals, polymer precursors, transition metal compounds) which play a relevant role in different areas of science and technology have been also realised.

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Dipartimento di Fisica e Unità INFM, Università di Roma III,Via della Vasca Navale 84, I-00146 Rome, Italy, 4 Sincrotrone Trieste, I-34012 Trieste, Italy, 5 Dipartimento di Chimica, Università degli Studi di Roma, “La Sapienza”, P.le A. Moro 5, I-00185, Rome, Italy 6 Dipartimento di Fisica, Politecnico di Milano and INFM Laboratorio TASC, I-34012 Trieste, Italy

The main requirements for the monochromator were high photon resolution over a wide energy range, stable spot size and high flux.

The design goal for the energy range was set as 20 to 800 eV. The resolution was required to be significantly better than the natural widths of the main atomic and molecular excitations in this range, and so the goal for the resolving power (∆E/E) was 10,000. A stable spot size means that changes in spot size, shape and position as a function of energy were to be minimised. This implied the use of a fixed exit slit and re-focusing system. The latter provides two additional advantages: firstly, the experimental station, which operates at pressures of 10-5-10-6 mbar, is further away from the optical elements of the monochromator. Secondly an almost circular focus can be provided, which is important when aligning several spectrometers and/or different excitation sources on the same target volume.

For the end-stations, the project foresaw the construction of two chambers available for users, and also to allow for the possibility that users bring their own chamber.

Both chambers are now available to users, and versatile support systems have been built for users who bring their own chambers. Both chambers have an exit port to which a windowless gas cell can be attached. The cell can be filled with gas independently of the sample in the main chamber, and used for photoabsorption spectroscopies. At the back of the gas cell, a calibrated photodiode can be inserted. Thus under suitable conditions, the user can measure simultaneously his/her spectrum; an absorption spectrum which gives a very precise calibration and a check of the monochromator resolution; and an I0 spectrum for normalisation to the absolute flux.

In this paper the beam-line and the end-stations are described in section 2, while a selection of the recent results of inner shell photoionisation experiments, electron-electron coincidence experiments and photoionisation of unstable and radical species are presented and discussed in sections 3-5.

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2. The beamline and the end-stations.

This section describes the characteristics of the beamline at Elettra and of the end-stations with a view to giving potential users the information they need to plan an experiment. A schematic diagram of the main components of the beamline is shown in fig. 1.

Most energy scanning spectroscopies are still done with fixed gap, although the gap of the undulator can be adjusted over a wide range manually or by software.

Changing the gap is a relatively slow process (about 2 minutes), due to the time taken for software overheads.

We are presently developing software to permit automated gap scanning in cooperation with the Elettra Beamline Control Group. Table 1 shows the results of a series of measurements of flux at the gas cell after the experimental chamber.

The optical concept of the monochromator and of the beamline has been published previously [2] as well as the first results [3]. In brief, there are two mirrors before the entrance slit; a plane mirror and a set of four spherical gratings (Table 2) between the entrance and exit slits.

Energy (eV)

Fig. 1. Schematic diagram of the beamline layout

2.a The beamline The light source of the beamline is an undulator of period 12.5 cm in section 6.2 of the Elettra storage ring; details can be found on the Elettra home page [1]. The lowest photon energy produced with the ring running at 2 GeV is 14 eV. The lower curve of fig. 2 shows a typical absorption spectrum with fixed gap: both the ion yield and the photodiode spectra are shown. The photodiode curve shows weak absorption due to the gas in the gas cell and indicates that absorption is still in the linear regime. The overall shape is determined by the variation of the flux due to the undulator output, and the full width at half maximum is about 4% of the peak energy.

Fig. 2. Photoabsorption spectra of the He doubly excited states below the N=2 threshold. Lower curve: ion yield spectrum, normalised to the flux.

Centre curve, “Undulator Gap 63.5”: photodiode signal.

Resolving power

Flux (photons/sec/100 mA)

45

>25,000

6.3•1010

65

>28,000

2.2•1011

86

> 10,000

1.5•1011

245

12,200

1.5•1010

401

>12,000

1.1•1010

540

10,000

2.0•1010

680

10,000

3.0•109

Table 1. Flux and resolution at selected energies.

Grating number

Lines/mm

1 1st order

Energy range (calculated)

400

20-50

2

1200

80-180

3

1200

160-430

4

1200

360-1000

1 2nd order

40-90

Table 2. Gratings and energy ranges.

The first mirror, the plane Switching Mirror, is used to steer the beam vertically so that it enters the entrance slit. This is important at the minimum slit width of 10 microns, but is a minor adjustment that is done once or twice a day. At large slit openings this is not normally necessary. The entrance slit is usually used at values between 10 and 30 microns, through which about 95% of the flux passes.

It is possible to mount an experimental end station at two different light exits (fig. 1). On the main line the light emerges from the fixed exit slit and is then refocused by two post-focusing mirrors. A first plane-elliptical mirror focuses vertically, while a second spherical mirror focuses the light horizontally. The beam exits at an angle of 4 degrees with respect to the horizontal, at a height of 180 cm. Thus the whole photon energy range (14-1200 eV) is available at the target , in an almost circular spot of few hundred microns.

Alternatively a toroidal deflection mirror can be inserted

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after the exit slit to steer the photon beam to the branchline (fig.1). The flux at higher photon energy decreases continuously with increasing photon energy due to the relatively large angle of incidence on the deflection mirror (6 degrees), and is about half the flux on the main line at 400 eV. The spot at the sample has an ellipsoidal shape, of approximate dimension 0.4 x 2 mm2. Some undulator curves showing flux and spectral distribution are shown in fig. 3.

The monochromator can be scanned in two different modes. In energy scan mode, both the mirror and grating of the monochromator are scanned simultaneously. This is the preferred mode of operation at low to medium

Fig. 3. Flux on the branch line as a function of photon energy (resolving power ~ 2000).

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energy resolving power, ∆E/E < 7000. The scanning of two mechanical systems (mirror and grating), doubles the contribution of mechanical errors to the degradation of the resolution. To avoid this, at higher resolving power fixed mirror scanning is preferred. The mirror is set to the average value for the range of energy to be scanned, and only the grating is scanned. A typical high resolution spectrum obtained in the region of helium double excitations is shown in fig. 2. This mode gives the highest resolving power, from 30,000 at 48 eV to 10,000 at 540 eV under standard conditions (see also refs. [3-5]).

It has the disadvantage that the defocus contribution to the resolution changes slightly over the scan. However at high resolution, the required step size is usually small and so the scan width is limited. If scans are confined to about 2% of the average energy, the resolution is approximately constant as the defocussing term is sufficiently small that it does not affect resolution. The resolution is so high that the mechanical scanning (which is sometimes in steps as small as 200 nm) is working at its limit, or beyond it, for example at the oxygen K-edge. In particular stick-slip behaviour and small jumps are observed which are probably due to the surface finish of the components of the drive screw. A piezo drive has been installed in series with the mechanical system to solve this problem. The piezo allows a minimum step size about ten times smaller than that achieved with mechanical scanning.

At the time of commissioning, all photoabsorption spectra taken between 90 and 900 eV showed resolution which was higher than or at least equal to published spectra [3-5]. At the nitrogen 1s->π absorption resonance, the ratio of the third peak height to the first minimum is often taken as a criterion of resolution. We obtained a value of 0.62±0.01, considerably lower than most published values [6-10]. For instance, Domke et al [9] used an SX700 monochromator and obtained a value of approximately 1 in first order, although their spectrum taken with second order light and masking of most of the grating was better. Watanabe et al [10] used a variable line space grating and their figure 6 seems to give a value of about 0.7.

They obtained an estimated resolution of 40.9 meV for an intrinsic line width of 117 meV. We estimate our resolution to be 35 meV for a line width of 115 meV. This performance at high energy has only recently been surpassed by the very newest beamlines [11, 12].

At low energies, the resolution is also very high, being approximately 1.6 meV and 2.2 meV at 45 and 65 eV (measured using neon and helium photoabsorption spectra respectively.) These results were obtained using first order light with no masking of the grating and the minimum slit settings, 10 microns. For helium we can do even better by using grating 3 and closing the slits to 5 microns, where we obtain a resolution of about 1.2 meV.

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This value is lower than the value reported from the Advanced Light Source [13], but it is competitive due to the high flux and ease of obtaining this resolution.The energy calibration has been carried out by measuring a series of absorption spectra at different mirror settings and selecting the value which maximises resolution. This provides a calibration point with a value of the photon energy, mirror and grating settings. A mathematical model has been developed [14] into which this data point is inserted, and then the functional describing the calibration curve is solved numerically. At least 3 data points per grating are necessary. Re-calibration after maintenance (which may change certain mechanical constants of the system) is simpler as it is assumed that the new calibration curve is the same as the previous one but that the grating and mirror are displaced in angle with respect to the old value. This assumption works reasonably well and a grating can be recalibrated with only two calibration points.The excellent resolution and flux have allowed a number of user groups to obtain outstanding results in the 48-72 eV energy range using fluorescence techniques [15-17]. The fluorescence signal is far lower than electron or ion signals and so users have combined the high flux of the beamline with the high efficiency of their own detectors to achieve these results.

In the higher energy range, the third and fifth harmonics of the undulator can be used. However at highest energy (800-1000 eV), it is sometimes convenient to use the insertion device in the wiggler mode. In this operation mode the gap is close to its minimum value and the flux becomes relatively smooth. The exact energy at which the wiggler mode becomes more advantageous than the undulator mode depends on machine conditions, which can vary.

A remaining problem which is still to be solved is the presence of higher order light at lowest energy. For the moment, a partial solution is to use aluminium and magnesium filters. In the longer term, an optimised grating for low energy will be installed (it is on order) and a branch line with a large angle of deflection will be installed.

2.b The end-stations The two originally planned end-stations are now available on the beamline.

The 800 mm I.D. multicoincidence end-station is lined with a 2 mm thick µ-metal shield, additionally screened by three pairs of coils; the residual magnetic field is less than 10 mG. Two independently rotatable arrays of hemispherical electrostatic analysers are housed in the chamber (fig. 4). Seven spectrometers are mounted on a turntable that rotates in a plane perpendicular to the direction of the incident beam. The polarisation vector of the radiation is in the same plane of these analysers.

Fig. 4. The multicoincidence end-station with the two turntables and the electron spectrometers.

Three other spectrometers are mounted on a smaller turntable that rotates around the direction of the polarisation vector of the radiation. This latter array allows photoionisation experiments out of the polarisation plane, to study, for example, non-dipole contributions to the photoionisation cross-section. All ten analysers are mounted at angular intervals of 30°.

The spectrometers are composed of two four-element lenses (194 mm long) that focus the photoelectrons from the target region onto the entrance slits of the hemispherical deflector. The lens system can be operated in two modes: a “low resolution” mode characterised by a ∆E/Ek=10-2, where Ek is the kinetic energy of the photoelectrons, and an angular acceptance in the dispersion plane of ±3°. This mode is most suited for coincidence experiments. The second mode is a “high resolution” mode with ∆Ek/Ek > 10-4 and an angular acceptance of ±0.5° used for non-coincidence photoelectron and Auger electron spectroscopies. The lens stack can work with retarding ratios of 2-5 and 5-100 in the low- and high-resolution modes, respectively. The mean radius of the hemispherical deflector is 33 mm and the gap 9.9 mm. These values have been chosen in order to obtain a maximum

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time spread of 2 ns in the trajectories of 100 eV electrons in the dispersing element.

An example of the performance of the analyser when operated in the high-resolution mode is shown in fig. 5, where the spectrum of Ar+ (3p-1) taken at 244.4 eV incident photon energy, the energy of the 2p3/2→4s resonance, is shown. The pass energy was 3 eV, corresponding to

quartz capillary mounted between the beam line and the chamber prevent contamination of the beamline. These design features allow the study of aggressive gases and vapours. Two VSW hemispherical photoelectron energy analysers (50 mm mean radius) can be mounted with a mutual angle of 90˚ on a turntable. The 4-element lens system of each analyser has an acceptance angle of ap-

Fig. 6. The VSW hemispherical analyser and the time-of-flight spectrometer of the ARPES end station.

Fig. 5. Ar+ (3p-1) photoelectron spectrum at 244.4 eV incident energy. A pseudo-Voigt function (full line) has been fitted to the data. Pass energy 3 eV; FWHM and analyser resolution 65 and 50 meV, respectively.

a retarding ratio of 76. The entrance and exit slits were 1 and 4 mm respectively. The FWHM of the measured peaks was 65 meV giving a resolution of 50 meV after the deconvolution of the contribution from the incident light. This shows that despite the small size of the hemispheres good energy resolution can be achieved with a reasonable count rate.

The experimental chamber of the ARPES (Angular Resolved Photon Emission Spectroscopy) end station consists of a 500 mm I.D. cylindrical vessel lined with a double µ-metal shield (fig.6). Gaseous and volatile liquid samples can be introduced into the ionisation region through a hypodermic needle mounted on an XYZ manipulator. Solids can be vaporised in a high temperature, anti-inductively wound oven, in which temperatures up to 1100˚ C can be obtained and maintained for several hours. The oven and the ionisation volume are enclosed in a cooled jacket designed to minimise contamination.

An efficient cryo and turbo pumping system and a

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proximately ± 3˚ and has been modified to suit high temperature experiments. Pass energies from 1 to 50 eV can be selected. A resolution of ~ 50 meV (FWHM) has been obtained for 25 eV kinetic energy electrons, using 2 eV pass energy, and 180 meV resolution for 130 eV kinetic energy electrons with 10 eV pass energy.

Valence and S 2p and C 1s core photoemission spectra have been obtained for solid and liquid organic samples [18,19]. Alternatively, a time-of-flight mass spectrometer can be used to study high temperature vapors and volatile liquid samples. Total-ion-yield spectra (NEXAFS) have been recorded for moderately large organic molecules (solid and liquid samples) in the sulfur 2p, nitrogen 1s, and carbon 1s threshold regions [20,18]. In small reactive molecules, partial-ion-yield and ion-ion coincidence spectra have been obtained. The same mass spectrometer can be rotated with respect to the polarization plane of the light to study the angular distribution of the fragment-ion emission in the decay of resonance states of small molecules.

The modular design of the ARPES chamber and large side flanges allow users’ spectrometers to be mounted. Various detectors for fluorescence, ion, and metastable species studies have been used successfully [15,16,21].

Two windowless photoabsorption cells are available and can be attached to the end-stations or to users’ apparatu-

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ses via a CF40 flange. A small one is housed in a six-way stainless steel cross and mounted on an x-y manipulator to allow easy alignment. Two plates 100 mm long and 20 mm wide are used to collect ions (or electrons depending on the bias voltage). The photoabsorption spectrum shown in fig.1 has been collected with this cell. The second one is a double-ion chamber, built to perform absolute measurements of the photoionisation cross section.

This consists of a cylindrical electrode at positive potential, and two ion collectors of well defined length L, similar to the design of Samson [22]. This approach minimises the difficulties due to the presence of higher order or stray light at the exit slit of the monochromator and the errors due to the inhomogeneous electric field in the ionisation cell. The pressure is measured by a calibrated Baratron.

The data acquisition system and the control of the endstations are based on personal computers. Versatile and user-friendly software packages in the LabVIEW G language (National Instruments™), in permanent communication with the Beam Line Control System (BCS) of Elettra, that controls the monochromator and the status of the beamline, have been developed to perform photoabsorption, photoemission, electron-electron and electron-ion coincidence experiments.

3. Inner shell photoabsorption, photoemission and fluorescence studies The very high energy resolution available, which is generally better than the natural width of inner shell excited states, can provide new spectroscopic information and detail about basic quantities such as transition energies, natural linewidths, relative intensities, spectral strengths and line shapes. Here we report some examples of this kind of investigation.

Until recently it was believed that the O 1s core hole width was too large to resolve vibrational structure in the core excited state, and hence to derive excited state parameters of oxygen containing molecules. We have shown that a full Franck-Condon analysis can be performed for the O2 and CO molecules [23], fig. 7, as well other diatomics such as the radicals OH and OD [24].

The parameters which were derived, such as lifetime width and vibrational quantum energy, are important data for a number of other spectroscopies such as fluorescence scattering and vibrational interference in Resonant Auger Raman spectra. If the beamline resolution and the width are not the limiting factors in resolving core excitation spectra, then the absence of fine structure must have a different physical origin. The antibonding states in the core excitation spectra of N2O and CO2 for instance were not vibrationally resolved, although the Rydberg states were [25]. We conclude that in this case the excitation to the anti-bonding states excites multiple

vibrational modes which overlap.This work has been continued with studies at the carbon and the oxygen K edges of a series of small organic molecules containing oxygen: H2CO, CH3CHO, (CH3)2CO, (CH3)2O, CH3OH and HCOOH. High resolution spectra of formaldehyde (H2CO) display clear vibrational structure in both energy regions. While the spectra in the region of the carbon 1s excitation agree well with previously published results, due to the high resolution vibrational structure of the oxygen K-edge π* excitation has been observed for the first time [26]. All unsaturated molecules in this family show a π resonance and a series of Rydberg states at both edges, and vibrational structure at the carbon edge, which will be analysed according to the Franck-Condon principle.

The near edge x-ray absorption spectra of CX4 (X = H, Cl, F) have been measured with high resolution at the 1s edges of carbon and fluorine, and the 2p edge of chlorine [27]. The absolute photoabsorption cross-section has been determined at the resonances and in the nearby continuum. In agreement with previous work, the antibonding valence peaks of the halides do not show any vibrational structure but sharp Rydberg states are observed at the C 1s edge. The C 1s line widths were compared to theoretical predictions of ion state line widths and reasonable values are found for CH4 and CCl4, but for CF4 the measured value is higher than the theoretical value. The discrepancy is assigned to mixing of valence band character in the Rydberg states, which is especially strong in CF4, and to multi-centre autoionisation. Finally the integrated oscillator strength below threshold is shown to vary in the order CF4 >CH4 > CCl4.

Other interesting materials that can be studied at the Beamline comprise transition metal compounds (TMC),

Fig. 7. O2 ion yield spectrum recorded in the first diffraction order. The solid line is a simulation using the parameters reported in [23], while the dashed line is a simulation using equivalent core model derived parameters [23].

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which are of fundamental importance in catalysis, sensors and a range of other applications. Although they have been extensively investigated by Synchrotron Radiation (SR) spectroscopy (XAS and PES) in the solid state, SR spectroscopy of gas phase TMCs is a barely touched subject. This is due to the experimental difficulties of dealing with these unstable and rather aggressive compounds. The interest in SR photoemission and absorption of TMCs in the gas phase is twofold. Firstly we expect to observe the full phenomenology exhibited in the solid state, with the advantages of the much higher resolution available and the absence of complications associated with solid state effects. Finer multiplet structure for example should be resolvable. Secondly we expect that precise new spectral data will lead to parallel development in theoretical models, for example to understand the involvement of d orbitals in the bonding, describe many body effects, and quantify oscillator strengths for metals and ligands.

The first series of compounds we have studied is the series of 3d transition metals TiCl4, VOCl3 and CrO2Cl2, in which we aim to understand the metal-oxygen and metal-chlorine bonding. The electronic structure of these compounds has been studied by mapping all of the electronic states accessible with photons in the energy range 20-1000 eV in absorption and emission. The NEXAFS spectra were recorded at the chlorine, titanium, vanadium and chromium L-edges and oxygen K-edges.

In the case of TiCl4 a much richer multiplet structure appears in the discrete part of the spectra, fig. 8 compared with earlier EELS results [28]. We obtain absolute cross sections that are higher than those obtained from EELS measurements [29] and, at the Cl 2p edge, consistent in three different molecules (TiCl4, VOCl3 and CCl4 [30])

Fig. 8. Absolute oscillator strength of TiCl4 at the Ti 2p threshold. The experiment is compared with the calculations by Decleva et al. [44], which do not account for the spin-orbit splittings.

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measured independently. This is surprising as the errors that may occur in photoabsorption (higher order light, line saturation) usually lower the apparent cross section, while those that can occur in EELS (quadrupole scattering) usually give values that are too high.

Photoelectron spectra were carried out using the ARPES chamber with a 50 mm mean radius electron energy analyser (VSW Ltd) as well as a second experimental chamber equipped with a 100 mm mean radius spherical electron analyser (CLAM, VG). The CLAM analyser was mounted at the magic angle, with differential pumping of the detector in order to achieve higher electron signal.

Full valence spectra have been recorded with the CLAM analyser at an overall energy resolution of 500 meV and satellites of inner valence states have been observed clearly. The photoelectron spectra of the core levels show the main lines and their satellite features [31], and correlation effects on the Cl 3s state appear to be particularly strong.

4. Photo-double ionization studied by electron-electron coincidence experiments In photo-double ionization (PDI) a single incident photon produces two photoelectrons that escape from the residual doubly charged ion core. Due to the single-particle nature of the dipole interaction, the electric field of the photon can act on one single electron only. The transfer of the photon energy to the second electron is then controlled by electronic correlation. Thus PDI is an ideal tool to trace the characteristics of the correlated motion of electronic systems.

The complete characterisation of the process implies the detection of the two photoelectrons in coincidence after energy and angular selection. The low value of the cross section of this process dominated by electron correlations, the need for an intense and tunable source in the VUV/soft X-ray region and the intrinsic difficulties of coincidence experiments hampered the study of PDI until the last decade. Here we will describe the results of some experimental studies of PDI that have taken advantage of the high resolution and flux of the beamline combined with the high efficiency of the multicoincidence end-station.

The first example is represented by the study of the PDI of He at 40 eV above threshold. PDI of He involves three charged interacting particles in the final state and represents the archetypal three-body Coulomb problem. We have studied the process in the case of unequal energy sharing between the two electrons, E1(2)>> E2(1) and upon the complementary kinematics obtained by the interchange E1↔E2=35↔5 eV. In the experiment the electron e1 is detected in a well defined direction (represented by the arrow in fig. 9), and therefore with a well-defined momentum. The coincidence angular distributions chan-

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ge shape and intensity depending on the direction and energy of e1 [32]. In fig. 9 an example of the complementary angular distributions, obtained when the electron e1 is detected in the direction of the polarisation vector of the incident radiation, is shown. The experiments have been compared with two amongst the most successful theoretical models of PDI: the 3C model [33] and the Convergent Close Coupling (CCC) model [34]. This first model [33] describes analytically the interaction in the final state between the three charged particles, while in the CCC model [34] a numerical approach to solve the PDI problem is adopted. Neither of the models describes the experiments satisfactorily. This shows that, despite the considerable theoretical progress, a universally applicable description of the three-body Coulomb problem has not been found.

In the case of the heavier rare gases, PDI may occur also via an indirect process. The indirect process is characterised by the formation of an excited state of the neutral or singly charged ion that eventually decays into the double continuum.

The indirect DPI is usually treated as a two-step process.

For example when the intermediate state is an excited state of the singly charged ion, it is assumed that the emission of a photoelectron occurs first and then an Auger electron is ejected. This relies on the fact that (i) the two free electrons can be distinguished via their kinetic energies, (ii) the contribution of the direct double ionization process is negligible and (iii) the intermediate state has a lifetime long enough to prevent any final state interaction in the continuum. A rather interesting case occurs when the tunability of synchrotron radiation is used to produce photoelectrons with kinetic energy close to the energy of the Auger electrons. Then, due to electron exchange, strong interference effects are expected in the angular and energy distributions of the outgoing electrons.

We have studied the following process [35] hν+Ne → Ne+[2s2p5(3P0)3p(2Se)] + eph(Eph, l=1)



Ne2+[2s22p4(1D)] + eAuger(EAuger=13.24 eV, l=2)

at hν=92.21 eV so that Eph= EAuger=13.24 eV. Thus at this energy the photo- and Auger electrons can be distinguished only because of their different angular momenta. Figure 10(a) shows the mixed non-coincidence photo/Auger electron spectrum measured at this photon energy and for ϑ=30°. The shape of this peak as well as its position does not change with the detection angle.

Figs. 10(b) and (c) show the coincidence energy distribution measured at mutual emission angles ϑ12=180° and 60°, respectively. In the measurements the kinetic ener-

gies of the two electrons were varied at fixed photon energy according to the relationship : hν-IP [Ne2+(1D)]=E1+ E2.

The data display a quite different shape at the two different mutual angles. The spectrum taken at ϑ 12=180° clearly shows a minimum when E1= E2=13.24 eV=EAuger.

According to the model of Vegh and Macek [36] this behaviour can be qualitatively understood as the signature of the interference effect, which depends on the total spin and parity of the electron pair. In the present case the theoretical description predicts destructive interference for antiparallel emission (ϑ12=180°) and constructive interference for the opposite condition of parallel emission.

These predictions are completely consistent with our experimental observation. These results, together with those obtained by tuning the photon energy close to hν=92.21 eV [37] in order to study the combination of coherence and final state effects, show that modern electron coincidence experiments have reached a level of detail comparable to the analogous quantum optics experiments devoted to the study of “coherence/incoherence” effects.

PDI can also be obtained by an inner shell excitation followed by a cascade process. An example of such a process is reported in fig. 11. The Ne 1s-13p(1P) inner shell excited state is produced by the absorption of a linearly polarised photon of 867.12 eV. The inner shell excited state decays via resonant Auger emission (E1=778.4 eV) to the Ne+ 2s2p5(1P)3p state, that is embedded in the Ne2+ continuum. In the second step the intermediate state decays via an Auger transition (E2=22.3 eV) to the Ne2+(1D) state [38].

Auger electron spectroscopy has always had a unique role in atomic and molecular physics for its capability to sample the electronic structure of both initial and final states and to highlight dynamical aspects of core hole relaxation. Recently Kabachnik et al [39] have shown theoretically that Auger processes can also be used to realize a “complete” experiment, in which all the matrix elements and their relatives phases can be determined. Ueda et al [40] have proved that coincidence experiments which detect the two partners of a cascade processes are the most suitable candidates to the purpose. We have measured separately the angular and energy distributions of the two electrons ejected in the cascade process and the angular distribution of the resonant Auger electron in coincidence with the second one detected at a fixed angle with respect to the polarisation of the incident photon beam. The results are shown in fig. 11. The coincidence angular distribution has been represented by a polynomial expansion (full line in fig. 12) [41], whose coefficients can provide information on the matrix elements and the phase shifts [39,40].

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5. Photoionisation of unstable species Third generation SR undulator beam lines are very attractive because they allow low density and highly reactive species to be studied in the gas phase at high resolution using more favorable experimental conditions. Investigations of unstable systems (transient and metastable species, free radicals and high temperature vapors) are of interest in many fields of applied and fundamental research since they play a key role in relevant processes in atmospheric chemistry, combustion, plasma chemistry,

The ground state electronic structure of O3 in the molecular orbital picture can be described as follows: 1a12 2a12 1b22

core MOs (O1s AOs)

3a12 2b22 4a12

inner valence MOs (O2s AOs)

5a12 1b12 3b22 4b22 6a12 1a22 2b10 7a10 5b20

valence MOs (O2p AOs)

The innermost 1a1 core orbital is the 1s central oxygen AO (OC 1s) while the 2a1 and 1b2 are almost even and odd combinations of the 1s terminal oxygen AOs (OT 1s).

Fig. 9. Polar plots of the He TDCS at 40 eV excess energy in unequal energy sharing conditions. The black and red lines are the predictions of the 3C [33] and CCC [34] theories.

astrophysics, gas-surface interaction. High-resolution studies are also important as they provide a stringent test for theoretical models in describing photoabsorption and ionization processes that are now observed experimentally in more detail. Recent investigations benefiting from the high photon flux and resolution of the beam line will be here described briefly.

The first example is represented by the study of the absorption and dissociation processes of the ozone molecule at the K-edge [42]. O3 is a C2v symmetry molecule in the ground state with oxygen atoms in two chemically distinct states, the “central” and the two “terminal” ones. The large difference in the chemical shift of the two kinds of atoms, 4.7 eV as measured by XPS, suggests that site-specific core excitation processes and decay dynamics of inner-shell resonant states could be selectively studied using high resolution photon sources.

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Fig. 10. Ne photoelectron/Auger electron non-coincidence spectrum (a) at hν=92.21 eV and θ=30° with respect to the direction of the photon polarisation. Photoelectron/Auger electron coincidence spectra at the same photon energy for θ 12=180_° (b) and 60°, respectively. The full lines in 9b) and (c) are a fit with the general formula by [36] convoluted with the apparatus function.

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The lowest unoccupied MO 2b1 (LUMO) has π* character, and represents the complement of the 1b1 π bonding orbital. The next two orbitals, 7a1 and 5b2, are unoccupied MOs with σ∗ antibonding character. At higher energies Rydberg and valence-Rydberg mixed type unoccupied orbitals are expected. The most intense transitions expected in the K-edge photoabsorption and the totalion-yield spectrum of O3 are electron excitations from the core MOs into the three relatively compact unoccupied valence MOs. Weaker transitions into the more dif-

tense bands at low energies dominate the whole spectrum. The former at 529.25 eV is due to the OT (1s) —> 3a” (π*) excitation while the latter, centered at 535.7 eV in the curve at 90˚, with intensity approximately double that of the first band, is ascribed to the two valence transitions OC (1s) —> 2b1 (π*) and OT (1s) —> 7a1 (σ*). This assignment is in accord with a recent photoabsorption work . The composite nature of this band has been clearly proved experimentally for the first time by the two spectra reported in fig. 11. The angular distribution

Fig. 12. Oxygen K-edge fragment-ion-yield spectrum of ozone recorded at 0˚ and 90˚ detection angles with respect to the polarization plane of the light.

Fig. 11. Scheme of the resonant excitation and cascade Auger decay of Ne (a). The non-coincidence angular distribution of the Auger electron ejected in the second step (b) and the coincidence angular distribution of the two ejected electrons (c).

fuse Rydberg orbitals are expected at higher photon energies below the two O1s ionization thresholds.

The high-resolution total-ion- and fragment-ion-yield spectra of ozone have been recorded at the K-edge. The fragment-ion-yield spectrum, reported in fig. 11, has been recorded at different detection angles with respect to the polarization plane of the light, namely at 0˚ and 90˚, using a narrow photon band pass (50 meV). Two in-

of the fragment ion emission from the two different valence resonances, with π* and σ* characters, is remarkably different. As it is clear from the strong change in the peak shape of the second band on going from 90˚ to 0˚, the OT (1s) —> 7a1 (σ*) resonance decays emitting fragment ions preferentially on the polarization plane of the light (0˚). This does not seem to be the case for the OC (1s) —> 2b1 (π*) resonance.

Four broad features are observed at higher photon energies. The interpretation of this spectral range has been supported by QDPTCI (quasi-degenerate perturbation theory configuration interaction) ab initio calculations. A reliable theoretical description of the experimental data in terms of excitation energy pattern and photoabsorption oscillator strengths in the whole spectral range was achieved only by using a high level ab initio approach.

The second example is a study of a high temperature vapor, namely the gas-phase photoemission study of 2mercaptobenzoxazole (MBO) [18]. This heterocyclic aromatic molecule, C7H5NOS, as well as other similar organic molecules, is of considerable importance for its interaction with metal surfaces. It finds application as a cor-

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rosion inhibitor and flotation collector, acting as a strong chelating agent. The knowledge of the electronic structure of the free molecule is an important piece of information with which to better understand the interaction and bond formation between the chelating molecule and the surface.

In the case of MBO two tautomeric forms may exist: the thione (NH) and the thiole (SH) forms, the first with a C=S double bond, the latter having an endocyclic double bond C=N and the hydrogen atom bonded to the sulfur atom instead of the nitrogen.

The electronic structure of the molecule is sensitive to the molecular structure and the different chemical bonds present in the two tautomeric forms.

The combined photoelectron spectra, recorded as a function of the energy of the synchrotron radiation, and ab initio calculations proved that only one of the two tautomers, the thione form, exists in the vapor phase.

The crucial experimental information was provided by the behavior of the valence photoionization cross sections as a function of the photon energy.

The behavior observed experimentally was compared with the theoretical atomic orbital composition of the molecular orbitals ionized. The NEXAFS spectra (totalion-yield) of the MBO free molecule were recorded at the carbon K-edge and the sulfur L-edges for the first time.

The results were compared with the solid state MBO NEXAFS spectra. The carbon 1s photoelectron spectrum of the molecule was also obtained, and the experimental ionization potentials of the different carbon atoms were found to be in good agreement with theoretical results obtained with the STEX method, thus proving the reliability of the theoretical approach in calculating the chemical shifts of the carbon atoms in the different chemical environments.

Recently, apparatuses producing transient, metastable and free radicals have been developed to study highly reactive species [43]. Several approaches have been adopted, such as microwave discharge and flash pyrolitic techniques. Finally, a high-resolution TPES (threshold photoelectron spectroscopy) analyzer has been developed to study reactive species.

6. Conclusion The Gas Phase Photoemission beamline at Elettra with its equipment allows a wide variety of experiments to be done in atomic, molecular and chemical physics. The commissioning of the beamline has shown that the performance is well above the specifications, in particular as far as the resolving power and the covered energy range are concerned. A selection of the first experiments performed has been used to show the potential of the facility and the outstanding results that have been obtained.

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Acknowledgments We thank all our colleagues at Elettra and at our home institutions for their support and technical assistance.

References [1] http://www.elettra.trieste.it [2] P. Melpignano, S. Di Fonzo, A. Bianco and W. Jark, Rev. Sci. Instrum. 66 (1995) 2125.

[3] R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, R. Krempaska, J. Slezak, K.C. Prince, R. Richter, M. Vondracek, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone, S. Stranges, M.-Y. Adam J. Electron Spectrosc. Relat. Phenom. 101103 (1999) 959; and K.C. Prince, R. R. Blyth, R. Delaunay, M. Zitnik, J. Krempasky, J. Slezak, R. Camilloni, L. Avaldi, M. Coreno, G. Stefani, C. Furlani, M. de Simone and S. Stranges, J. Sync. Rad. 5 (1998) 565-568.

[4] M. Coreno, L. Avaldi, R. Camilloni, M. de Simone, J. Karvonen, R. Colle and S. Simonucci, Phys. Rev. A 59 (1999) 2494 [5] K.C. Prince, M. Vondracek, J. Karvonen, M. Coreno, R. Camilloni, L. Avaldi and M. de Simone, J. Electron Spectrosc. Relat. Phenom.

101-103 (1999) 269 [6] C.T. Chen, Y. Ma and F. Sette, Phys. Rev. A 40 (1989) 6737.

[7] C. Quaresima, C. Ottaviani, M. Matteucci, C. Crotti, A. Antonini, M. Capozi, S. Rinaldi, M. Luce, P. Perfetti, K.C. Prince, C. Astaldi, M. Zacchigna, L. Romanzin and A. Savoia, Nucl. Instr. and Methods A364 (1995) 374 .

[8] D. Cvetko, L. Floreano, R. Gotter, M. Malvezzi, L. Marassi, A. Morgante, G. Naletto, A. Santaniello, G. Stefani, F. Tommasini, G. Tondello and A. Verdini, Proc. SPIE 3150 (1997) 86.

[9] M. Domke, T. Mandel, A. Puschmann, C. Xue, D.A. Shirley, G. Kaindl, H. Petersen and P. Kuske, Rev. Sci. Instrum. 63 (1992) 80.

[10] M. Watanabe, A. Toyoshima, Y. Azuma, T. Hayaishi, Y. Yan, A. Yagishita, Proc. SPIE 3150 (1997) 277.

[11] Y. Saito, H. Kimura, Y. Suzuki, T. Nakatani, T. Matsushita, T. Muro, T. Miyahara, M. Fujisawa, K. Soda, S. Ueda, H. Harada, M. Kotsugi, A. Sekiyama, and S. Suga, Rev. Sci. Instr. 71 (2000) 3254.

[12] O. Schwarzkopf, M. Borchert, F. Eggenstein, U. Flechsig, C. Kalus, H. Lammert, U. Menthel, M. Pietsch, G. Reichardt, P. Rotter, F. Senf, T. Zeschke, and W.B. Peatman, J. Electron Spectr.

Related Phenomena 103 (1999) 997; and F. Senf et al, Nucl. Instr. and Methods A, in press.

[13] K. Schulz, G. Kaindl, M. Domke, J.D. Bozek, P.A. Heimann, A.S. Schlachter, and J.M. Rost, Phys. Rev. Lett. 77 (1996) 3086.

[14] J. Krempasky, R. Krempaská, A. Bianco, A. Abrami, R. Pugliese, F. Billè, J. Karvonen, M. Coreno and M. de Simone, Proc. SPIE 3150 (1998) 76 [15] F. Penent, P. Lablanquie, R.I. Hall, M. Zitnik, K. Bucar, S. Stranges, R. Richter, M. Alagia, P. Hammond, and J. Lambourne, Phys. Rev. Lett. 86 (2001) 2758, accepted.

[16] T. W. Gorczyca, J. E. Rubensson, C. Såthe, M. Strom, M. Agaker, D. Ding, S. Stranges, R. Richter and M. Alagia, Phys. Rev. Lett. 85 (2000) 1202.

[17] P. Lablanquie, F. Penent, R. I. Hall, J. H. D. Eland, P. Bolognesi, D. Cooper,G.C. King, L. Avaldi, R. Camilloni, S. Stranges, M. Coreno, K.C. Prince, A. Muheleisen and M. Zitnik, Phys. Rev. Lett., 84 (2000) 431-434.

[18] G. Contini, V. Di Castro, S. Stranges, R. Richter, and M. Alagia, J. Phys. Chem. A, 104 (2000) 9675-9680.

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[19] G. Polzonetti, V. Carravetta, A. Ferri, P. Altamura, M. Alagia, R. Richter and M.V. Russo, Chem. Phys. Lett., (2001), in press.

[20] V. Caravetta, G. Iucci, A. Ferri, M.V. Russo, S. Stranges, M. de Simone, and G. Polzonetti, Chem. Phys., 264 (2001) 175-186.

[21] J.-E. Rubensson, C. Såthe, S. Cramm, B. Kessler. S. Stranges, R. Richter, M. Alagia, and M. Coreno, Phys.Rev.Lett., 83 (1999) 947950.

[22] J.A.R. Samson and L.Yin, J. Opt. Soc. Am. B6 (1989) 2326 [23] M. Coreno, M. de Simone, K.C. Prince, R. Richter, M. Vondracek, L. Avaldi, R. Camilloni, Chem. Phys. Lett. 306 (1999) 269.

[24] M. Alagia , R. Richter and S. Stranges, in preparation.

[25] K.C. Prince, L. Avaldi, M. Coreno, R. Camilloni e M. de Simone J. Phys. B: At. Mol. Opt. Phys., 32 (1999) 2551.

[26] M. de Simone, M. Coreno, P. Franceschi, R. Richter and K. C. Prince, in preparation.

[27] M. de Simone, M. Coreno, M. Alagia, R. Richter and K. C. Prince, submitted for publication.

[28] A. T. Wen and A. P. Hitchcock, Can. J. Chem. 71 (1992) 100.

[29] W. Chan, G. Cooper and C. E. Brion Phys. Rev. A 44 (1991) 186.

[30] M. de Simone, M. Coreno, M. Alagia, R. Richter and K. C. Prince, in preparation [31] B. Wallbank, J. S. H. Q. Perera, D. C. Frost and C. A. McDowell, J. Chem. Phys. 69 (1978) 5405.

[32] P. Bolognesi, R. Camilloni, M. Coreno, A. Kheifets, J. Berakdar and L. Avaldi submitted to J. Phys. B:At. Mol. Opt. Phys. (2001)

[33] F. Maulbetsch and J.S. Briggs, J. Phys. B: At. Mol. Opt. Phys. 27 (1994) 4095 [34] A.S. Kheifets and I. Bray, Phys. Rev. Lett. 81 (1998) 4588 [35] S. Rioual, B. Rouvellou, L. Avaldi, G. Battera, R. Camilloni, G. Stefani and G. Turri, Phys. Rev. A 61 (2000) 044702 [36] L. Vegh and J.H. Macek, Phys. Rev. A 50 (1994) 4031 [37] S. Rioual, B. Rouvellou, L. Avaldi, G. Battera, R. Camilloni, G. Stefani and G. Turri, Phys. Rev. Lett. 86 (2001) 1470 [38] G. Turri, G. Battera, L. Avaldi, R. Camilloni, M. Coreno, A. Ruocco, R. Colle, S. Simonucci, G. Stefani J. Electron Spectrosc.

Relat. Phenom. 114-116 (2001) 199 [39] N.M. Kabachnik, I.P. Sazhina and K. Ueda, J. Phys. B:At. Mol. Opt. Phys. 32 (1999) 1769 [40] K. Ueda, Y. Shimizu, H. Chiba, Y. Sato, M. Kitajima, H. Tanaka and N.M. Kabachnick, Phys. Rev. Lett. 83 (1999) 5463 [41] V. Schmidt Electron Spectrometry of Atoms using Synchrotron Radiation, Cambridge University Press 1997, chpt. 4 [42] S. Stranges, M. Alagia, G. Fronzoni, P. Decleva, J. Phys. Chem. A, 105 (2001) 3400-3406.

[43] M. Alagia, M. Coreno, M. de Simone, R. Richter, and S. Stranges, J. Electon Spectrosc. Relat. Phenom., 114-116 (2001) 85-92.

[44] P. Decleva , G. Fronzoni, A. Lisini and M. Stener Chem. Phys. 186 (1994) 1

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PROGETTO E.S.S. Articolo ricevuto in redazione nel mese di Aprile 2001

RESEARCH AND DEVELOPMENT FOR THE TARGET SYSTEM OF THE ESS PROJECT G.S. Bauer PaulScherrer Institut CH-5232 Villigen

Abstract The concept developed for the 5 MW European Spallation Source (ESS) employs a completely novel target system based on liquid mercury in a steel container and subjected to a pulsed beam at a repetition rate of 50 Hz and with an energy content of 100 kJ per pulse. This clearly needs comprehensive R&D on various fronts, starting from computational simulations via materials compatibility problems, heat transfer questions all the way to the effect of pressure waves and possible cavitational effects and, last but not least, specific effects of the particular radiation spectrum the structural material is exposed to. In parallel, R&D efforts are going on to predict and optimise the performance and design of the moderator system to provide the future users with as closely as possible the energy and time structure of the neutrons they desire. This includes efforts to provide novel concepts of highly efficient cold moderators and, possibly, pulses from a source directly fed by the long pulses from the linac. The paper summarizes work done so far and points out questions that still need to be resolved as well as opportunities to do so.

1. Introduction Being in the fortunate situation of having at their disposition the world’s leading continuous and pulsed neutron sources as well as a number of national and local facilities, Europe’s scientists have come to appreciate neutron scattering as a valuable tool to carry out research in a large and growing variety of fields. The community has grown to a respectable size and has founded various national societies which work together under the umbrella of a European association, ENSA. In this situation it is natural that thought is being given to the question of how the research opportunities in the field can be maintained or improved in view of an obvious trend to shut down ageing reactor neutron sources and of the long lead time required to realise new large projects.

ENSA has, therefore, welcomed an initiative from several European research institutions which, with support from the European Union had engaged in a project study for a new, high performance neutron source called the “European Spallation Source” (ESS), which would satisfy

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the needs of neutron scatterers during the decades to come. ESS would be a pulsed spallation neutron source at an unprecedented proton beam power level of 5 MW, thus exceeding ISIS by about a factor of 30. The study was finished in 1996 and its results were published in three volumes, giving an overall summary [1], the scientific case [2] and the technical concept [3]. Unfortunately funding to continue the work was very limited in the following years. Therefore important research necessary for the realisation of the project could only be carried out on a very limited sale, based on contributions from some interested laboratories and some support from the 4th European framework program. In the mean time both, the United States of America and Japan have engaged in similar projects, which incorporate several of the concepts developed for ESS, in particular the use of mercury as a liquid heavy metal target and a horizontal slab target geometry. The US project, SNS, has been approved for realisation at Oak Ridge National Laboratory and construction for the 2 MW facility is under way. The Japanese project, JSNS, which has been incorporated in a joint JAERI-KEK high power accelerator project with multiple uses was approved in late 2000. JSNS, in its first stage will use 1 MW of beam power, but options for later upgrade are in the planning.

These three initiatives have greatly boosted world wide interest in the next generation neutron sources and also helped to stimulate pertinent research on an international level. However, ESS, aiming at a beam power level significantly higher than SNS and JSNS will face additional challenges which require further R&D work.

After a brief summary of the technical concept of ESS as worked out in the project study, the present paper outlines target and moderator related R&D, emphasising questions that still need to be answered and points out activities and opportunities to carry out such research.

2 The ESS project study 2.1 The ESS concept and accelerator system The European Spallation Neutron Source (ESS) is intended to become the next regional source in Europe, like the high flux reactor of the Institut Laue-Langevin has been for the past decades. When the basic

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Fig. 1. Sketch of the ESS facility showing the linac building with an achromatic 180° bend leading to the compressor ring from where the beam is distributed to two target stations.

specifications of ESS were decided, this was in full appreciation of the fact that a 5 MW beam power-short pulse source would pose interesting challenges on the accelerator side as well as, and in particular, on the target side. After examining various options for the different components of the system, the study group formed in 1992 came up with a concept of a 1.334 GeV linac followed by two compressor rings and two target stations. The two rings are foreseen because on theoretical grounds one expected a limit of about 2x1014 protons per ring due to space charge effects, whereas the average current of 3.75 mA at 50 Hz results in 4.63 x 1014 protons per pulse. The two rings are filled sequentially.

Chopping the 1.2 ms long pulse in the linac into 360 ns long micro pulses with 240 ns long gaps between them allows to fill the rings in such a way, that a charge free gap can be maintained for activating the extraction kickers. With 1000 micro pulses injected into one ring, a kicker magnet is activated during a 100 msec long gap to direct the beam to the other ring. When both rings are full, extraction occurs over a single turn in each ring and the two 400 ns long sub-pulses are combined to a 1 msec

long double pulse to be transported to the target.

Moderation of the neutrons generated in the target is too slow for the two sub-pulses to affect the thermal or cold neutron pulses.

The need to have two compressor rings to provide the pulses necessary to meet the users’ demands has a profound influence on the whole accelerator system.

Without going through the reasoning for the various choices in detail, we give a brief description of the concept, as it resulted from the feasibility study [3]1: Because injection into the rings is by charge exchange and there are no sufficiently powerful H- sources available, two ion sources are required to produce the necessary current of 104 mA in the linac. Their beams will be combined in a funneling section at an energy of 5MeV, after pre-acceleration in two radio frequency 1

Since the end of this study, new aspects such as a different injection scheme into the rings, super conducting cavities and different rf-sources have been considered, which may change the layout of the linac as well as of the rings. This has not yet lead to a new accepted design and will not be considered here, since it does not affect the target stations.

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quadruopole (RFQ) structures, each with beam chopping to generate the micro pulses. This results in doubling of the bunch frequency from 175 MHz in the RFQs to 350 MHz in the following drift tube linac (DTL), which accelerates the beam to 70 MeV. The bulk of the accelerating structure is a coupled cavity linac (CCL) with an rf-frequency of 700 MHz that takes the beam up to its final energy of 1334 MeV. The total length of the linac is little over 700 m with 663 m taken up by the CCL structure. All accelerator facilities (linac, 180° achromatic bend, compressor ring and beam transport line) are buried under concrete and dirt shielding. There are two target stations (high repetition rate, 50 Hz and low repetition rate, 10 Hz) which will share the proton pulses in a 4:1 ratio. Fig. 1 shows a sketch of the whole facility.

The beam is injected into the ESS targets horizontally, which not only makes the beam transport simpler and

Fig. 3. Schematic representation of the ESS target system, mounted on a trolley that also contains the drain tank and can be rolled back into the hot cell for maintenance. The double walled protective shroud around the target is cooled separately and is connected to the drain tank to return spilled mercury in case of a leak in the container.

Fig. 2. Calculated neutron yield for tungsten, tantalum (both with 20% coolant fraction) and mercury targets in a large lead reflector [3].

Fig. 4. The ESS mercury target and its immediate environment of reflector and moderators.

safer than vertical injection, but also has the advantage of an easier target handling system. Before entering the 50 Hz target station the beam passes through a large hall where the option has been left open to install other scientific instrumentation that might use part of the beam. This might either be meson targets or other test installations to carry out spallation related research.

2.2 The ESS target stations The two target stations, although receiving different time average beam power will have to cope with the same proton pulses of 100 kJ energy content. Since this is a more stringent condition than an average power dissipation of 1 or 4 Megawatts, they will be of essentially identical design as far as the targets and their handling systems are concerned. Differences may result in the moderator layout and in the beam lines, because the low repetition rate target station is mainly intended for very

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slow neutron work, which requires longer separation between pulses to avoid frame overlap in time of flight measurements and, in order to optimise resolution, will occasionally also use very long flight paths. Good cryogenic moderators are, therefore, at a premium, and an initiative has been started, to develop an advanced system based on solid pellets of high slowing down power which are transported through the moderator vessel continuously to limit radiation damage and to release stored energy at regular intervals (cf. Ch. 9) After some initial scoping studies [4], during which various target options, including rotating ones were considered, the ESS target team decided in favour of a liquid metal target using mercury. It had become clear that the high absorption cross section of mercury for thermal neutrons is not a disadvantage for pulsed

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sources and that otherwise mercury has very favourable properties for use as a spallation material [5], [6]. Among them are its high density, relatively low specific activation and the fact that it is liquid at room

Fig. 5. Plan view of the overall layout of the ESS target stations

temperature and therefore does not require auxiliary heating or pose the risk of damaging the container by volume changes upon solidification. As can be seen from Fig. 2, a mercury target is also expected to yield a higher neutron leakage from its surface than either tantalum or tungsten with 20 % coolant in their volume.

The horizontal beam injection and the liquid state at room temperature also favour a design concept [6], where the target container is surrounded by a separately cooled shroud that is connected to a dump tank in case the actual target container springs a leak or breaks. A very schematic representation of the ESS mercury target system is shown in Fig. 3.

The closed mercury loop is mounted on a movable

Fig. 6. Calculated pulse for the ESS coupled water moderator with lead reflector (solid line). The dashed line in the insert gives the estimate for a decoupled and poisoned moderator.

support to be able to roll it back from its operating position into a service hot cell for maintenance. The support trolley also contains a shielded drain tank into which the whole target loop can be emptied when opening the container becomes necessary in order to exchange any component, in particular the snout that is exposed to the high radiation field during operation.

A cut away view of the target container and its surroundings is shown in Fig. 4. The small H2O or supercritical H2 moderators are located above and below the target and are viewed by horizontal beam tubes in a way that avoids direct sight on the target. The surrounding reflector material is heavy water cooled lead.

Computational fluid dynamics studies carried out to optimise the flow in the beam interaction area [7] showed that three inlet and one outlet channels for the mercury provide for sufficient window cooling and at the same time allow to reduce re-circulation and vortex formation in the target region to a level which does not result in overheated zones. In this concept, the inlet flow through the bottom channel provides for the necessary flow across the beam window and the two side inlet channels provide the bulk of the mass flow and prevent recirculation in the beam interaction zone.

The plan view of the overall layout of the ESS target stations given in Fig. 5 shows that the inner shielding block, which is essentially steel and contains the beam shutter systems, is placed eccentrically in the overall shield to account for the anisotropy of high energy neutron emission in the spallation process and for the difficulty of shielding against these neutrons.

In the forward direction relative to the proton beam the remote handling cell for the target loop will be located, in which also the moderator-reflector unit can be maintained. This unit, together with a large part of its shielding plug, must be lifted out of its operating position vertically and must be transported in a shielding cask to the maintenance cell through a high bay area on top of the target block. A beam window separating the accelerator vacuum from the atmosphere in the target vault will be located upstream of the target and will be designed for remote replacement via the high bay area above the target shield.

Clearly, the heavily shielded beam transport line and the service cells on the floor level in immediate contact with the target block occupy a significant faction of the circumference of the target block, but the usually rather stretched geometry of instruments on pulsed sources and the extensive use of neutron guides will allow to accommodate at least nine direct beam lines on either side of the target, or even more, if neutron guide bundles can be employed. With its two target stations and high neutron flux (cf. Fig. 6), ESS will be an extremely powerful facility, well suited to meet the users´ demands

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to the next generation neutron sources. Its design and construction will, however, pose several technological challenges on both the accelerator as well as on the target side, and will also generate a wealth of information relevant to possible future accelerator driven devices in nuclear technology.

3. Some General Remarks on Liquid Metal Targets Up to now liquid metal targets have not been employed in spallation neutron sources. There is, however general consensus among target developers that, while up to 1MW of beam power solid targets are feasible from a heat removal point of view, their limits will not be far above this level. For higher power neutron sources liquid metal targets are the option of choice. They offer a number of potential advantages over volume cooled solid targets: - Higher heat removal capability due to the fact that the heated material is transported rather than the heat.

- Higher spallation material density in the volume due to absence of cooling channels which tend to dilute the target the more, the higher the power density.

- No or a minimum amount of water with its associated problems (radiolysis, production of short lived positron emitters and 7Be, etc.) in the proton beam.

- No life time limit caused by radiation damage in the target material. Only the structural material, which need not be chosen for high neutron production but can be selected for high radiation resistance, will suffer from this effect.

- Significantly lower specific radioactivity in the target material due to the large mass used and perfect mixing, making an emergency cooling system unnecessary and allowing afterheat removal by very simple means in a region where it does not affect the design of the neutron source proper.

- Significantly lower inside pressure in the target than in a water cooled system, putting less stringent requirements on the strength of the container wall.

Of course, the significance of all these arguments increases with increasing beam power.

Since liquid metal targets are a novel concept, there is no pertinent experience and a number of issues must be considered. The choice of materials is essentially limited to lead (Tm = 327 °C), lead bismuth eutectic (Tm = 125 °C), lead magnesium eutectic (Tm = 250 °C) and mercury (Tm = -38,8 °C). Lead based liquid metal targets have a high boiling point and will be good, if high operating temperatures can be afforded (or are desirable) and if thermal neutron absorption in the target should be low.

For low operating temperatures and when a high fast flux is the main goal, mercury can be used. Its main disadvantage is a low boiling point of only 356.6 °C at 1 atmosphere of pressure. While all heavy liquid metals are similar in terms of their specific heat (0.12 (Hg) - 0.15

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(PbBi) J/gK), there are differences, for example in their wetting behaviour towards solids, the solubility of foreign species etc., which may affect practical details of their use. This concerns heat transfer from and to walls, which is important for the cooling efficiency for beam windows and the sizing of heat removal systems.

Although little is known, there will also be differences in their corrosive behaviour towards the solid structural materials. It is obvious that, before liquid metal targets can actually be used in spallation sources, these questions need to be investigated. This is why the ESS team has proposed and has engaged in an R&D phase after completion of the project study.

4. Fluid dynamics and heat transfer studies 4.1 CFD simulation and flow optimisation A three-dimensional model was used to assess, by computational fluid dynamics (CFD), the feasibility of different concepts for creating the coolant flow patterns necessary to avoid excessive temperatures within either the fluid or the target structure. Fluid density variation was calculated by using the volumetric expansion coefficient for mercury to enable buoyancy to be

Fig. 7. Computed temperature con-tours (oC above inlet) in the ESS target with three inlet channels supplied by mercury of constant pressure.

included, otherwise all material properties were assumed to be independent of temperature (the Boussinesq approximation). As the Reynolds number in the target is of the order of 106, the flow is highly turbulent and the high Reynolds number k-ε turbulence model with standard coefficients was used throughout.

Conditions were assumed to be steady state, with no flow fluctuation allowed. Calculations were carried out using the thermal-hydraulics code CFX4, known previously as CFX-F3D [8] or CFDS-FLOW3D and supplied

by AEA Technology, Harwell, UK. A detailed account on the CFD studies for the ESS target, which lead to the design shown in Fig 4, can be found in ref. [7]; here only

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the most important results will be summarised briefly.

Although the beam will be pulsed, time-averaged energy deposition, in both window and fluid, was applied in the model, giving the same overall deposition rate with time as in the real target. This is an acceptable approximation as the pulse interval is very much shorter than the time constants of the fluid and thermal processes involved.

(For special problems resulting from the pulsed heat input see Ch. 5). According to neutronics calculations [3] the total heat deposition is 2.8 MW in the fluid and 11 kW in the window. Shell and flow guide materials were assumed to be ferritic (HT-9) steel. The window consisted of a two dimensionally curved dome, of thickness 3 mm where it joined the walls, but tapering to 1.5 mm at its centre. Boundary conditions were set by defining plane velocity and temperature distributions at inlet, and plane pressure at outlet. Total fluid mass flow rate was set at 175 kg/s and inlet temperature was chosen somewhat arbitrarily at 100°C, giving a mixed mean outlet temperature of ~217°C. This was a compromise between having a low fluid pressure drop within the target while keeping the peak fluid temperature safely below the boiling point of mercury.

To simplify construction and operation, it is desirable to supply mercury to both, side and lower inlets from the same pumped supply and at the same pressure. In this case of equal inlet pressure, the zone of highest fluid temperature occurs on the symmetry plane. Calculated temperature contours are shown in Fig. 7 a and b for the vertical and horizontal planes containing the peak temperature.

Based on a simple heat balance, the mean fluid tempera-

ture rise through the target is 97.1°C. With equal inlet pressures, the peak fluid temperature rise to 143°C above inlet temperature is less than twice this mean increase, pointing to reasonably good mixing. With some refinement of the design, it appears that this particular choice of lower and side baffle shapes, sizes and locations is quite promising, and a common supply to the target inlets is possible.

In summary, the following results and conclusions can be drawn from this first round of CFD calculations: Parameter studies with a target design including lower and side entry flows showed that it should be possible to supply fluid to lower and side inlets from the same manifold, at the same pressure, without needing to introduce flow-throttling devices. With equal inlet pressures, total coolant mass flow rate of 175 kg/s, and heat deposition of 2.8 MW in the fluid and 11 kW in the target window, peak window and fluid temperatures were almost equal at 143°C above inlet, and total system pressure loss between inlet and outlet was close to 2.1 bar.

Pressure loss is concentrated at the nozzle-like exits from the inlet flow guides. Yet, at the expansion downstream of these exits there is such a large pressure drop that the target outlet pressure must be kept above about 3 bar to prevent cavitation. This indicates that it might be worthwhile to study in more detail the degree of tapering of the target hull.

Peak fluid temperature is very sensitive to asymmetry in side-inlet flows. Therefore, such asymmetry should be reduced to a minimum. Displacement of the proton beam within limits should be acceptable, without causing too

Fig. 8. Plexiglas model of the ESS Mercury target for flow tests with water (left) and flow field in the mid-plane measured by UVP for a flow distribution of 2:1:2 between the left, lower and right inlet channels (right). The upward flow along the window is not visible in this projection.

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high an increase in target temperatures.

Changing the target structure material to austenitic steel did not affect the peak fluid temperatures, but window peak temperature rose by 30OC. Calculations were very encouraging, but two areas of uncertainty require further investigation: (1) Firstly, window cooling at the centre has been shown to depend strongly upon the position of the zone of interaction between the flows from either side. In reality, the interaction zone is likely to be highly turbulent. The turbulent fluctuations may improve the heat transfer between window and coolant over and above that predicted by the k-ε turbulence model for the mean flow field. Experiments are needed (and under preparation) to quantify this effect.

(2) The second uncertainty, and one which also requires experimental investigation, concerns the accuracy of the wall functions for momentum and heat transfer at the window, where the coolant passes over a curved surface during its expansion from the inlets.

4.2 Flow studies on a water model of the ESS target In view of the complexity of the problem, it is important to check the theoretical results by experiment. For simplicity, this is usually done on a water model first, before the actual final test is performed. Ultrasonic velocity probe (UVP)-Doppler technique [9] and optical flow visualisation were used to study the flow distribution in the ESS target.

The model used for the ESS target studies was produced at Forschungszentrum Jülich and was shipped to PSI for the experiments. It is shown in Fig. 8. In order to allow visualisation experiments as well as UVP measurement it is made of Plexiglas. On its inside the model is an exact geometrical reproduction of the ESS target as presently designed. The three water flows from the bottom and side inlet channels collide in the target window region, where the flow structure is expected to be very complicated, as discussed above.

Due to the need of sequential data taking the vector maps one can obtain by the UVP technique represent time

Fig. 9. Examples of temperature distributions measured by infrared thermography on a hemispherical, electrically heated beam window for natural convection (a and c) and pumped flow (b) in the target and by a symmetric (c) and skew gap (a and b) formed by the inner guide tube. Isotherms for 3.6 and 7.2K temperature drop are highlighted to illustrate different cooling conditions. The bright areas on the sides are in the vicinity of the electrodes through which the rf-current was applied.

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averaged flow fields in the model. Measurements are limited to two dimensional-2 component flow mapping.

Several values for the ratio of flow rates of three inlet channels (left : bottom : right) were used keeping the total flow rate constant at 0.88 litres/sec. While a more complete account on the results is given in ref. [10], the findings of these experiments can be summarised as follows: The 3D water experiments - at low flow rate as imposed manly by the stability of the Plexiglas model - showed that rather large re-circulation regions are generated in the reaction zone with a complicated flow pattern, at least under the low flow rate conditions applied. A change in the rate ratio of the three inlet channels results in a change in the size and intensity of the re-circulation regions, but they never disappear, nor does a certain asymmetry in the flow pattern. It would be expected that different patterns of flow fields may appear with higher flow rates, but so far, no experimental data have been obtained. In order to establish the optimum target geometry and flow conditions, further studies have to be done, both experimentally and numerically.

the importance of a flow across the target window.

Relative to a situation where natural convection near the window prevailed and a pumped bulk flow was used to move the mercury in the target, the heat transfer coefficient between the window and the mercury could be improved by a factor of five by using a pumped bypass flow which was directed across the window. The measured temperature drop in this case was 0.5 K/(W/cm2). The same value was also obtained for a geometry representative for the ESS target window cooled in cross flow.

IRT has, so far, been applied only for the SINQ target geometry but has proven extremely valuable in visualising the temperature distribution on the target window with very good spatial and temporal resolution.

Examples for measured distributions showing the effect of differently shaped flow guide tubes near the target window are given in Fig. 9. While electric skin effect heating was used in these tests, more realistic power input systems are under preparation. Similar investigations are also planned for the ESS target window.

4.3 Heat transfer measurements Heat transfer between the liquid metal and the walls is critical for proper cooling of the beam entrance window as well as for heat removal from the target. The heat transfer coefficient depends critically on the flow conditions near the wall, but also on other factors such as wetting and geometry. In order to examine this problem in a realistic situation, i.e. a representative geometry, mercury as liquid metal and relevant flow velocities, two methods were developed, namely heat emitting temperature sensitive surfaces (HETSS) [11] and infrared thermography (IRT) on curved surfaces [12] and used in collaboration with the Institute of Physics at the University of Latvia (IPUL) in Riga.

The HETSS technique was first applied to a geometry relevant for the SINQ target and allowed to demonstrate

4.4 Outlook on flow and heat transfer studies Although the confidence derived from the above investigations was important in supporting the decision for a mercury target, a large body of work remains to be done for the final design. Following these first studies, little progress has been achieved in this area until recently, due to lack in funds and manpower. In 2000, experiments on the water loop (FZJ) and mercury loop (IPUL Riga) started again. New computational fluid dynamics analyses have been initiated in collaboration with the Nuclear Research and Consultancy Group in the Netherlands using FLUENT and STAR-CD codes. The efforts include computational and experimental twophase flow studies for pressure wave mitigation (see below). Feasibility and conceptual design studies in FZJ have recently received new impetus by a collaboration

Fig. 10. Calculated stress on the wall of a cylindrical 20 cm diameter target after a 100 kJ proton pulse of 1 ms duration without gas admixture but cavitation setting in at zero pressure (left) and with gas admixture (3 vol% He, right) [14].

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entered with EMPRESSARIOS and SENER (Spain). These activities include theoretical and experimental work on target and mercury loop components and remote handling systems. In parallel, the collaboration with IPUL continues and intensive exchange of information is exercised with the SNS and JSNS projects in the US and Japan. However, in order to gain the necessary experience in running and maintaining a mercury system and to investigate ESS specific questions, a full size mercury loop at one of the laboratories participating in the ESS R&D effort will be indispensable. This loop shall be almost identical to the actual target loop and will serve as a test and training facility before, during and after the construction of the ESS targets.

5. Pulsed Power Input Since short pulses are a prime design goal in pulsed sources, the effect is that the time average power of perfectly manageable level is deposited in large “chunks”.

Even for a high repetition rate source as ESS, the power in one proton pulse is 100 kJ and about 60% of this is deposited in a small volume of the order of a few liters during 1 µs. It was pointed out immediately when mercury was first proposed as target material for ESS, that this was the source of potential problem [13]: During the short heating time the liquid has no way of accommodating the resulting expansion by displacement, conduction or convection and hence a high pressure level builds up which is the source of a pressure wave travelling outward and eventually hitting the container. There it generates dilatational stress upon impact. Subsequently, the whole system will experience pressure and stress oscillations until the energy has been dissipated. Although the exact magnitude of this effect depends on geometric and other design details and on position, its order of magnitude could be readily calculated for a cylindrical geometry and it was found to be in a regime, where the

endurance limit of the container material might play a role. A more detailed analysis was carried out later [14], taking into account the direct power deposition in the beam window as well as that in the liquid metal. The calculations showed that, from the direct heat input in the window rather high stress levels perpendicular to the wall surface resulted and that a high frequency stress wave starts to travel along the wall as soon as the pulse occurs. By comparison, the pressure wave from the liquid arrives at the surface only at a later time, depending on the distance it has to travel, and it generates a lower frequency of oscillation. These calculations also allowed to take into account the effect of possible voiding (cavitation) in the liquid metal, i.e. the failure to sustain tensile stress. If voiding was to occur at zero pressure, i.e. the fluid had no tensile strength at all, this would, under certain conditions, result in a roughly two times higher stress than if no voiding occurs.

One way considered to mitigate the effect of these pressure waves was to try and inject gas bubbles in a suitable distribution in order to “soften” the liquid metal, i.e. increase its compressibility. It was found that, if the compressibility was the average of the static compressibility of the two components, the magnitude of the wall stress would be reduced dramatically at a gas concentration around 3 vol%. Fig. 10 shows calculated stress values for a 20 cm diameter cylindrical target exposed to proton pulses of the same order of magnitude (total and power density) as expected for ESS. It is obvious that, under the assumptions made, admixture of helium would reduce the wall stress from a dangerously high level of 450 MPa in the case where voiding occurs at zero pressure to a modest level of 2 MPa. Although efforts are going on at the ESS project to generate such bubbles in the liquid metal, one must be careful about using static properties for the highly dynamics effects in question. At the time being, gas injection can-

Fig. 11. Measured and calculated strain levels (left) and axial deformation velocity (right) on the surface of the stainless steel container of a 20 cm diameter mercury target after impact of a 30 kJ proton pulse. (Experimental data for the strain at z=0.17 m are from the ORNL team and for the velocity at z=0.3 m from the JAERI team of the ASTE collaboration; calculations for the case without voiding, L. Ni)

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not yet be considered as a safe way to control the pressure wave problem. In an attempt to obtain experimental information, an international collaboration was organised to carry out full power measurements on a full scale target on the Alternating Gradient Synchrotron (AGS) in Brookhaven (ASTE Collaboration). First results have been obtained for the stress after a 30 kJ proton pulse at one point at z = 0.17 m on the surface and for the axial acceleration of the wall at another at z = 0.3 m (z being measured along the beam axis from the point where the beam hits the target). In both cases calculated data for the case without voiding reproduce the measured amplitude but do not succeed in describing the frequency behaviour in detail (Fig. 11). This is, however, not surprising because the target container actually used had a much more complicated structure than the one modeled.

Nevertheless the agreement of measured amplitudes with those calculations in the gas-free case seems to indicate that voiding does not occur, at least at 30 kJ pulse power. Efforts to obtain reproducible data on a simple target geometry are continuing.

It is intended to extend these measurements also to a target geometry representative of the ESS target and to test options to mitigate the stress on the walls. Successful and reliable stress mitigation is crucial for the feasibility of a liquid metal target at a pulse power level above 50 kJ. Another problem related to the pulsed heat input is the possibility of enhanced erosion of the walls. It is well known that collapsing bubbles generated by cavitation in a liquid can have a detrimental effect on solid material in their vicinity. During the rarefaction phase of the pressure wave the liquid metal goes into tension. This might lead to formation of cavities. Cavitation might also be the result of the extremely high power density in “thermal spikes” (energy deposition in the “damage cascade” by primary knock-ons (PKAs) and recoil nuclei). Near the wall collapsing cavities might have deleterious effects on the solid metal (pitting, erosion, destruction of the protective oxygen layer etc.). Although the first analysis of the ASTE results quoted above seem to indicate that, at roughly 1/3 of the power level of ESS, voiding did not affect the wall stress, this is still insufficient evidence. In particular, it should be noted that, in a split Hopkins bar experiment that generated an estimated pressure level of 80 MPa in the mercury, severe pitting was observed on the wall that clearly increased with the number of pressure cycles [15]. This problem certainly mandates careful investigation of the target containers to be used in future ASTE runs.

6. Radiation effects in structural materials The ESS target and its immediate surroundings will be subject to unusually high radiation levels. The effects of radiation on steels and other materials are very well

studied in fission and fusion reactor research, and a general image has evolved on what kind of changes are to be expected under certain irradiation conditions. They are all consequences of two kinds of interactions of the radiation field with the material: a. Atomic interaction by transfer of kinetic energy to individual atoms of the solid („primary knock-ons“).

These atoms can be displaced from their lattice site and can, if their energy is high enough, displace also other atoms from their positions, thus creating a cascade of damage in a small volume in the solid. In this cascade region there will be empty lattice sites („vacancies“) and atoms that come to rest at positions, which are not regular lattice sites („interstitials“).

Many of the initially created vacancy-interstitial pairs will recombine while the lattice is still locally very „hot“, i.e. the atoms are vividly oscillating. Thus, the net number of lattice defects remaining is much smaller, than the number of atoms actually initially displaced from their sites. This is why, after tens of „displacements per atom“ (dpa), the lattice can still be more or less in tact. The dpa-level, which is commonly used as a measure of radiation damage is calculated on the basis of the total energy transferred to the material divided by the mean energy required to displace one atom from its site in the particular material. The dpa number is being used as an easy reference to characterise the amount of radiation a material had been exposed to, accounting for its microscopic properties to some extent. It turns out that, in a fission or fusion environment radiation effects such as hardening or loss of ductility usually scale well with dpa levels. This may, however no more be true when high gas production levels are involved, as in the spallation case. b. Nuclear interactions can lead to significant alterations of the materials composition. While in fission and fusion reactor spectra, whose neutrons have energies generally well below 14 MeV, this is predominantly neutron capture and secondary reactions from the decay of radioactive products, the spallation spectrum containing protons and neutrons of high energy, up to a GeV, will generate a much broader range of reaction products resulting from the spallation reaction itself and some high energy fission taking place in the material. In particular, the cross section for hydrogen and helium production increases steeply with neutron energy (and atomic mass), resulting in much higher production rates of these species in a spallation spectrum. While helium is usually considered as nearly immobile because it occupies substitutional sites in the crystal lattice, hydrogen can diffuse through the lattice relatively easily and be trapped at certain locations. This may lead to segregation to grain

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boundaries or gas bubble formation and have a strong effect on the mechanical properties of the material. Of course also enhanced mobility of other species due to the dpa damage may be important, especially in alloys that have been stabilised in a certain state, i.e. contain metastable structures.

The temperature of irradiation is, of course, an important factor. One reason is the amount of spontaneous recombination. Another one is the mobility of certain species in the lattice as mentioned above, which is strongly temperature dependent. It has been found that, based on their reduced irradiation temperature, most metals show the same kinds of reactions to irradiation, which range from hardening via radiation creep and void swelling all the way to helium embrittlement at higher temperatures when helium atoms become mobile (see, e.g. ref [16]). While in a spallation source the target temperature will generally be fairly low, there will always be certain range of temperatures in a target or its structural material. Irradiation conditions will, therefore, vary locally. This is particularly important if one tries to run, for example a beam window, above the hardening regime in the beam centre. Towards the edges of the

beam the temperature will always be lower and creep in the ductile centre may result in reversed stress when the beam is turned off. So, clearly one must be very careful when judging the situation only by the most highly loaded point; the regions around it may be in more of a risk than the hot spot itself.

Of course, the degree to which the above effects play a role depends very strongly on the particular material in question. Several steels have been developed, that are fairly resistant to radiation damage and also some aluminium alloys have been used to several tens of dpa.

Unfortunately tungsten and its machinable alloys, like W5%Re, otherwise favoured target materials, seem to suffer from a rapid increase of their ductile-to-brittle transition temperature under irradiation [17]. As mentioned before, until recently there were almost no data on the effects of spallation radiation on materials, but with the recent interest in powerful spallation sources and with the advent of suitable opportunities, such as the LASREF-facility at Los Alamos, mainly in conjunction with the APT-project [18], [19] and the SINQ target irradiation program [20], this situation is now rapidly changing. As a first step components used over long time

Fig. 12. Tensile behaviour of samples taken from components used in the proton beam of LAMPF and ISIS. Damage levels given result from protons only; in some cases, especially the tantalum from the ISIS target, a similar, additional damage may have been inflicted by the neutrons produced.

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in existing facilities and now available have been examined at FZ JĂźlich and PSI [21]. These were mostly intensely water cooled components, so their irradiation temperature was generally low. Nevertheless, first results give quite interesting data on the behaviour of different kinds of steel and tantalum, as shown in Fig. 12 [22].

While for austenitic 304L stainless steel and martensitic steel of the DIN 1.4926-type the hardness increases, but significant ductility is retained up to 6 dpa, the artificially hardened Iconel 718 shows severe embrittlement with a

total loss of ductility between 10 and 20 dpa. A surprising result was obtained for the pure tantalum taken from a spent ISIS target plate: Even after 13 dpa (from protons alone; probably a similar damage level resulted from the neutron flux) this material, although significantly hardened, still retains as much as 15 % ductility. This clearly demonstrates that it is important to have data for exactly the materials and operating conditions in question. One important feature in spallation neutron sources are the frequent large amplitude thermal cycles

Fig. 13. Dose rates measured around the SINQ target # 3 after 6800 mAh of irradiation and 9 months of cool down (left) and arrangement of test rods inside the target (right).

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due to accelerator trips that seem to be difficult to avoid.

It is unclear whether these have any effect on the radiation damage.

In order to clarify these questions, ESS has joined other projects in an irradiation program going on at the SINQ target at PSI (STIP - SINQ Target Irradiation Program). In the first phase of this program 1500 specimens of different types (from miniature tensile to TEM disks) and several steel rods and lead filled steel tubes were incorporated in the Zircaloy target used at SINQ in the years 1998 and 1999. The target was removed from the beam position after having received a total of 6800 mAh of beam and was dismantled for removal of the test specimens in the autumn of 2000. Despite a cooling time of nine months the dose rates measured were still very high, as shown in Fig. 13.

Also shown in Fig. 13 are the positions of the test rods inside the target and their use. The test rods are being dismantled (end of 2000) and the samples are prepared for shipment to the participating laboratories for examination. Gamma scans on Zircaloy rods have helped to assess the precise effective beam distribution on the target which, by the way, agrees well with the results of beam-optical calculations. The beam and temperature history of the irradiation has been recorded over the whole period the target was in use.

Since the specimens were placed in the STIP-1 target a number of new questions have arisen, which lead the collaboration to launch a second round of irradiation, STIP-2 in the following target, This target was put in operation in March 2000 and will be irradiated through December 2001, unless a problem arises before that date.

The anticipated total charge will exceed 10 Ah and the fast neutron component per proton in the spectrum is almost doubled relative to STIP-1 due to the use of lead instead of zircaloy as a spallation material. Of particular interest to ESS are some capsules filled with mercury in which tensile specimens and TEM disks are placed together with steel samples that are stressed by an aluminium core due to its thermal expansion. These will be the first examples of high load irradiation of steel samples in contact with mercury. Neutron radiographs taken before the irradiation showed that the mercury was not wetting the specimens initially. After the end of the irradiation another neutron radiograph will be taken for comparison before opening the capsules.

An important aspect of irradiation in a spallation spectrum may be the simultaneous generation of hydrogen and helium at high levels. While experiments have shown that hardening due to helium alone will play a role relative to the dpa-inflicted hardening only at a helium concentration above 1 at% at low temperatures [23], evidence seems to exist that hydrogen, which would otherwise diffuse easily through the lattice, may be

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trapped by helium present in the lattice [24]. In steel this evidence is more or less circumstantial at present, but also the hydrogen distribution in Zircaloy rods from the SINQ target as determined by neutron transmission radiography seems to point in the same direction [25].

This effect and its possible consequences on materials behaviour, although of particular importance in the spallation environment, is not yet clear. It is also for this reason that the results expected from the steel rods in the STIP experiments are of particular interest. The very high dose levels around the target listed in Fig 13 show that most of the work to be carried out in the area of structural materials relies on the availability of hot cells with experienced and skilled personnel and elaborate test facilities.

7. Liquid Metal-Solid Metal Reactions Although radiation damage and potentially the fatigue resulting from the pressure waves are currently considered the likely life time limiting factors in the ESS target, there are other issues that should not be under estimated. While, from the point of view of radiation damage in structural materials there may be arguments to select somewhat elevated operating temperatures, one will generally try to run spallation source targets for research neutron sources at the lowest possible temperature for a variety of reasons, not the least one being the amplitude of thermal cycling during beam trips. Nevertheless, there will be a hot spot at the beam entrance window and a hot and cool region at the heat exchanger. This might, in principle, give rise to one or both of two phenomena: liquid metal corrosion and liquid metal embrittlement. The state of knowledge, poor

Fig. 14. Summary plot of the corrosion of steels in Mercury as a function of their total content in Ni, Cr and Mn.[27]

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as it is, has recently been reviewed [26] and only a brief account will be given here.

Liquid metal corrosion (LMC) is effectively a mass transport phenomenon from hot to cold regions of the loop. It can happen if the wall material or one of its major constituents has a highly temperature dependent solubility in the liquid. In that case dissolution will occur at the hot point and at the cooler parts of the loop the dissolved component will segregate to the walls. This leads to weakening of the material in the hot regions on the one hand and may result in flow blockage in the heat exchanger on the other. Although no data seem to be available for the temperatures in question, some information may be drawn from data on higher temperatures. Of the major constituents of steel, iron has a low and weakly temperature dependent solubility in mercury. The solubility of nickel is highest in heavy liquid metals, although in mercury it has only a weak temperature dependence. The temperature dependence of the solubility of chromium in mercury is stronger than that of the other two. As a consequence it is even lower than that of iron below 450°C. Based on these findings, low Nickel content steels were selected as preferred candidates for the container of the ESS target, although the argument does not seem to be very strong. A summary plot shown in Fig. 14 [27], seems to indicate that, at total concentrations of Ni, Cr and Mn of less than 15 %, no severe corrosive attack must be anticipated for steels at temperatures up to 500°C by mercury.

Particularly low corrosion rates were observed for 5%Cr0.5%Mo-1%Si low carbon steel [28].

Furthermore, it was reported that steel with high nitrogen or carbon content were efficiently protected from corrosion if Zr or Ti was present in solution in the mercury, probably due to the formation of insoluble Tiand Zr-carbides or nitrides on the surface. In summary, at least without possible amplification effects of simultaneous stress and irradiation, corrosion by

mercury is not expected to be a problem for a properly chosen container material, for which a rather wide selection seems to exist. These findings are also confirmed by experiments carried out at PSI and SNS (ORNL). There is no information, however, whether or not the situation would be different under irradiation.

This should be examined with priority.

Liquid metal embrittlement (LME) is a well known phenomenon, for example for aluminium and mercury. It has been observed preferentially in systems, where the solid and the liquid metal do not form intermetallic compounds [29] but other cases have been reported, too [30]. From simple thermodynamic arguments it was concluded [29] that binary systems in which LME is most likely to occur have a low temperature dependence of the solubility of the solid component in the liquid, which is precisely what one would choose to minimize corrosion.

In contrast to corrosion, liquid metal embrittlement does not depend on a temperature gradient in the liquid, but may be strongly affected by the temperature level. In fact, it seems to occur preferentially in a temperature range of 50K around the melting point of the liquid metal.

The reason why LME is a particular concern is because it is strongly affected by stress on the solid.

Phenomenologically it is explained by a liquid solid bond, which is strong enough to weaken the solid solid bond to a level below or near the yield strength of the solid metal. Any stress on the solid may thus lead to crack propagation, often along grain boundaries where the order is perturbed or where foreign atoms may catalyse the process. The phenomenon does not imply transport of dissolved atoms over long distances, but rather a rearrangement in the immediate vicinity of the tip of the propagating crack. It has certain similarities with the well known phenomenon of stress corrosion cracking, which is also known to be “assisted“ by irradiation (irradiation assisted stress corrosion cracking, IASCC). However, no information at all seems to exist on

Fig. 15. Tensile test results for a notched specimen of 91-type martensitic steel in air and molten lead. The area under the stress-strain curve plotted in the graph at the right is proportional to the total energy required to break the specimen, because the stress given is the ratio of the load and a cross section, which is assumed constant.

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the influence of simultaneous irradiation and stress for LME. In contrast to radiation embrittlement, LME is effectively a weakening of the grain boundaries that progresses from the surface and reduces the stress bearing thickness of the material. So, while radiation embrittlement can lead to enhanced crack propagation, if such cracks are present, but otherwise does not have a negative effect on the elastic properties of most materials, LME will eventually lead to component failure, even if the design originally was very conservative with respect to the elastic stress regime.

Fig. 16. Measured neutron spectra from different moderator materials at 20 K [33]

The above remarks need not mean that there is a high risk for LME or corrosion to destroy the target container, even under its rather severe operating conditions, especially since low nickel content martensitic steels are favoured for the ESS target container. There simply isn’t enough information available to judge the situation reliably, and serious R&D work is needed on this topic. The issue has received a new twist, though, by the recent observation of what was considered as evidence for LME by lead on a notched sample of martensitic steel with a non-standard heat treatment [31]. As can be seen from Fig. 15, the energy required to break the specimen in a tensile test was found to be reduced to almost 1/4 of its value in air when the testing was done in molten lead at a temperature of 350°C. Reduced fracture toughness was observed in range up to 80K above the melting point of lead (327°C). Unfortunately no further data in air are given for this temperature range to show conclusively the role of the lead in this effect.

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Clearly, this problem needs to be studied in detail for the materials foreseen as candidates for the ESS target. It is particularly worrisome that virtually nothing is known about the effect of irradiation on LME. An enhancement effect might result from - the dpa damage - local stress set up by thermal gradients - enhanced diffusion under irradiation - production of new elements - a “pumping” effect on the grain boundaries due to thermal cycling as a consequence of beam trips - or other, unidentified effects.

It is in recognition of this problem that the MEGAPIE collaboration, which aims at running a PbBi pilot target at the SINQ facility [32] has decided to launch an irradiation experiment in liquid metal at the old PSI 72 MeV injector cyclotron. In this experiment, called LiSoR (Liquid-Solid-Reaction under irradiation), specimens will be irradiated in flowing lead-bismuth eutectic under stress and beam-generated temperature gradients.

Tensile tests under irradiation as well as tests over an extended time span under static stress are foreseen. The outcome of these tests are crucial for the MEGAPIE project, but, although PbBi is generally considered more aggressive than mercury, it would be highly desirable to carry out similar tests in mercury for the ESS case. The irradiation in the SINQ target mentioned above is not a sufficient proof of no risk, even if no attack of the mercury on the steel should be found.

8. Cold Moderator development A subsystem of growing importance on any new neutron source will be its cryogenic moderators because, on the one hand the demand for low energy neutrons is rapidly increasing and on the other hand cold moderators offer an extended slowing-down regime with superior line shape as compared to what is obtained in thermal equilibrium of neutrons with an ambient temperature moderator. The user demand as manifest in volume II of the ESS study report [2] shows that for the high and low repetition rate target stations 15 out of 24 respectively 12 out of 12 proposed instruments require cryogenic moderators. Currently the only design for such moderators capable of withstanding the load levels anticipated for ESS is supercritical hydrogen, which was chosen as reference concept for ESS. Methane, which is known to yield a much better performance (Fig. 16, [33]) is very susceptible to radiation damage and can probably not be used in fluid form at 5 MW of beam power.

Currently the liquid methane in the ISIS moderator is changed on line every 2 days, corresponding to 0.3 MWd, but still the moderator vessel must be replaced twice a year, i.e. after about 10 MWd because of build-up of polymerisation products at its walls. Clearly, the

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corresponding intervals, about 30 times shorter, would be unacceptable at the ESS. The ESS team has, therefore, launched an effort to develop a new, advanced moderator concept, which has grown into an international collaboration (ACoM, [34]).

The term advanced cold moderators is meant to apply to such systems which have a potential to enhance the performance of cold moderators over what is currently achievable with liquid or superfluid hydrogen on any particular type of source but cannot be realised at present with existing technologies. Development is desirable in two ways: - A moderator material should be employed which does not or only at a very slow rate lead to the buildup of residues on the walls of the moderator vessel and which comes as close as possible to the performance of solid methane.

- For every neutron energy there exists a moderator temperature at which the intensity output is maximum. Therefore a system that could be operated in a wide temperature range would provide unparalleled flexibility to adjust to evolving needs.

- If realised, this would be a more efficient way of improving source performance than any potential increase in source power.

According to current thinking the system would be based on heterogeneously cooled pellets of methane or a clathrate (e.g. methane hydrate) that would be thermally annealed at appropriate intervals to release stored energy. Pellets would also be replaced continuously or batch wise at a rate required by the accumulation of nonannealable radiation damage, which would degrade performance. Data on the excitation frequency spectrum of methane hydrate, have recently become available [35], which clearly show the presence of intense low energy peaks at 1 meV and higher (similar to those in solid methane), as well as the full frequency spectrum of water.

The hope that methane hydrate might make a good moderator is largely based on this finding. From the data shown in Fig. 16 one might expect that a combination of the respective spectra would result in a moderator with outstanding performance in the whole energy range for a system based on methane trapped in H2O-ice (methane hydrate) and cooled with liquid or supercritical hydrogen. In order to ultimately realise such a system, a large programme of work has to be carried out.

One important problem in realising such a system is the transport of the pellets into and out of the moderator vessel at a rate that allows annealing outside the radiation field before too much damage builds up and

Fig. 17. Scheme of a pelletised moderator system with continuous helical screw transport of the pellets and batch wise annealing outside the moderator vessel.

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with minimal or no interruption of the source operation.

Within the current R&D efforts in the ESS team two methods of pellet production are being investigated: by the extrusion technique at FZJ. This technique takes advantage of developments from the fusion research program. Although it can produce one pellet at a time the production rate should be fairly high with a suitably designed extruder. The pellets as produced are not round and a technique will have to be developed to make them spherical. Again, it should be possible to adopt existing industrial procedures. by the moulding technique at PSI. Spherical ice pellets have been produced successfully in a test setup, which allows simultaneous production of 100 pellets by freezing liquid into a mould and releasing the pellets afterwards by opening the mould and briefly heating it to melt their surface. If successful with methane, this technique can be extended to a much higher production rate in a straightforward manner.

With respect to Pellet transport systems, again, two different ways of moving the pellets through the moderator vessel at the desired rate are under development: The fluidised bed technique at FZJ [36]. This technique allows to transport the pellets vertically upward in accordance with the present design of the cold moderator system for ESS (vertical insertion from above, cf. Fig. 4).

It requires the moderator vessel to be filled to a level of only 2/3 of its volume and there remains some uncertainty about whether or not the emptying is complete in routine operation (where no observation is possible) and how the technique would work with supercritical hydrogen. Furthermore, operation of the source will be interrupted during the emptying and filling process until equilibrium temperature has been reached again.

A helical screw transport system examined at PSI. This system operates in a continuous fashion removing pellets at a constant rate from the bottom of the moderator vessel and replacing them from a top inlet. Its principle is shown in Fig 17. A room temperature functional model made of Plexiglas and using polyethylene pellets is in operation, but an engineering design of a system working at cryogenic temperatures would yet have to be carried out. So far, this model transports pellets horizontally into and out of the moderator vessel and vertically through their “annealing” position. Operation with a 90° tilt might be possible but a design of the moderator vessel and the transport helix would have to be developed which ensures even transport of the pellets through the vessel and reliable vertical transport.

Studying other methods to replace the moderator material in the vessel at the desired rate would certainly be beneficial.

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Experiments had been started but are now on a hold at PSI to produce high density methane hydrate directly from the gas phase.

In view of the large potential gain associated with such a system (a factor of 3 in cold neutron intensity and an extension of the slowing down regime with its narrow pulses through the whole thermal energy range without requiring poisoning or decoupling of the moderator), its development should be given very high priority. A test facility to measure and verify the performance of different moderator systems in a realistic ESS geometry has recently been completed at FZJ on the 2 GeV proton synchrotron COSY. Even if the system finally developed would require redesign of parts of the target shield in order to be accommodated, it would be worth the effort, because gaining a factor of 2-3 by increasing the proton power is out of reach. This is, why its development is urgent.

9. Neutronic Optimisation While much important data will be obtained from experimental studies, it is virtually impossible to cover the full range of parameters. It is, therefore, essential that the experimental work be accompanied by theoretical efforts leading to models which enable (a) the interpolation and extrapolation of the experimental data to ranges not covered by the experiments (b) the transfer of data from related R&D in other fields to spallation conditions (c) optimisation studies to be carried out covering a large multi-dimensional parameter space and (d) provide data for the engineering work not accessible to experimental studies, such as power deposition and radiation levels in various regions around the target, predictions about thermal and mechanical loads etc.

In order to assess the loads experienced by the ESS target structural materials, high energy particle transport (HET) codes and structural mechanics codes were and continue to be employed to calculate energy depositions, atomic displacement rates, neutron and foreign element production rates, temperature and stress distributions, induced radioactivity levels and after heat, etc. In parallel, the HET codes were validated by experiments (NESSI collaboration). This work is near its completion as far as the production of hadrons, mesons, and light nuclei is concerned. There remain, however, still considerable discrepancies among the various codes with respect to spallation product generation in thick targets, a question, which is important when it comes to assess the potential long term corrosive effect of the liquid metal or its radioisotope vector at the end of its use. Efforts in this respect are going on in Russia under the umbrella of ISTC-supported projects, but these mainly relate to relatively thin targets for practical reasons. Some important data can also be expected to

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result from the analysis of the (small) samples of mercury presently under irradiation in the STIP-2 phase at PSI and an examination of samples from the mercury targets used in the ASTE collaboration. The latter will have been subjected to a relatively low dose of protons of different energies (1.5 to 24 GeV) over a long time span, though, which may make it difficult to use the results for code validation. It would be highly desirable to irradiate small removable samples of mercury at each of the energies.

10. Mercury Loop Design and Manufacturing Selecting liquid mercury as target (and cooling) material for ESS introduced an innovative component in the ESS project. Although this means more R&D work, the advantages gained by that choice are considered worth the effort. This view is confirmed by the adoption of the ESS liquid Hg concept for the high power spallation sources SNS in USA and JSNS in Japan. Alongside with

Fig. 18. Estimated pulse shape that would be obtained if the unchopped linac pulse from the ESS accelerator would be injected in the ESS reference target-moderator system.

Fig. 19. Illustration of the effect of the reflector characteristics on the pulse shape obtained for different pulse lengths on a long pulse spallation source.

materials issues, pressure wave mitigation and flowoptimisation as discussed before, R&D work towards an optimum engineering design and the manufacturing of the mercury loop and its periphery is of great importance. The activities must include both computational studies (finite element analysis) and experimental and conceptional work, such as the evaluation of available and newly developed components on existing loops, safety considerations (e.g. avoidance of and recovery from spills), manufacturing techniques, remote handling concepts, etc. Apart from the Institute of Physics in Riga, Latvia, and, more recently, at SNS in Oak Ridge practically no experience in operating large liquid Hg loops existed up to now, and this is in non-radioactive conditions only. The new collaboration with EMPRESSARIOS and SENER (Spain) mentioned above includes theoretical and experimental work on target and mercury loop components and remote handling systems.

Finally, a collaboration with ORNL on the SNS Hg test loop has been established, providing access to the operating experience by exchange of personnel. ESS is also providing special loop components, e.g. radiationhard flow meters. However, in order to gain the necessary experience in running and maintaining a mercury system and to investigate ESS specific questions, a full size mercury loop at one of the laboratories participating in the ESS R&D effort will be indispensable.

This loop will be as identical as possible to the actual target loop and shall serve as a test and training facility before, during and after the construction of the ESS targets.

11. Target Systems overall optimisation 11.1 Impact of new cold moderator system The ESS target station was designed for supercritical hydrogen moderators to be inserted from the top. As mentioned above, there might be an incentive to consider horizontal insertion of a pelletised moderator system, which would require substantial redesign of the target block. However, even if a reliable solution for vertical pellet transport without interruption of the moderator operation can be found, substantial modifications to the cold source insert will become necessary. This decision pending, both options, for horizontal and vertical insertion, should be considered in order to provide a qualified basis for the overall optimisation of the targetmoderator-reflector system.

11.2 Advanced shutter design with in shield neutron optical components Very recently large progress was and still is being made in the development of neutron optical components, largely based on multilayer systems. This enables a much

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better utilisation of the neutrons emerging from a moderator, at least in the thermal and cold energy regime, while at the same time providing for efficient background suppression. However, it is essential to bring up the neutron optics as close as possible to the moderator and have it precisely aligned with the neutron guide system outside the target block. In order to be able to take advantage of this development in ESS it is necessary to develop a shutter system that can hold such components and position them with very high precision and reproducibility each time the shutters are opened, while blocking the beam efficiently when closed. Such a system has been proposed for SNS with vertically moving shutters. It remains to be examined whether the rotating disk shutters presently planned for ESS offer similar opportunities. In this context it is important to examine and improve as much as possible the radiation hardness of the neutron optics and to develop a replacement concept which allows to exchange these components in a usefully short time with minimum radiation exposure of the operating crew.

11.3 Reflector optimisation In the current ESS reference design a heavy water cooled lead reflector has been chosen for the short pulse target station. This was in view of its good performance for coupled moderators. In the frame of an overall system optimisation this question should be revisited because of the well known storage properties of lead for slowing-down neutrons. In a short pulse (decoupled and poisoned) moderator situation the long leakage time of slowing down neutrons from the reflector may lead to undesired tails on the moderator pulses. A reflector with better moderating properties may be advantageous. In an extreme case even a heavy water reflector might be an option.

11.4 Long pulse target station The ESS study included two target stations, one for high repetition rate short pulse and one for low repetition rate work, mainly with cold neutrons. In addition, the idea of using a target with direct injection from the linac without beam compression is now being under discussion again.

While the first concept of this kind [37] was geared towards instruments that could use a high repetition rate [38], more recent thinking is directed towards low repetition rate instruments [39], [40]. While this means more energy in each pulse and hence more pulse-to-pulse thermal cycling in the target, one very important advantage, from the target design point of view, of such a concept still remains: the problem of pressure wave generation is practically non-existent due to the long period over which the protons are injected. In a straight forward manner, such a target system could be identical

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to the short pulse target, but fed with more neutrons per pulse, because there is no need to chop the beam as with injection into a ring. This results in roughly 30% more time average flux and a pulse shape as shown in Fig. 18.

It should be noted that this pulse was simply derived from the ESS short pulse for a coupled moderator as shown in Fig. 6, which was analysed in terms of three exponential decay modes (which might roughly be associated with the original slowing down in the moderator, a storage (life) time in the moderator and a return time from the reflector). The results are shown in the insert of Fig. 18 and were used to calculate the anticipated pulse shape for a full linac pulse injected in the same moderator-reflector system.

In order to illustrate the influence of the reflector system on the pulse shape and to provide an idea on the kind of optimisation possible, we use experimental results obtained in the frame of the SNQ study [41]. In this work three different reflector configurations (lead and graphite with water premoderator and heavy water) were used with a liquid hydrogen moderator. The respective time average fluxes and the dominant flux decay constants were determined for each configuration. Using these parameters, the effect of the reflector system on the pulse from a liquid hydrogen moderator is illustrated in Fig. 19.

It is obvious that the peak flux obtained depends also on the pulse length. Since the pulse duration has a direct effect on the operating cost of the accelerator, careful thought must be given to this question, which is also related to the minimum distance from the moderator at which choppers can be installed.

Thus, although the design of the target-moderatorreflector system of a long pulse target station could, in principle, be the same as that for a short pulse target, there may be different optimisation criteria that apply, in particular since the emphasis will be even more on the use of cold neutrons. This concerns the pulse separation, the pulse duration and, of course, the moderator reflector system. Depending on the final pulse length chosen, the experimental techniques on this target station may require substantial phase space tailoring of the neutron beams by neutron optical methods and choppers. These concepts need to be worked out in detail and their effect on the choice of target station components such as the reflector system, the shutters and even the moderators and possibly their positioning relative to the target (wing, slab or flux trap geometry) must be studied. Prototypic development of the new components is indispensable.

As far as the mercury target system is concerned, it needs to be studied to what extent the high absorption cross section of mercury is a disadvantage in an overall optimisation scheme for an LPNS, taking into account usersâ&#x20AC;&#x2122; needs as well as operational aspects.

Clearly, including an LPNS in the considerations for ESS

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constitutes a major deviation from the original concept and the consequences of this move must be studies in detail not only with regards to the target station itself, but also with respect to its effect on the accelerator system as well as on capital and operating cost. Nevertheless, working out the details of an LPNS target station is a valid exercise within the ESS R&D phase, in order to enable a qualified decision between different options that might be possible within the set cost limit.

and highly rewarding goal, it is hard to imagine that it will be the ultimate facility. Further progress, however, will require the concerted effort of all groups and countries interested in it, since neutron scatterers and neutron source designers will continue to depend on developments going on elsewhere. This is why CONCERT deserves the support of the neutron scattering community. If nothing else, it will help us to make more qualified decisions about ESS.

12. Concluding Remarks Although the technical concept for ESS has been worked out in substantial detail in the Technical Study published in 1997 [3], several new aspects with respect to the overall systems optimisation have surfaced in the mean time, in addition to the necessary R&D work already identified in the Study. This is why the ESS Consortium has decided to spend two to three more years on R&D and systems optimisation. The work to be accomplished in this period is substantial and any European research institution, which is willing and able to make a contribution to this is extremely welcome. The work is being carried out in close collaboration also with the US and Japanese projects, which have made great progress in recent years and which use concepts very similar to the ones developed for ESS in several respects. Yet, the goals of ESS are even more ambitious and this means that we need to go beyond these projects in various aspects also in our research.

In this context it is also important to note that other potential users for high power accelerators are becoming increasingly vocal. In view of the cost and effort associated with the construction and operation of such a facility, there is an obvious need to examine possible synergies and opportunities for joint efforts. It is for this reason that CEA of France and ESS have initiated a study to explore this potential under the name of CONCERT (Combined Neutron Centre for European Research and Technology). As a starting point, this study tries to serve all or most of the present potential customers of high current accelerators in Europe by one single facility.

While it is likely that considerable cost savings may result from one accelerator with a longer duty cycle as compared to several independent installations, provided a dependable and robust design for such an accelerator can be found, there are also other aspects that need to be considered. These include licensing issues, operational and reliability needs and others. For these and other reasons it is likely that, in the end, not all possible applications of high power accelerators can be combined under one umbrella, but it is important to identify possible synergies also among the end users and to include a more long term aspect in the considerations.

Although, at present, ESS seems like a very ambitious

13. Acknowledgements This paper is based on the work of and on discussions with a large number of colleagues whose contributions are greatly appreciated, although they cannot be enumerated individually here. Particular thanks go to H. Ullmaier from Forschungszentrum Juelich for his encouragement to write the paper and for critically reading the manuscript.

References 1. ESS, A Next Generation Neutron Source for Europe; Vol. I The European Spallation Source; ESS Council; ISBN 090 237 6551 (1997) 2. ESS, A Next Generation Neutron Source for Europe; Vol. II, The Scientific Case; ESS Council; ISBN 090 237 6608 (1997) 3. G.S. Bauer, T. Broome, D. F. Filges, H. Jones, H. Lengeler, A. Letchford, G. Rees, H. Stechemesser, G. Thomas (eds.) “ The European Spallation Source Study” Vol. III, The Technical Study; ISBN 090 237 6659 (1997) 4. G.S. Bauer, F. Atchison, T.A. Broome, H.M. Conrad “A Target Development Program for Beamhole Spallation Neutron Sources in the Megawatt Range” Int. Conf. on Accelerator-Driven Transmutation Technologies and Applications, Las Vegas, July 1994 American Institute of Physics Conf. Proc. 364,(1995)105-116 5. G.S. Bauer: “Mercury as a Target Material for Pulsed (Fast) Spallation Neutron Sources” Proc. ICANS-XIII, Paul Scherrer Insitut, PSI-Proceedings 95-02 (1995) T12, 547-558 6. G.S. Bauer “Liquid Metal Target Station” report ESS-95-20-T (1995) 7. T.V. Dury, B.L. Smith, G.S. Bauer “Design of the ESS Liquid Metal Target Using Computational Fluid Dynamics“ Nuclear Technology 127 (1999) pp 218-232 8. CFX4 Release 4.2 User Manual, CFDS, AEA Technology, Harwell, UK (June 1997).

9. Y. Takeda, Instantaneous velocity profile measurement by ultrasonic Doppler method, JSME International Journal, Fluids and Thermal Engineering, Vol.38, No.1, (1995) pp 8-16 10. K. Haga, Y.Takeda, G.S. Bauer and B. Guttek , “Flow Study on the ESS target water model“, Proc. ICANS-XIV, ANL98/33, ISSN1560-858X, (1998) pp 329-341 and ESS report 98-76-T (1998) 11. I. Platnieks, G.S, Bauer, O. Lielausis Y. Takeda, “Measurements of Heat Transfer at the Beam Window in a Mockup Target for SINQ Using Mercury“ in Proc ICANS XIV, Argonne National Laboratory, ANL-98/33 (1998) pp. 382-395 12. J.A. Patorski, G.S. Bauer and S. Dementjev “Two dimensional and dynamic (2DD) method of visualisation of the flow characteristics in a convection boundary layer using infrared thermography” in Thermosense XXII, Proc. of SPIE Vol. 4020

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(2000) pp 240-251 and http://www1.psi.ch/www_gfa_hn/asq/ asqhome.html 13. K. Skala and G.S. Bauer “On the Pressure Wave Problem in Liquid Metal Targets” Proc. ICANS-XIII, Paul Scherrer Insitut, PSI-Proceedings 95-02 (1995) 14. L Ni, G.S. Bauer: “Dynamic Stress of a Liquid Metal Target Container Under Pulsed Heating“ ASME Journal of Pressure Vessel Technology 120, (1998) pp 359-364 15. M. Futakawa et al., private communication (data presented at the 2nd workshop on Spallation Target and Moderator Technology at JAERI, Nov. 13-15, 2000) 16. H. Ullmaier and F. Carsughi “Radiation damage problems in high power spallation neutron sources” Nucl. Inst. Meth. in Phys. Res. B101 (1995) 406-421 17. K. Krautwasser, H. Derz, E. Kny, in High Temperatures and Pressures, 22,25 (1990) 18. S.A. Maloy, W.F. Sommer, R.D. Brown, J.E. Roberts, J. Eddleman, E. Zimmermann G. Willcutt “Progress Report on the Accelerator Production of Tritium Materials Irradiation Program“ Proc.

Symp. On Materials for Spallation Neutron Sources, Orlando, Fla. TMS (1998) pp 131-138 19. M.W. Capiello, „Target/Blanket design for the Accelerator Production of Tritium Plant“ Proc. Topical meeting on Nuc. Appl. of Accelerator Technology, ANS .Order No 700249 , ISBN 0-89448629-2 (1997) pp 129-135 20. G.S. Bauer, Y. Dai and L. Ni “Experience and Research on Target and Structural Materials at PSI“ Proc. Int Workshop on JHFScience - N Arena, KEK (1998) and PSI-Bericht 98-04 (1998) 21. S. Becker, F. Carsughi, H. Cords, H. Derz. H.J. Dietze, R. Duwe, T. Flossdorf, M. Huehnerbein, G. Kueppers, G. Pott, H. Ullmaier, T. Broome, P. Ferguson, S.A. Maloy, W.F. Sommer “Post Irradiation Investigations at the FZJ“ Proc 2nd Int Workshop on Spallation Materials Technology, internal report FZ-Juelich, Jul-3450, pp 414-443 22. F. Carsughi, data presented at ICANS XV, Tsukuba, Oct.2000.

23. J.D. Hunn, E.H. Lee, T.S. Byun and L.K. Mansur “Ion-irradiationinduced hardening in Inconel 718” paper presented at IWMST-4, to be published in J. Nucl. Mat 24. F. Garner; data presented at the 4th Int. Workshop on Spallation Materials Technolog, Schruns, Austria, Oct. 8-13, 2000.

25. E. Lehmann, PSI, pivate communication 26. J.R. Weeks “Compatibility of Sructural Materials with Liquid Lead-Bismuth and Mercury“ Proc Symp. Materials for Spallation Sources, TMS (1998) ISBN 0-87339-361-9 27. J.F. Nejedlik and E.J. Vargo, „Kinetics of Corrosion in a Two-Phase Mercury System“ Corrosion 20 (1964) pp. 384-391 28. A.H. Fleitman and J:R: Weeks, “Mercury as a Nuclear Coolant“ Nucl. Eng. Des.16 (1971),pp 266-278 29. J.R. Weeks “On the Occurrence of Liquid Metal Embrittlement in Specific Binary Systems“ private communication; BNL-internal paper 12923 (1968) 30. E.E Glickman, Yu.V. Goryunov, N.A. Molchanova, V.E. Panin “Life time under Load and Fracture Kinetics of Copper in the Presence of Mercury“ Sov. Phys. J. SOPJAQ 24(3) (1981) pp 199-294 31. C. Nicaise, A. Legris, J.B. Vogt and J. Foct “Embrittlement of the Martensitic Steel 91 by Molten Lead” paper presented at the IWSMT-4, Schruns Oct. 6-10, 2000 and submitted for publication in J. Nucl. Mat. 32. G.S. Bauer, M. Salvatores and G. Heusener, “MEGAPIE, a 1 MW

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Pilot Experiment for a Liquid Metal Spallation Target” accepted for publication in J. Nucl. Mat. (proceedings of IWMST-4, 2000) 33. K. Inoue, H. Iwasa ,Y. Kiyanagi, J. At. Energy Soc. Japan, 21 (1979) 865 34. G.S. Bauer “The ACoM Collaboration - An International Effort to Develop Advanced Cold Moderators“ Proc ICANS-XIV, ANL98/33, ISSN1560-858X, (1998) pp 101-110 35. F. Trouw, ANL, private communication 36. H. Barnert-Wiemer, K. Doering, H. Stechemesser “Development of a Cold Pellet Moderator for ESS“ Proc. ICANS XIV ANL98/33, ISSN1560-858X, (1998) pp 557-568 37. G.S. Bauer, H. Sebening, J.-E. Vetter and H. Willax (eds.)

“Realisierungsstudie zur Spallations- Neutronenquelle” KFA Jülich and KfK Karlsruhe (1981) report Jül-Spez-113/KfK3175 38. R. Scherm and H. Stiller (eds), “Proceedings of the Workshop on Neutron Scattering Instrumentation for SNQ, Maria Laach, 3-5 Sept. 1984; report Jül-1954, KFA Jülich (1984) 39. J. Alonso, R. Pynn. T. Russell, L. Schroeder (eds.) Proc. Workshop on Neutron Instrumentation for a Long-Pulse Spallation Source, LBL37880 / UC-406 / CONF 9504205 (1995) 40. F. Mezei, ed. „Workshop on Neutron Scattering Instruments at Long Pulse Spallation Sources” Special Issue of J. Neut. Res. Vol 6, Nos.1-3 (1997) 41. G.S. Bauer, H.Conrad, W. Fischer, K. Grünhagen and H, Spitzer: “Leakage flux, life-time and spectra of cold neutrons from H2moderators with various reflectors” Proceedings ICANS VIII, report RAL-85-110 (1985) p.344-354 42. L. Pienkowski, F. Goldenbau,, D. Hilscher, U. Jahnke, J. Galin and B. Lott, “Neutron multiplicity distributions for 1.94 to 5 GeV/c proton-, antiproton-, pion, kaon-, and deuteron-induced spallation reactions on thin and thick targets” Phys. Rev. C 56 1909 (1997) 43. G.S. Bauer, “ Some General Reflexions on “Long Pulse“ Neutron Sources“, J. Neut. Res., 3 (1996), pp. 253-271 44. G.S. Bauer “Options for Neutron Scattering Instruments on Long Pulse Neutron Sources”, Proc. ICANS XIII, PSI-Proceedings 95-02 (1995) pp 339-354

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S.I.S.N. The European scenario in neutronics is moving, and many new developments of interest to our community are taking place. It may be worthwhile to stop and analyze the situation, in order to evaluate these developments and see what contribution our community can give.

The European Spallation Source (ESS) Project is advancing, and much of the preliminary work to arrive at the point when an executive project will be formulated is nearing completion. Even though the actual time scale is still long term for such a project, its possible implementation should color much of the strategic planning of the international neutron scattering community, hence of the Italian community also.

From this point of view I see two important issues: 1. The renewal of the convention between the Consiglio Nazionale delle Ricerche (CNR) and the ISIS facility at the Rutherford-Appleton Laboratory near Oxford (UK).

2. The collaboration between INFM (Istituto Nazionale per la Fisica della Materia) and INFN (Istituto Nazionale di Fisica Nucleare) to use the Legnaro accellerator as a medium-small size spallation source.

ISIS is the world’s best neutron spallation source; the projects for a new, much improved target station (ISIS2) are well advanced. Thus the renewal of the convention at this time could have important implications for the medium term strategies of our community. The renewal of the agreement could imply the necessity that the Italian community build a new spectrometer, as was the case in the past (the PRISMA and TOSCA spectrometers). Considering that the ISIS facility is in the process of building a second target with higher neutron production, the challenge of innovating instrumentation expressly plan-

ned for a spallation source would be very interesting, also as a testing ground for future, ESS-related developments. Concerning the second point, if the ongoing projects and work could actually materialize in a national neutron source, it would really be an historic event; a development which our community has awaited for decades. A national source would finally place Italy as a major player in the international community. But the beneficial effects would also be strategically important for the growth of our national community, especially for attraction and formation of young, motivated researchers, and for the unification of the Italian Physics community to work together on an important project. This is particularly important at this historical stage, when Physics is on the defensive on many fronts in our society.

Another important scenario is that of the changes taking place at the Institut Laue-Langevin in Grenoble (Fr).

ILL still is the best overall neutron source in the world, and its Millennium Programme is in full swing.

When such programme will be completed, the world panorama in neutron science will be changed; approximately twenty entirely new or strongly upgraded instruments will be operational.

Our community has been essentially absent from the first stages of the Millennium Programme, but there is still time to participate in the proposal of innovative instrumentation.

From this point of view particularly interesting would be a proposal to build a new reflectometer, which was recently made by ILL researchers. Such instrument would bridge the gap with the ESRF (European Synchrotron Radiation Facility) X-ray reflectometers, and actually be superior, given the additional advantage of isotopic substitution (and particularly hydrogen-deuterium) typical of neutron spectrometry. This possibility would greatly help the growing Italian community which is involved

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in the physics of nanostructures. It would also help in attracting to the field scientists from other disciplines (especially biology and biochemistry).This brings me to the final point I wish to discuss. ILL is in the process of revising the functioning of its Scientific Council, with the express purpose of favoring extension of the user base to other disciplines, and to give additional support to exceptional, highly innovative, high risk research, relative to more standard, or “bandwagon” proposals. This opens up new possibilities for proposals, and scientific inputs, which are not in the mainstream of the moment.

ILL will be interested in helping the formulation, the preparation and eventually the realization of out of the ordinary, “different” proposals which have the potential for attracting new scientists and new science to Grenoble. Our community should take advantage of this.

When discussing all these possibilities for the future development of our community, we should not forget however that none of this will actually happen if our community does not grow. We desperately need many more young researchers who will become competent in the several aspects of neutron science, but more specifically in beamline and instrument development. We still have much to do from this point of view.

In many cases, even for interesting jobs and positions, it is difficult to find qualified participants. We must make the search and formation of young researchers the topmost of our priorities. This implies efforts at all levels for our community.

Recently the problem of the decrease of science (and particularly physics) students has come to the forefront as a major problem for science worldwide. In Italy the INFM, INFN and SIF (Società Italiana di Fisica) have formed a Commission to study the problem and make proposals. Clearly in order to increase the number of young researchers in

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neutronics we must both broaden the disciplinary base of users and increase the number of science students in general. Thus our community should follow closely the activity of the major institutions on this front and eventually contribute to

their work. This however will help in the long run. For more immediate results, we must try to increase the information about the opportunities offered by neutron science in our Universities, and be particularly present in the processes of re-

newal of the didactic offer in Physics and other disciplines in the framework of the reform of the University degrees and courses which is taking place in Italy at this time.

S.I.L.S.

nale di Fisica della Materia e dal Consorzio Interuniversitario per le Biotecnologie. Durante il Convegno fu dato mandato ad un gruppo di lavoro presieduto dal prof. Carlo Calandra di preparare lo statuto della Società. Il gruppo di lavoro presentò al Convegno degli Utenti di Luce di Sincrotrone tenutosi nel 1992 la proposta. Nella riunione fu dato mandato al prof. C. Furlani di fondare legalmente la Società, cosa che avvenne il 15 dicembre del 1992. Presidenti della SILS sono stati il prof. C. Furlani dal 1992 al 1995, il prof. C.M. Bertoni dal 1996 al 1999; attualmente è Presidente il prof. S. Mobilio. L’attività della SILS consta principalmente nella pubblicazione del Bollettino “SILS Notizie” con cadenza semestrale, nella organizzazione del Convegno Annuale della Società e nella organizzazione della Scuola Nazionale di Luce di Sintrotrone con cadenza biennale.

SILS NOTIZIE è un utile strumento di informazione sulla attività della Società; sui principali eventi nazionali ed internazionali connessi con la Luce di Sincrotrone quali Convegni, Scuole, sviluppo di nuovi progetti, risultati scientifici particolarmente rilevanti. Ha una ampia diffusione e costituisce un utile mezzo di diffusione delle informazioni relative alle attività e possibilità offerte dalla radiazione di sincrotrone anche al di fuori della SILS, in quanto inviato non ai soli Soci, ma ad una Comunità ben più ampia.

Il Convegno annuale della SILS ha lo scopo di aggiornare la comunità scientifica italiana sullo stato dell’arte della ricerca con radiazione di sin-

crotrone e sugli ultimi risultati scientifici di rilievo, favorire l’incontro di ricercatori appartenenti a diverse aree scientifiche, diffondere informazioni sulle possibilità di ricerca e sui criteri di accesso per gli utilizzatori alle facilities. Il programma scientifico, che si articola in due giornate e mezzo, prevede relazioni su invito, comunicazioni e sessione poster sulle ricerche di particolare rilievo nei diversi settori di applicazione della radiazione di sincrotrone. Generalmente sono invitati relatori stranieri ad illustrare lo stato dell’arte a livello internazionale nei diversi campi di applicazione. La Società premia i migliori contributi scientifici presentati da giovani ricercatori. Il Convegno ha carattere itinerante ed è tenuto tra la fine di giugno e l’inizio di luglio. L’ultima edizione si è tenuta a Palermo, la prossima sarà a Firenze.

Durante il Convegno si tiene la Assemblea dei Soci durante la quale il Presidente illustra lo stato della Società e le azioni intraprese. In Assemblea vengono anche discusse le candidature per le cariche sociali (presidenza e giunta esecutiva) la cui durata è biennale.

La Scuola Nazionale di Luce di Sincrotrone di cui nel settembre di quest’anno si tiene la sesta edizione offre a persone già operanti nel campo della Luce di Sincrotrone o interessate ad entrarvi una panoramica attuale delle caratteristiche e potenzialità della stessa. Le possibilità di ricerca con radiazione di sincrotrone vi sono affrontate sia da un punto di vista teorico che sperimentale e viste nella loro connessione a varie discipline (chimica, fisica, biologia, scienze del-

La Società Italiana Luce di Sincrotrone ha lo scopo di riunire gli utenti italiani di luce di sincrotrone fornendo una tavola di discussione e di confronto ad una Comunità estremamente differenziata negli interessi scientifici. Nella SILS la Comunità degli utilizzatori ritrova la sua unità che le consente di individuare le proprie esigenze scientifiche ed organizzative relativamente all’uso delle sorgenti di radiazione di sincrotrone. La SILS infatti ha per statuto lo scopo di: • promuovere l’attività di ricerca nel campo della radiazione di sincrotrone e delle sue applicazioni; • coordinare l’attività degli utenti italiani presso sorgenti italiane ed europee, coadiuvandoli nei rapporti con gli Enti incaricati della gestione di tali sorgenti; • dare pareri scientifici e tecnici relativamente alla organizzazione di facilities di radiazione di sincrotrone ed alla progettazione o all’ammodernamento di nuove sorgenti; • fornire agli Enti di ricerca pareri sullo sviluppo e le prospettive della radiazione di sincrotrone.

L’idea di costituire una Società che raggruppasse tutti gli utenti di radiazione di sincrotrone italiani nacque nel 1991 durante il Convegno sulle “Prospettive e Programmi di Utilizzazione della luce di sincrotrone presso le macchine di Grenoble e Trieste” organizzato dalla Commissione per il Coordinamento delle Attività Italiane presso l’ESRF, dal Consiglio Nazionale delle Ricerche, dal Consorzio Interuniversitario Nazio-

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Marco Fontana Presidente della SISN

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la terra) e a diversi tipi di materiali.

Direttori della Scuola sono i prof. S. Mobilio e G. Vlaic. La Scuola, svolta nello stupendo scenario marino di S. Margherita di Pula, si articola in circa 70 ore di lezione, in cui sono trattati come argomenti la produzione e proprietà della radiazione di sincrotrone, le facilities di ELETTRA e ESRF, le attività italiana a ESRF; vengono introdotti gli elementi fondamentali delle metodologie sperimentali quali l’assorbimento di raggi X (EXAFS e XANES), la diffrazione di raggi X (cristallo singolo, polveri, scattering anomalo, MAD, DAFS), le

spettroscopie di fotoemissione lo scattering a basso angolo ed illustrate le applicazioni delle singole spettroscopie a vari tipi di materiali, tra i quali, biomolecole, materiali magnetici, catalizzatori, superfici. La Scuola ha un notevole successo raccogliendo un notevole numero di studenti, quest’anno oltre la cinquantina; segno questo dell’interesse che la radiazione di sincrotrone suscita nella Comunità scientifica.

Al momento a livello internazionale esiste un notevole fermento di iniziative e proposte allo scopo di realizzare sorgenti di radiazione di sin-

NEUTRON RESEARCH AWARDED

BY THE FRENCH ACADEMY OF SCIENCE The French Academy of Sciences and the Institute de France-Aventis Foundation, in occasion of the V edition of the Scientia Europaea Forum, held in Strasbourg from 9 to 13 September 2000, assigned the 2000 prize to a research group selected over more than 400 nominations from 35 European countries.

In the opinion of Prof. Guy Ourisson, President of the French Academy of Sciences, the prize acknow-

ledges the merits, at international level, for innovative scientific studies of interdisciplinary character.

In occasion of the award official event Igor Landau, member of Aventis, leader in life sciences and in Pharmaceutics, declared: "These researchers have dealt with topics which are not common, confirming that the most of future science will be focused surely on the interfaces among several disciplines and cultures".

crotrone di quarta generazione tipo laser nel campo dell’ultravioletto e dei raggi X. Anche a livello italiano una tale possibilità è prevista dal Piano Nazionale della Ricerca del MURST. In tale dinamica situazione la SILS pur non avendo alcun ruolo istituzionale, svolgerà senz’altro un ruolo di notevole importanza in quanto fornirà indicazioni e suggerimenti agli Enti di Ricerca ed alle falicities sulle necessità e sui desiderata della Comunità degli utilizzatori.

Settimio Mobilio Dip. Fisica, Università Roma Tre e INFN Lab. Naz. Frascati

The three "laureates" of the 2000 prize are Salvatore Magazù, physicist of Messina University, Zoe Pikramenou, chemist of Birmingham University and Beata Vertessy, biologist at the Institute of Enzymology of Hungarian Academy of Sciences.

The research concerns with the role of a particular enzyme, deoxyuridine triphosphate nucleotidohidralose, dUTPase for short, of paramount importance for the reparation of DNA. Concerning the neutron contribution to the awarded interdisciplinary research, it has played a central role in determining a structure-function connection in the dUTPase.

Small Angle Neutron Scattering, using the contrast variation method, was employed to characterize the conformational changes of the enzyme (whose molecular weight resulted of ~ 49 kDa) in the two states called "open" and "closed".

The mobility of the apoenzyme (open conformation) C-arm, was characterized by Quasi Elastic Neutron Scattering measurements using selective deuteration and a working hypothesis for the movement of the C-terminal arm was proposed.

Ulderico N. Wanderlingh Università di Messina

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CONSUNTIVO DELL’ATTIVITÀ DI RICERCA svolta nel periodo 1985-2000 nell’ambito dell’Accordo Internazionale CNR-CCLRC, per l’utilizzo della sorgente di neutroni a spallazione ISIS presso il Rutherford Appleton Laboratory (UK) Il Consiglio Nazionale delle Ricerche sostiene fin dal 1985, l’accesso dei ricercatori italiani che utilizzano tecniche di spettroscopia neutronica e di MuSR (Muon Spin Resonance) presso la sorgente pulsata di neutroni ISIS operante al Rutherford-Appleton Laboratory (Oxford, U.K.). Questo è stato possibile grazie ad un accordo decennale stipulato con il SERC (Science and Engineering Research Council) e successivamente rinnovato con il CCLRC (Council for the Central Laboratory of the Research Councils) fino al marzo dell’anno 2002.

Attraverso questo accordo internazionale l’Ente: - garantisce a tutta la comunità italiana l’accesso alla strumentazione di ISIS, con una percentuale di utilizzo pari al 5% del tempo totale disponibile. I dati riportati nella Tabella I, relativi alle percentuali di tempo macchina effettivamente assegnate per l’attività di ricerca dei gruppi italiani, dimostrano come, nel corso degli ultimi anni, i ricercatori italiani abbiano usufruito in media di una percentuale ben superiore a quella prevista dall’accordo. È importante sottolineare che ad ISIS l’assegnazione di tempo sperimentale ai gruppi di ricerca italiani avviene su base competitiva, attraverso la presentazione di proposte di esperimenti, che vengono vagliate e selezionate da apposite Commissioni Internazionali di esperti. Inoltre l’atmosfera culturale e scientifica del centro di ricerca è particolarmente vivace e stimolante e costituisce un ambiente adatto per la formazione qualificata di giovani ricercatori da inserire successivamente nelle strutture di ricerca e sviluppo operanti del nostro paese; - finanzia direttamente lo sviluppo di strumentazione da installare ad

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ISIS, progettata e costruita presso i sta facilità viene realizzata per mezpropri organi di ricerca, anche in colzo di una fascio di muoni polarizzati laborazioni con gruppi universitari. formato a partire da un bersaglio In particolare si ricordano il ProgetTABELLA 1 to PRISMA realizzato presso l’ISM ISIS Giorni % Giorni % (Istituto di StrutRound richiesti assegnati tura della Materia, Frascati) nel perio88/1 222,3 11,8 100,1 10,5 do dal 1984 al 1991 ed il Progetto 89/1 203,0 10,3 55,1 6,0 TOSCA, avviato 89/2 198,3 11,4 64,8 9,8 nel 1996, realizza90/1 198,8 10,7 33,0 5,5 to presso l’IEQ (Istituto di Elettro90/2 170,3 8,8 53,5 7,6 nica Quantistica, 91/1 176,0 7,9 61,8 7,9 Firenze) e recente91/2 169,3 8,1 57,4 5,8 mente completato.

Entrambi gli spet92/1 135,3 4,7 49,8 4,6 trometri PRISMA 92/2 167,4 7,3 61,3 7,5 e TOSCA, sono 93/1 172,0 6,1 45,9 4,9 oggi parte integrante del parco 93/2 158,6 6,7 51,9 5,1 strumenti operan94/1 112,3 4,6 39,4 4,2 te presso la sor94/2 123,6 5,0 51,1 4,9 gente ISIS. Inoltre CNR finanzia di95/1 190,4 8,2 72,4 6,5 rettamente una 95/2 141,8 5,7 56,5 4,7 unità di personale 96/1 119,7 5,3 53,3 4,6 (ex-art 36) in servizio presso ISIS, 96/2 143,7 5,9 57,7 4,7 che, quale respon97/1 102,0 4,6 56,3 4,5 sabile dello spet97/2 164 7,5 71 6,0 trometro TOSCA, svolge sia attività 98/1 123 5,7 52 4,1 sperimentale sia 98/2 189 8,8 92 6,6 di supporto per i ricercatori italiani 99/1 127 6,6 68 5,6 che effettuano 99/2 205 10,6 105 7,3 esperimenti pres00/1 242 11,8 104 8,7 so tale apparecchiatura; 00/2 192 8,6 93 7,3 - garantisce inoltre 01/1 189 10,1 93 7,4 l’accesso alla strumentazione di MuSR (Muon Spin Totale 4335,8 1699,3 Resonance). Que-

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sottile lungo il fascio primario. I muoni vengono quindi distribuiti su tre strumenti. La gestione della facilità avviene con le modalità competitive comuni a tutti gli strumenti neutronici e l’utilizzo vede una significativa presenza italiana. Essa si è accresciuta rispetto ai primi anni, nei quali pure il contributo italiano allo sviluppo delle apparecchiature fu determinante.

Con la stipula di questo accordo internazionale ed il cospicuo investimento che l’Ente ha garantito nel corso degli ultimi 15 anni il CNR ha permesso un considerevole sviluppo della comunità scientifica italiana che impiega le tecniche di diffusione dei neutroni. Tale comunità, che nella prima metà degli anni ’80 contava circa una decina di unità, si è estesa a circa 200 ricercatori, CNR ed universitari, che svolgono di routine attività di ricerca utilizzando le tecniche di spettroscopia neutronica e di MuSR. Questi ultimi conducono sia attività di ricerca di base, su un ampio spettro di tematiche scientifiche (vedi Allegato I), sia di sviluppo e realizzazione di strumentazione (vedi Allegato II). In particolare l’accordo CNR-ISIS ha favorito, nel corso degli

anni, l’instaurarsi di una efficace collaborazione italo-inglese che ha tra l’altro permesso il finanziamento, su fondi della Comunità Europea, di altri progetti di ricerca nel campo della spettroscopia neutronica. A questo proposito si ricordano i progetti VESUVIO, XENNI e TECHNI (vedi Allegato II). È importante ricordare che l’Europa proprio grazie ad ISIS detiene una leadership nel settore della spettroscopia di neutroni con sorgenti pulsate, infatti tale sorgente rappresenta la più intensa sorgente a spallazione di neutroni operante nel mondo. Questa è generata da un fascio di protoni da 800 MeV per una potenza totale del fascio pari a 0.36 MW. Tenuto conto che nel prossimo decennio si prevede una riduzione della disponibilità complessiva di fasci di neutroni, per la programmata chiusura di diverse sorgenti attualmente operative basate su reattori a fissione, la comunità scientifica internazionale ha messo a punto diversi progetti per la costruzione di nuove sorgenti pulsate. In ambito Europeo gli studi di fattibilità per la realizzazione di una sorgente a spallazione di seconda generazione sono coordinati da

un organismo, l’ESS R&D Council, costituito da un consorzio di Enti e Istituzioni Europee, a cui, nel 1998, a riconoscimento della attività svolta dai suoi organi di ricerca ad ISIS, il CNR è stato invitato ad aderire.

L’anno scorso questo consorzio ha finalizzato entro il 2003 la predisposizione del progetto per una nuova sorgente pulsata di neutroni in Europa, l’European Spallation Source (ESS), da sottoporre ai governi dell’Unione Europea, come illustrato nella relazione allegata, ‘Partecipazione del CNR al Progetto European Spallation Source (ESS) in ambito ESS R&D Council’, del delegato CNR in ambito ESS R&D Council, prof. M. Fontanesi.

Per maggiori informazioni sulla attività di ricerca e sviluppo di stumentazione svolte dalla comunità italiana sia ad ISIS che in ambito ESS si rimanda ai siti web http://www.isis.rl.ac.uk/ http://www.ess-europe.de/ Le attività di ricerca e di sviluppo di strumentazione ad ISIS, per quello che concerne il CNR, sono coordinate da una apposita Commissione per la Spettroscopia Neutronica.

Italy % time Requested

14

Allocated

12 10 8 6 4 2

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00/2

99/2

98/2

97/2

96/2

95/2

94/2

93/2

92/2

91/2

90/2

89/1

88/1

0

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Investimenti CNR nell’ambito dell’attuale accordo con il CCLRC per l’utilizzo della sorgente ISIS Nel decennio 1985-1995, l’impegno del CNR è quantificabile in: • 20000 ML di investimento oneroso per l’accesso alla sorgente ISIS; • 3000 ML di investimento per il progetto e la realizzazione di strumentazione per spettroscopia neutronica da installare ad ISIS (Progetto PRISMA e sviluppo di cristalli monocromatori).

Nel periodo 1996-2001 l’impegno del CNR è quantificabile in: • 12300 ML di investimento oneroso per l’accesso alla sorgente ISIS; • 3200 ML di investimento per il progetto e la realizzazione di strumentazione per spettroscopia neutronica da installare ad ISIS (Progetto TOSCA e sviluppo di cristalli monocromatori); • 1 unità di personale (art 36 distaccato ad ISIS presso lo strumento TOSCA).

Proposta per il rinnovo dell’Accordo Internazionale CNR-CCLRC Premessa Il CCLRC ha recentemente approvato un progetto, denominato ISIS II, che prevede la costruzione di una nuova targhetta per la produzione di neutroni ‘freddi’ da affiancare a quella già operante ad ISIS e che utilizzerà lo stesso acceleratore di protoni da 800 MeV al Rutherford Appleton Laboratory. Alcuni dettagli tecnici di questa proposta sono riportati in Allegato III e nel brochure ‘A second target Station at ISIS’ preparata dal CCLRC, anche allegato a questa relazione.

Questa nuova struttura sperimentale permetterà la produzione di intensi fasci di neutroni “freddi”, che permetteranno di effettuare studi complementari a quelli attualmente possibili ad ISIS; in particolare si prevede un programma di ricerca specificatamente dedicato allo studio ad alta risoluzione della struttura e della dinamica nella materia soffice, nei

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materiali biologici e nei materiali avanzati. Questo favorirà un ulteriore progressivo ampliamento dello spettro delle tematiche scientifiche che sono attualmente oggetto di ricerca ad ISIS. A questo proposito è importante ricordare che, pur prevalendo nel complesso le ricerche nel settore fisico, negli ultimi anni uno spazio significativo è stato ricoperto da ricerche in ambito chimico, biologico, biomedico, mineralogico e in materiali avanzati per l’ingegneria.

In questi ambiti l’attività di spettroscopia neutronica presso ISIS ha fatto emergere un insieme di rilevanti programmi di ricerca, fornendo tra l’altro significativi risultati per la comprensione delle proprietà dei polimeri, surfattanti e proteine in interfacce e soluzioni, materiali con magnetoresistenza gigante, superconduttori ad alta temperatura. Il progetto ISIS II rappresenta un importante occasione di sviluppo dello studio di questi sistemi ed una opportunità per la comunità di ricercatori italiani operante in questi settori di ricerca. Contemporaneamente anche la comunità MuSR sta sviluppando un programma di adeguamento del fascio e del parco strumentazione. Essa prevede miglioramenti significativi nel tasso di conteggio, nella dimensione del fascio (ossia nella possibilità di studiare campioni piccoli), oltre che un nuovo strumento dedicato ad alti campi magnetici e con migliore risoluzione temporale. Il progetto è descritto in dettaglio nella brochure “Opportunities for New Science with Pulsed Muons - Development of the ISIS Muon Facility” e, più succintamente, nell’Allegato III. Proposta Il progetto ISIS II prevede un aumento di potenza dell’acceleratore di protoni di ISIS, e quindi una ottimizzazione prestazioni della sorgente con la conseguente realizzazione di strumentazione idonea per ricerche nel settori sopra descritti. La Commissio-

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ne CNR di Consulenza del Presidente per la Spettroscopia Neutronica e la Società Italiana per la Spettroscopia Neutronica, hanno esaminato in dettaglio tale progetto, in diverse riunioni, valutato le opportunità scientifiche che esso renderà disponibili e le conseguenti ricadute per la comunità scientifica italiana. A tale proposito in vista della scadenza dell’accordo CNR-CCLRC previsto nel marzo 2002 entrambi gli organismi auspicano un rinnovo di tale accordo, tenuto conto: • del soddisfacente utilizzo di ISIS da parte della comunità italiana, anche superiore alla percentuale spettante prevista dall’accordo (vedi Tabella I); • dell’aumento del numero di ricercatori italiani in questo settore, negli ultimi anni sono stati complessivamente circa 500, tra CNR ed universitari, dei quali circa 200 svolgono di routine attività di ricerca utilizzando la spettroscopia di neutroni; • che le attività di sviluppo di strumentazione che hanno accompagnato i precedenti accordi hanno permesso di creare nel nostro paese una comunità con considerevoli capacità tecniche nella costruzione di nuova strumentazione per spettroscopia di neutroni e MuSR sulle sorgenti a spallazione, permettendo l’inserimento di ricercatori CNR nelle attività di ricerca e sviluppo di strumentazione dell’ESS R&D Council (vedi relazione Partecipazione del CNR al Progetto European Spallation Source (ESS) in ambito ESS R&D Council).

• della rilevanza che le sorgenti di neutroni a spallazione rivestono per lo sviluppo della ricerca di base ed applicata nel panorama europeo, come risulta da numerose note di indirizzo predisposte in ambito di Organisation for Economic Co-operation and Development (OECD) Global Science Forum (vedi relazione ‘Partecipazione del CNR al Progetto European

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Spallation Source (ESS) in ambito ESS R&D Council’ allegata).

Per questi motivi la Commissione di Studio per il Coordinamento delle Attività di Spettroscopia Neutronica del CNR ha già proposto, in occasione della stesura del piano triennale dell’Ente e più recentemente al Presidente del CNR (vedi Verbale allegato), che il nuovo accordo mantenga la partecipazione del CNR ad ISIS allo stesso livello dell’attuale, con una percentuale di utilizzo della strumentazione pari al 5% del tempo totale disponibile. È inoltre auspicabile che, come per il precedenti, il nuovo accordo preveda, oltre ad una quota onerosa, la costruzione di una apparecchiatura sperimentale o l’aggiornamento di una già esistente ad ISIS. In questo contesto va ricordato che il progetto ISIS II prevede una sorgente di neutroni con un fascio di intensità superiore del 50 % rispetto a quella attuale, e che l’incremento della corrente dell’acceleratore sarà disponibile verso la fine del 2002. Quindi la nuova apparecchiatura potrebbe anche essere scelta tra quelle previste nel progetto ISIS II (vedi Allegato III e brochure ‘A second target Station at ISIS’ allegata). Per quanto riguarda la costruzione di un nuovo strumento si sottolinea come le competenza per la predisposizione di un progetto siano disponibili in ambito CNR sia presso l’Istituto di Elettronica Quantistica di Firenze, che ha realizzato, nell’ambito del presente accordo lo strumento TOSCA, sia presso l’Istituto di Tecniche Spettroscopiche di Messina, che ha comunicato alla Commissione Neutroni la sua disponibilità alla realizzazione di strumentazione per spettroscopia di neutroni ad ISIS. È poi assai importante che anche la comunità italiana di spettroscopia muonica mantenga un contributo attivo alla costruzione della nuova strumentazione. Sarà possibile negoziare con ISIS se contribuire con una parte alla strumentazione già prevista o se pianificare un ulteriore sviluppo del parco stru-

menti preesistente (ad esempio DIZITAL, originariamente costruito in Italia). Le competenze necessarie sono presenti sia nel gruppo universitario di Parma, sia in quello di Pavia che nell’Istituto CNR- MASPEC di Parma per ciò che riguarda gli alti campi magnetici ed il trattamento e manipolazione campioni in condizioni di UHV. Relazione predisposta dalla Commissione di Studio per il Coordinamento delle Attività di Spettroscopia Neutronica del CNR

ALLEGATO 1: Istituti CNR ed unità di ricerca universitarie Consiglio Nazionale delle Ricerche: IEQ - Ist. Elettronica Quantistica (Fi) ITS - Ist. Tecniche Spettroscopiche (Messina) ISM - Ist. Struttura della Materia (Roma) ISM - Ist. Spettroscopia Molecolare (Bologna) ISM - Ist. Spettroscopia Molecolare (Bologna) MASPEC - Ist. Materiali Speciali per l'Elettronica e Magnetismo Università: Dip. Chimica (Fi) Dip. Fisica (Fi) Dip. Cristallografia (To) Dip. Scienza dei Materiali (An) Dip. Fisica (Mi-Bicocca) Dip. Scienza Materiali (Mi-Bicocca) Dip. Fisica (Me) Dip. Fisica (Tor Vergata Roma) Dip. Fisica (Roma Tre) Dip. Fisica (Parma) Dip. Fisica (Perugia) Dip. Fisica (Modena) Dip. Scienze della Terra (Mi) Dip. Scienze Ambientali (Vt) Dip. Chimica Fisica (Ve) Dip. Fisica (Politecnico Mi) Facoltà Medicina (Mi) Dip. Fisica (Ge) Dip. Fisica (Trento) Dip. Chimica (Sassari) Dip. Fisica (Cagliari) Dip. Fisica (Palermo)

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Dip. Chimica-Fisica (Pavia) Dip. Fisica (Pavia) Linee di ricerca 1. UNI - Dip. Scienze Mineralogiche e Petrologiche di Torino Studio di materiali di interesse geologico (Prof. G. Ferraris) 2. UNI - Dip. Scienza dei Materiali di Milano-Bicocca Studio delle proprietà vibrazionali in conduttori ionici (Prof. M. Catti) 3. UNI - Dip. Scienze della Terra di Milano Analisi Strutturali in mineralogia e cristallografia (Prof. G. Artioli) 4. UNI - Dip. Fisica di Milano-Bicocca Sviluppo di rivelatori per scattering di neutroni epitermici (Prof. G. Gorini, Dr. M. Tardocchi) 5.UNI - Dip. Fisica Politecnico di Milano Sviluppo di rivelatori a stato solido per neutroni (Prof. C. Petrillo) Sviluppo di cristalli monocromatori per neutroni (Prof C. Petrillo) 6. UNI - Dip. Chimica e Biochimica Medica Studio di membrane biologiche con tecniche di diffrazione a basso angolo (Prof. M. Corti) 7. UNI - Ist. Chimico Farmaceutico Tossicologico di Milano Studio della catalisi molecolare di sistemi idrogenati (Prof. A. Albinati) 8. UNI - Dip. Fisica di Trento Proprietà dinamiche e strutturali di sistemi vetrosi (Prof. A. Fontana) 9. UNI - Dip. Chimica Fisica di Venezia Studio di proprietà strutturali di leghe metalliche (Dr. R. Frattini) Studio di sistemi mesoscopici con tecniche di diffrazione a basso angolo (Prof. A. Benedetti) 10. UNI - Dip. Fisica di Parma Proprietà dinamiche e strutturali di sistemi macromolecolari di interesse biologico (Prof. A. Deriu, Prof. F.Cavatorta, Prof. G. Albanese, Dr. M.T. Di Bari) Proprietà magnetiche e strutturali di sistemi con proprietà di magnetoresistenza gigante e colossale (Prof. A. Deriu, Dr. I. Bergenti)

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Proprietà di liquidi molecolari complessi (Prof. M. Fontana) Proprietà dinamiche e strutturali di sistemi magnetici (Prof. G. Amoretti) Proprietà elettroniche degli ossidi dei metalli di transizione (Prof. C. Bucci, Prof. R. De Renzi, Prof. M. Riccò) Proprietà di fullereni e fulleridi (Prof. C. Bucci, Prof. R. De Renzi, Prof. M. Riccò) La ricerca è svolta in collaborazione con il il MASPEC e con ISIS. 11. UNI - Dip. Fisica di Modena Proprietà termiche, magnetiche ed elettroniche di materiali magnetici e superconduttori (Prof. O. Moze) 12. Ist. Spettroscopia Molecolare Bologna Spettroscopia vibrazionale di molecole coniugate basate su tiofeni e fullereni (Dr. G. Ruani, Dr. S. Degli Esposti) 13. UNI - Dip. Fisica di Ancona Interazioni magnetiche fondamentali e proprietà di materiali magnetici (Prof. R. Caciuffo).

Studi microstrutturali di materiali di interesse tecnologico ed industriale mediante tecniche neutroniche e di raggi X (Prof. G. Albertini).

Misure di tensioni residue mediante diffrazione di neutroni ed X (Prof. F. Rustichelli).

Proprietà strutturali di molecole biologiche (Prof. F. Mariani).

14. CNR - Ist. Elettronica Quantistica di Firenze Costruzione e ottimizzazione dello spettrometro TOSCA (localizzato ad ISIS) (Responsabile: Dr. M. Zoppi) Progettazione e costruzione della Stazione Sperimentale Italiana ad ISIS (Progetto INES) (Responsabile: Dr. M. Zoppi).

Proprietà magnetiche di catene unidimensionali di magneti molecolari (Dr. M.G. Pini, Prof. A. Rettori) La ricerca viene svolta in collaborazione con il Dipartimento di Chimica Inorganica di Firenze (Prof. D. Gatteschi) e con il Dipartimento di Scienza dei Materiali di Ancona (Prof. R. Caciuffo).

Proprietà strutturali e dinamiche

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delle fasi condensate dell’idrogeno (Dr. U. Bafile, Dr. M. Celli, Dr. M. Zoppi ) La ricerca è svolta in collaborazione con il Dr. D. Colognesi, ricercatore CNR distaccato ad ISIS. Struttura e dinamica dei liquidi semplici (Dr. U. Balucani, Dr. U. Bafile, Dr. M. Celli, Dr. M. Zoppi) La ricerca è svolta in collaborazione con il Dipartimento di Fisica di Firenze (Prof. F. Barocchi), il Dipartimento di Energetica di Firenze (Prof.

R. Magli) e con il Dipartimento di Fisica di Trento (Prof. R. Vallauri).

Diagnostica non distruttiva di bronzi archeologici (Dr. S. Siano, Dr. U. Bafile, Dr. M. Celli, Dr. M. Zoppi) La ricerca è svolta in collaborazione con il Dipartimento di Fisica di Modena (Prof. O. Moze) e con il Rutherford Appleton laboratory (Dr. W. Kockelmann).

15. CNR - Ist. Ricerca Onde Elettromagnetiche di Firenze Indagini di spettroscopia vibrazionale in materiali inorganici contenenti acqua e/o gruppi idrossilici di interesse per i beni culturali (Dr. Mauro Bacci).

Studi strutturali in cuprati superconduttori (Dr. Luciano Cianchi).

16. UNI - Dip. Fisica di Firenze Proprietà strutturali di liquidi semplici (Prof. F. Barocchi, Dr. G. Pratesi, Dr. F. Formisano, Dr. E. Guarini).

La ricerca è svolta in collaborazione con il CNR-IEQ di Firenze, con il Dipartimento di Chimica e Biochimica Medica di Milano (Prof. R. Magli), e con il Dipartimento di Fisica di Genova (Prof. G. Casanova).

Proprietà dinamiche e strutturali di sistemi vetrosi (Prof. F. Barocchi, Prof. M. Sampoli, Dr. G. Pratesi) Proprietà strutturali di sistemi colloidali e micellari (Prof. D. Senatra, Prof. C. Gambi) 17. UNI - Dip. Chimica Fisica di Firenze Proprietà strutturali e dinamiche di soluzioni macromolecolari e colloidali (Prof. P. Baglioni, Dr. D. Berti) 18. UNI - Dip. Fisica di Perugia Sviluppo di cristalli monocromatori

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per spettroscopia neutronica (Prof. F. Sacchetti) Sviluppo di rivelatori per neutroni (Prof. F. Sacchetti).

La ricerca è svolta in collaborazione con il gruppo di spettroscopia di neutroni del CNR-IEQ di Firenze con il Politecnico di Milano (Prof. C. Petrillo).

19. UNI - Dip. Chimica di Sassari Amorfizzazione meccanica di leghe metalliche (Prof. S. Enzo).

La ricerca è svolta in collaborazione con il Dipartimento di Chimica-Fisica di Venezia (Dr. R. Frattini).

20. CNR - Ist. Struttura della Materia di Roma Studio della dinamica vibrazionale dell’idruro metallico PdH (Dr. F. Cilloco).

La ricerca è svolta in collaborazione con il CNR-IEQ di Firenze (Dr. M. Zoppi).

Studio di sistemi polimerici in soluzione solida con sali ionici (Dr. V. Rossi Albertini).

21. UNI - Dip. Fisica Roma Tor Vergata Sviluppo di rivelatori per neutroni epitermici (Prof. C. Andreani, R. Senesi, A. Pietropaolo).

Realizzazione di uno spettrometro per scattering di neutroni epitermici ad ISIS - Progetto VESUVIO (Prof. C. Andreani, Dr. D. Colognesi, Dr. E. Degiorgi, Dr. R. Senesi) Studio delle proprietà dinamiche di singola particella in fluidi quantistici e molecolari (Prof. C. Andreani, Prof.

E. Pace, Dr. D. Colognesi, Dr. E. Degiorgi, Dr. R. Senesi) La ricerca è svolta in collaborazione con il Dr. D. Colognesi, (CNR-ISIS), con Dr. M. Zoppi (CNR-IEQ) e con Università di Roma Tre Dip. Fisica (Prof. M. Nardone e Prof. M.A.Ricci).

22. UNI - Dip. Scienze e Tecnologie Chimiche Univ. Roma “Tor Vergata” Studio della dinamica dell’acqua in domini polimerici confinati (Prof. G. Gaio Paradoss) 23. UNI - Dip. Fisica “E. Amaldi”- Roma Tre Studio della struttura microscopica e della dinamica traslazionale e vibra-

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zionale di soluzioni acquose sopraccritiche (Prof. M.A. Ricci, Dr. C. Oliva, Dr. A. Isopo, Dr. A.Botti).

Studio della struttura microscopica e della dinamica traslazionale e vibrazionale dell’acqua in geometria confinata (Dr. F. Bruni, Prof. M.A. Ricci, Dr. P. Gallo, Prof. M. Rovere, Dr. F. Venturini).

Indagini strutturali e spettroscopiche di materiali di interesse per i Beni Culturali (Prof. M. Nardone).

La ricerca 3) è svolta in collaborazione con l’Istituto per la Conservazione della Carta e il Dipartimento di Geologia di Roma Tre. 24. UNI - Dip. Scienze Ambientali di Viterbo Studio della dinamica di macromolecole di interesse biologico e del ruolo del solvente (Prof. S. Cannistraro, Prof. A.R. Bizzarri).

25. CNR - Ist. Tecniche Spettroscopiche di Messina Proprietà strutturali e dinamiche di elettroliti polimerici (Dr. G. Di Marco, Dr. F. Aliotta, Dr. M.E. Fontanella).

Proprietà strutturali e dinamiche di soluzioni polimeriche e micellari (Dr. F. Aliotta, Dr. M.E. Fontanella, Dr. C. Vasi).

26. UNI - Dip. Fisica di Messina Dinamica vibrazionale e proprietà strutturali di soluzioni proteiche (Prof. U. Wanderlingh, Prof. R. Giordano).

Dinamica vibrazionale di sistemi vetrosi (Prof. G. Carini).

Proprietà strutturali e dinamiche di soluzioni macromolecolari di interesse biologico (Prof. P. Migliardo, Prof. S. Magazu).

27. UNI - Dip. Fisica di Palermo Studio della dinamica di soluzioni acquose di trealosio (prof. L. Cardone, Prof. A. Cupane).

Struttura e dinamica di macromolecole e sistemi micellari (Prof. R. Triolo, Prof. E. Caponetti).

28. UNI - Dip. Chimica-Fisica di Pavia MuSR in spinelli di manganese di interesse per la sensoristica (Dott. P. Ghigna).

29. UNI - Dip. Fisica di Pavia

MuSR in sistemi magnetici basso dimensionali (Prof. P. Carretta).

MuSR in sistemi magnetici molecolari ad alto spin (Prof. F. Borsa, Dott.

A. Lascialfari, Prof. D. Gatteschi - Firenze).

MuSR in cuprati superconduttori (Dott. A. Lascialfari).

30. UNI - Dip. Fisica di Cagliari MuSR in cuprati magnetici, progenitori dei superconduttori (Prof. P. Manca).

ALLEGATO 2: Attività di Ricerca e Sviluppo di strumentazione per spettroscopia di neutroni e MuSR svolta dalla Comunità Italiana nel periodo 1985-2000 Con finanziamento CNR: 1. Progetto PRISMA PRISMA I: progettazione e costruzione di uno spettrometro con sistema a 16 rivelatori, chopper Nimonico e inclusione di rivelatori a grafite.

PRISMA II: Doppio banco di analizzatori e rivelatori.

PRISMA III: Rivelatori per diffrazione per studi di scattering diffuso.

PRISMA IV: Istallazione della guida per neutroni

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2. Progetto TOSCA TOSCA Phase 1 TOSCA Phase 2 Sviluppo di un sistema di filtro a Para-idrogeno.

Con finanziamento della Comunità Europea a gruppi Universitari ed INFM: 3. Progetto VESUVIO: Progettazione e costruzione di uno spettrometro anelastico di neutroni epitermici (finanziamento Comunità Europea).

4. Progetto DIZITAL: costruzione della spettrometro per misure di rotazione e rilassamento in campi magnetici longitudinali, trasversali e nulli (finanziamento INFM).

5. Progetto EMU: Development of a second European MUsr spectrometer (finanziamento Comunità Europea).

6. Progetto SLOWMU: Development of an Ultraslow Muon Source for the EC Muon Facility at ISIS. 7. Sviluppo di tecniche pulsate e lo sviluppo di sorgenti fredde.

8. Progetti XENNI e TECHNI: Sviluppo di rivelatori per neutroni termici ed epitermici (finanziamento Comunità Europea).

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ALLEGATO 3: Descrizione tecnica del progetto ISIS II L’installazione della stazione con la seconda targhetta (ISIS II) prevede un upgrade dell’acceleratore di ISIS con un incremento della corrente del fascio da 200 mA a 300 mA. Il guadagno del 50% sarà ottenuto con l’aggiunta di quattro nuove cavità di accelerazione nel sincrotrone e richiederà solo cambiamenti minimi agli schermaggi ed al sistema di raffreddamento della attuale targhetta di Tantalio (ISIS I).

L’aumento della corrente verrà utilizzato per la seconda targhetta, per esempio indirizzando un impulso ogni cinque del fascio di protoni su questa ultima. Questo permetterà di ottenere un fascio su ISIS II con frequenza di 10 Hz e potenza di circa 36 KW. Con questa potenza di fascio relativamente modesta, la targhetta e il moderatore potranno essere ottimizzati per produrre flussi di neutroni “freddi” per impulso molto più alti di quelli disponibili presso ISIS I, con conseguente miglioramento della qualità, risoluzione ed intensità dei fasci di neutroni “freddi” disponibili sulle beam lines.

Per la produzione di neutroni sarà utilizzata una targhetta di Tantalio.

Sono previsti tre moderatori: uno a metano liquido a 100K e due a metano solido a 25 K, il primo per la produzione di impulsi stretti per studi ad alta risoluzione e l’altro, per la produzione di impulsi più larghi, a più alta intensità. Gli impulsi di neutroni “freddi” avranno una intensità almeno un ordine di grandezza maggiore di quelli disponibili presso l’attuale stazione ISIS I. Schema della ISIS Facility comprendente l’acceleratore e le stazioni con le due targhette ISIS I e ISIS II Il completamento della stazione per la seconda targhetta è previsto entro tre anni dall’inizio dei lavori con una spesa stimata di ~ £ 60M.

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Strumentazione prevista per ISIS II Il disegno della stazione per la seconda targhetta sarà ottimizzato per applicazioni che richiedono un alto flusso di neutroni “freddi” con durata dell’impulso ed intervallo di lunghezze d’onda relativamente ampi (per esempio per studi di scattering a basso angolo, riflettometria, e spin echo) e per applicazioni ad alta risoluzione sia in diffrattometria che in spettroscopia. Il fattore di guadagno sarà ~20 per studi di scattering a basso angolo e di riflettometria, e ~10 per quanto riguarda la spettroscopia e la diffrazione ad alta risoluzione.

Il fascio secondario di muoni dovrà restare lungo il fascio che porta alla prima targetta. Godrà dell’incremento di corrente, di un ulteriore aumento di intensità prodotto da nuovi quadrupoli magnetici di larga accettanza e da un nuovo disegno del bersaglio di produzione, oltre che della riprogettazione del trasporto magnetico. Ciò consentirà un aumento complessivo di un ordine di grandezza del numero di particelle, la conseguente possibilità di segmentare temporalmente il fascio e la riduzione sensibile della sezione del fascio sul campione.

Per effettuare questo tipo di studi è stato già individuato un parco strumenti, un certo numero dei quali è di diretto interesse per le attività specifiche della comunità italiana operante nell’ambito della spettroscopia neutronica ed in particolare in ambito CNR. Questi includono: SANS – disegnato per essere il migliore strumento per scattering a basso angolo, in grado di accedere simultaneamente ad un intervallo di momenti trasferiti, Q, eccezionalmente ampio per questa classe di strumenti. fm-SANS – strumento per scattering a basso angolo con uno specchio focalizzante che permette di accedere valori di Q molto bassi rispetto a quelli accessibili con la strumentazione convenzionale.

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HRPD-II – disegnato per diventare il diffrattometro a più alta risoluzione operante al mondo (Dd/d ~0.03%).

LMX – diffrattometro per cristallografia di molecole complesse, ad esempio di interesse biologico. hr-ENGIN – diffrattometro per studi ad alta risoluzione su materiali avanzati per l’ingegneria. hr-SXD – diffrattometro a cristallo singolo per studi ad alta risoluzione temporale e spaziale.

WISH – diffrattometro a polveri a grande intervallo angolare in grado di accedere un ampio intervallo in Q. MUSICAL – strumento ad alta risoluzione per studi di scattering ad alti angoli.

HERBI – spettrometro a cristallo analizzatore con alta risoluzione in energia ed un ampio intervallo dinamico.

INTER – riflettometro con alta flessibilità nella scelta della risoluzione e del flusso, ideale per studi in funzione del tempo nell’intervallo sulla scala dei secondi.

Pol-REF – riflettometro a neutroni polarizzati in un ampio intervallo di lunghezze d’onda. off-SPEC –un riflettometro operante sia in geometria verticale che orizzontale.

NIMROD – un diffrattometro che combinando l’uso di rivelatori a basso ed alto angolo permette lo studio dell’ordine locale e a medie distanze nei materiali cristallini e disordinati.

DIPOL –diffrattometro a scattering diffuso con opzione di analisi di polarizzazione.

LET –spettrometro a chopper per basse energie trasferite. cn-CAS – spettrometro a cristallo analizzatore dedicato a studi a cristallo singolo ad alta risoluzione.

NSE – neutron spin echo con risoluzione nell’intervallo meV-neV ed ampio intervallo dinamico.

MuSR e EMU godranno dei miglioramenti del fascio.

TESLAMU - nuovo strumento ottimizzato per gli alti campi magnetici

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longitudinali, per esperimenti di rilassamento e di level crossing.

Ricostruzione della struttura cristallina del neurotrasmettitore Acetilcloline effettuata ad ISIS.

Opportunità scientifiche I recenti significativi sviluppi della ricerca che utilizza fasci di neutroni “freddi” permetterà lo studio delle proprietà strutturali e dinamiche in diverse aree tecnologicamente rilevanti, quali la materia soffice, le scienze biologiche e biomediche e i materiali avanzati.

In particolare per quanto riguarda le aree dei materiali avanzati e della materia soffice, l’interesse è attualmente rivolto allo studio di sistemi complessi a molti componenti e a molte fasi, all’utilizzo di portacampioni complessi per lo studio di sistemi fuori dall’equilibrio e in condizioni estreme di temperatura e pressione. In molti di questi sistemi la scala dimensionale di interesse varia da quella molecolare a quella mesoscopica. Questo determina la necessità di aver accesso ad un ampio intervallo di lunghezze d’onda con particolare enfasi sui neutroni “freddi”. Gli studi di cinetica (che vanno dallo studio della cinetica delle reazioni chimiche allo studio della tensione superficiale dinamica) e dei sistemi a molti componenti e a molte fasi richiedono anch’essi un ampio intervallo spettrale e un alto flusso di neutroni “freddi”.

Sviluppi scientifici attesi in alcune aree specifiche: Soft Condensed Matter Superfici, proprietà di interfacce e di bulk in sistemi complessi (polimeri, surfattanti, colloidi) - Studi di interfaccia: autoorganizzazione e ordinamento in miscele complesse di surfattanti, polimeri e proteine all’interfaccia, con particolare enfasi sui processi cinetici e sui sistemi a molti componenti ad interfacce tecnologicamente rilevanti (liquido-liquido e liquido-solido).

- Processi in solidi soffici: relazione tra struttura microscopica e proprietà di bulk (rheology) in fluidi rilevanti dal punto di vista industriale.

- Autoorganizzazione: struttura di mesofasi liotropiche, microemulsioni, con enfasi sulla dinamica dei cambiamenti di fase, l’associazione e la dissociazione e l’autoorganizzazione in fluidi supercritici.

- Hopping e dimensionalità del trasporto in polimeri conduttori con T1 muonici.

Sistemi biomolecolari Farmaci, interazione membrane-proteine, bio-compatibilità e funzionalità, tecnologia del cibo.

- Membrane: organizzazione strutturale delle membrane e sistemi membrane-proteine.

- Tecnologia del cibo: studio della distribuzione del solvente e dei cambiamenti strutturali in sistemi complessi durante il processo di formazione, assorbimento delle proteine e stabilità colloidale, meccanismo foam and gel formation. Studi di invecchiamento.

- Assemblaggio macromolecolare: studi a bassa risoluzione in sistemi macromolecolari, in sistemi non trattabili con cristallografia ad alta risoluzione, virus, glyco-proteine, folding di proteine, interazione proteine-nucleoacidi, struttura del solvente.

- Farmaci: determinazione della struttura di nuove droghe, dove il ruolo dell’idrogeno è essenziale

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nella comprensione dell’interazione droga-recettore, ingegneria molecolare.

- Dinamica molecolare in proteine attraverso i rilassamenti muonici longitudinali.

Materiali avanzati Sistemi organici ed inorganici complessi, clatrati, intercalati, zeoliti, materiali nanostrutturati, superconduttori ad alta temperatura, materiali con magnetoresistenza gigante, films magnetici e a molti strati.

- Dettagli strutturali in materiali GMR / CMR, ossidi non-stechiometrici, piezoelettrici, ferroelettrici e materiali ad espansione termica negativa.

- Materiali multi-cristallini compositi, quali quelli a fasi geologiche miste, manganiti magnetoresistive, superconduttori ad alta temperatura.

- Studi in condizioni di pressioni e temperature estreme, catalisi, reazioni chimiche in situ (quali per esempio elettrochimica e funzioni delle batterie).

- Strutture di nano-materiali, processi sol-gel.

- Struttura di materiali complessi: ruolo delle zeoliti nei materiali a scambio di ioni e nelle catalisi, elettroliti innovativi, formazione di clatrati, materiali nano-strutturati.

- Determinazione di nuove strutture magnetiche: materiali con stati magnetici non classici.

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Partecipazione del CNR al Progetto European Spallation Source (ESS) in ambito ESS R&D Council Premessa Negli ultimi anni lo sviluppo della ricerca, fondamentale ed applicata, nel campo della materia condensata ha conosciuto in tutto il mondo un impulso senza precedenti. Le ricadute tecnologiche e le innumerevoli applicazioni delle ricerche in questo vastissimo settore sono sotto gli occhi di tutti. Si è di fatto sviluppata una nuova disciplina che va sotto il nome di Scienza dei Materiali e oggi la ricerca di punta in questo settore è volta alla realizzazione di materiali ‘‘artificiali’’ cioè con composizioni e caratteristiche strutturali microscopiche controllate e ottimizzate ad hoc per ogni specifica applicazione.

I nuovi materiali così ottenuti hanno prestazioni enormemente superiori a quelle dei materiali tradizionali e questo nuovo tipo di approccio alla ‘scienza dei materiali’ trova oggi applicazione nei campi più svariati: dalla microelettronica e optoelettronica (strutture complesse di materiali semiconduttori per applicazioni nei microchip ad altissima integrazione, e ossidi di litio per batterie ricaricabili che hanno portato ad enormi progressi nel campo dei computer e dei nuovi sistemi miniaturizzati di telecomunicazione), alla metallurgia (leghe metalliche per applicazioni in condizioni estreme, materiali intelligenti a memoria di forma), alla cosidetta ‘soft matter’ (materiali polimerici, surfattanti, nuovi solventi atossici e chimicamente inerti), ai materiali per applicazioni biomediche (composti ceramici con elevata biocompatibilità per gli impianti artificiali, materiali organici per microchirurgia, veicolatori di farmaci).

Questo nuovo tipo di tecnologia richiede la conoscenza ed il controllo dettagliato delle proprietà dei materiali e della loro organizzazione su

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scala atomica e molecolare. Per ottenere queste informazioni è necessario impiegare tecniche di indagine altamente sofisticate, in grado di determinare con elevata precisione le caratteristiche strutturali e dinamiche microscopiche dei materiali; tra queste un ruolo preminente hanno la spettroscopia neutronica (N) e la radiazione di Sincrotrone (LS). Queste tecniche sono disponibili oggi in Europa presso grandi Centri Internazionali di ricerca: ESRF – European Synchrotron Radiation Source a Grenoble per quanto riguarda la LS, e l’ILL (Institut Laue Langevin a Grenoble) ed ISIS (Rutherford Appleton Laboratory presso Oxford), per quanto riguarda la spettroscopia neutronica.

Ovviamente le applicazioni delle tecniche di LS e di N, non sono limitate alla scienza dei materiali, ma hanno un’importanza riconosciuta strategica in molti paesi avanzati, in tutti i campi che riguardano lo studio della materia condensata.

I principali Enti di Ricerca Italiani nel campo della Fisica e Chimica dei Materiali, CNR – Consiglio Nazionale delle Ricerche e INFM – Istituto Nazionale per la Fisica della Materia, partecipano alla gestione di questi Centri, in particolare rispettivamente ISIS ed ILL per quanto riguarda la spettroscopia neutronica, garantendo così ai ricercatori italiani l’accesso ai Centri Internazionali di ricerca che costituiscono un ambiente adatto per la formazione qualificata di giovani ricercatori da inserire successivamente nelle strutture di ricerca e sviluppo operanti del nostro paese. In queste strutture l’assegnazione di tempo per esperimenti ai diversi gruppi di ricerca dei paesi partecipanti avviene su base competitiva, attraverso la presentazione di proposte di esperimenti, che vengono vagliate e selezionate da apposite Commissioni Internazionali di esperti.

L’impiego della diffusione dei neutroni per lo studio della struttura e della dinamica microscopica della

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materia è tutt’ora in forte sviluppo in tutto il mondo. E’ importante ricordare che l’Europa detiene una leadership in questo settore in quanto ha realizzato, la più intensa sorgente a spallazione di neutroni, denominata ISIS operante da circa quindici anni, con potenza di fascio di 0.36 MW, presso il Rutherford-Appleton Laboratory (Oxford, U.K.). ISIS è stata utilizzata ampiamente dai ricercatori italiani grazie ad un accordo che il CNR ha stipulato nel 1985, con il Council for the Central Laboratory of the Research Council (CCLRC) Inglese. Come parte integrante dell’ accordo con il CCLRC, il CNR ha finanziato in questi anni la costruzione di due nuovi spettrometri per neutroni (denominati PRISMA e TOSCA), realizzati interamente in Italia, presso i suoi Istituti, ed attualmente operanti presso la sorgente ISIS. La comunità italiana è impegnata nel settore della spettroscopia neutronica, sia presso reattori sia presso sorgenti a spallazione, nella conduzione di ricerche su tematiche specifiche e nella realizzazione di nuova strumentazione per neutroni.

Per quanto riguarda il futuro prossimo è in discussione in ambito internazionale la costruzione di nuove sorgenti di neutroni a spallazione.

Tenuto conto che nel prossimo decennio si prevede una riduzione della disponibilità complessiva di fasci di neutroni, per la programmata chiusura di diverse sorgenti attualmente operative basate su reattori a fissione, e tenuto conto delle buone prestazioni della sorgente ISIS, la comunità scientifica internazionale ha messo a punto, negli ultimi anni, diversi progetti per la costruzione di nuove sorgenti a spallazione, che sono state valutate in sede di organismi internazionali (Ref. vii). In particolare negli Stati Uniti il Congresso ha recentemente approvato (Giugno 2000) un finanziamento di 278 milioni di $ per la realizzazione di una nuova sorgente a spallazione, denominata SNS, basata su un accelerato-

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re di protoni da 1 GeV per una potenza totale di 2 MW. Contemporaneamente, vale la pena rilevare che il Giappone ha iniziato il finanziamento (per oltre 1 miliardo di $) di un progetto congiunto JAERI-KEK che include tra le altre cose, la realizzazione di una sorgente di neutroni a spallazione basata su un acceleratore di protoni da 3GeV. Gli studi di fattibilità per la realizzazione di sorgenti a spallazione di nuova generazione in Europa sono: Il progetto ESS (European Spallation Source). Prevede la realizzazione di un acceleratore da 1.3 GeV per una potenza totale di 5 MW, con un costo valutato tra 1000 MECU e 1500 MECU. Lo studio di fattibilità di questa sorgente è stato preparato da un consorzio di istituzioni Europee e di enti di ricerca, denominato ESS Council, e i risultati sono disponibili, dalla fine del 1996, in un documento in tre volumi disponibili al sito web http://www.isis.rl.ac.uk/ess/. Successivamente, nel 1997, è stato costituito l’ESS R&D Council, http:// www.fz-juelich.de/ess/, con l’obiettivo di predisporre un progetto tecnico definitivo, sia della sorgente sia della strumentazione. Fanno parte dell’ESS R&D Council quattordici istituzioni scientifiche e Laboratori appartenenti a diversi paesi europei, e per l’Italia ne fanno parte il CNR (delegato prof. M. Fontanesi) e l’INFM (delegato prof. F. Barocchi).

Recentemente il Council ha deciso la costituzione di un Project team che, sotto la direzione di un Project leader ed in stretta collaborazione con i laboratori associati al Council, avrà l’incarico di preparare entro l’anno 2003 il progetto realizzativo per la costruzione della sorgente ESS, per sottoporlo ai Governi della Unione Europea (Allegato I). Dal 1998, CNR è stato invitato ad aderire all’ ESS R&D Council ed a partecipare direttamente alle attività di sviluppo di strumentazione per la sorgente ESS. A seguito della proposta formulata

dal delegato dell’ente Prof. M. Fontanesi in ambito ESS R&D Council il Consiglio Direttivo dell’ente ha approvato a gennaio del 2000 le modalità di partecipazione CNR in questo organismo per il periodo 2000-2003.

Alle attività di ricerca e sviluppo di strumentazione in ambito ESS partecipano per il CNR i ricercatori dell’IEQ di Firenze. Un gruppo operativo dell’ IEQ è stato recentemente anche chiamato a collaborare allo sviluppo di strumentazione presso la sorgente neutronica SNS (Oak Ridge, USA) che verrà costruita nei prossimi anni negli Stati Uniti.

Il progetto Austron. Prevede la costruzione di una sorgente con una potenza del fascio di protoni pari a 0.5 MW. Per quanto riguarda la realizzazione di AUSTRON l’investimento previsto è pari a 337 MECU (distribuito sui 7 anni necessari per il completamento del progetto), e il costo operativo è pari a 36.6 MECU/anno a partire dall’entrata in funzione della sorgente, stimata intorno al 2007. Il governo austriaco si è dichiarato disponibile a finanziare 1/3 del costo complessivo del progetto, ma nei diversi incontri bilaterali a livello governativo che gli austriaci hanno avuto in Europa è emersa una chiara indisponibilità da parte di paesi, presso i quali sono già operanti sorgenti di neutroni, quali la Francia, l’Inghilterra, la Svizzera e la Germania, mentre un qualche interesse è stato espresso da parte dell’Italia in un incontro con la delegazione austriaca che si è tenuto al MURST il 22/3/99 (Ref. (x)).

Il progetto ISIS-II. Il progetto ISIS-II prevede la realizzazione, entro il 2004, di una seconda targhetta presso il Rutherford Appleton Laboratory (UK) dove è già in funzione la più intensa sorgente a spallazione di neutroni europea ISIS. Una parte del fascio di protoni di ISIS, con potenza di 60 KW, verrà inviato su una nuova targhetta che verrà ottimizzata

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per strumentazione che richiede neutroni con lunghezze d’onda maggiori di quelle attualmente disponibili ad ISIS. Il costo previsto è valutato intorno ai 150 MECU. Il CLRC prevede di finanziare con propri fondi la realizzazione della targhetta e degli edifici, mentre è intenzionata a proporre entro la fine dell’anno agli attuali enti europei partner di ISIS, e tra questi il CNR, la realizzazione e gestione delle linee sperimentali, attraverso accordi di programma.

Attività della comunità italiana Per quanto riguarda l’inserimento dei ricercatori italiani nei progetto di cui al paragrafo precedente, va rilevato che negli Enti CNR ed INFM operano oggi ricercatori con le competenze adeguate per partecipare attivamente ed in modo propositivo sia alle attività di ricerca sia allo sviluppo di strumentazione presso una sorgente a spallazione di neutroni.

Infatti i ricercatori attivi presso questi enti, svolgono da anni in stretta collaborazione, una vasta e documentata attività di ricerca presso la sorgente di neutroni a spallazione ISIS. Il CNR: • garantisce l’accesso a tutta la comunità italiana, con una percentuale di utilizzo della stumentazione ad ISIS pari al 5% del tempo totale disponibile (vedi Tabella 1 del documento allegato ‘Consuntivo dell’attività di ricerca svolta nel periodo 1985-2000 nell’ambito dell’Accordo Internazionale CNRCCLRC, per l’utilizzo della sorgente di neutroni a spallazione ISIS presso il Rutherford Appleton Laboratory (UK)’); • finanzia direttamente lo sviluppo di strumentazione, progettata e costruita presso i proprio organi di ricerca. In particolare si ricordano il Progetto PRISMA realizzato presso l’ISM – Istituto di Struttura della Materia (Frascati) nel periodo 1986/1991 ed il Progetto TOSCA, in fase di realizzazione presso l’IEQ

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– Istituto di Elettronica Quantistica di Firenze, avviato nel 1996 e la cui conclusione è prevista nel 2001.

Investimenti CNR Per quanto riguarda gli investimenti del CNR per attività di ricerca e sviluppo di interesse per i progetti di nuove sorgenti a spallazione, si ricorda che nel periodo 1985-1995 l’impegno complessivo del CNR è stato, in media, di 4 uomini anno/anno con un investimento per la strumentazione pari a 1.5 MECU (Progetto PRISMA e sviluppo di cristalli monocromatori). In aggiunta nello stesso periodo l’accesso alla sorgente ISIS ha comportato un investimento oneroso pari a 10 MECU. Nel periodo 1996-2001 l’impegno complessivo del CNR sarà in media di 4.5 uomini anno/anno, comporterà un investimento per la strumentazione pari a 1.5 MECU (Progetto TOSCA e sviluppo di cristalli monocromatori) e un investimento oneroso per l’accesso alla sorgente ISIS pari a 6 MECU. Il CNR provvede anche alla formazione di personale per mezzo di borse di ricerca, dottorati di ricerca, contratti a tempo determinato e scuole.

Investimento italiano Al momento attuale il contributo italiano nel suo complesso è il risultato di attività di ricerca svolte da gruppi CNR in collaborazione con gruppi, INFM e non, localizzati presso le Università. I fondi destinati a tali attività in ambito ESS R&D, elencati in Tabella 1, derivano direttamente dal CNR (per quanto riguarda la quota di accesso ad ISIS e per la strumentazione TOSCA) e dalla Comunità Europea (VESUVIO e Sviluppo di rivelatori). Le attività di ricerca italiana CNR ed INFM nel loro complesso possono essere così riassunte: Tabella 1 Ente Attività di ricerca CNR: Progetto TOSCA, strumentazione per studi di spettrosco-

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pia molecolare.

INFM: Sviluppo di rivelatori a stato solido.

INFM: VESUVIO, strumentazione per spettroscopia di neutroni agli eV. CNR/INFM: Sviluppo di cristalli monocromatori a grande area.

L’attività italiana è valutata complessivamente in 11.5 uomo anno/per anno corrispondente ad un investimento di 2.6 MECU. I progetti italiani elencati in Tabella 1 sono descritti in dettaglio al sito web:http://www.fz-juelich.de/ess Conclusioni Il CNR è l’unico Ente che ha permesso alla comunità italiana l’utilizzo della sorgente di neutroni a spallazione europea ISIS finanziando sia l’accesso alla facility, fin dalla sua entrata in funzione nel 1985, sia la costruzione di nuova strumentazione.

L’Ente dispone di gruppi qualificati di ricercatori esperti che potrebbero inserirsi efficacemente nelle diverse fasi realizzative del progetto ESS previste nel prossimo triennio e descritte in Allegato I della relazione ‘Consuntivo dell’attività di ricerca svolta nel periodo 1985-2000 nell’ambito dell’Accordo Internazionale CNR-CCLRC, per l’utilizzo della sorgente di neutroni a spallazione ISIS presso il Rutherford Appleton Laboratory (UK). Questi ricercatori hanno anche in questi anni operato validamente per la formazione di nuovo personale per attività di ricerca e di sviluppo di strumentazione presso ISIS. La partecipazione alla fase realizzativa del progetto ESS permetterà ai nostri ricercatori di operare fin dalla fase di avvio a stretto contatto con gli altri colleghi europei assicurando anche il necessario know-how nel campo della spettroscopia neutronica con sorgenti a spallazione, settore che presenta interessanti prospettive di sviluppo nel prossimo decennio. A questo proposito, a conclusione del Meeting

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of the OECD Committee for Scientific and Technological Policy, che ha avuto luogo a Parigi il 22-23 Giugno 1999 e in un recente documento stilato in ambito OECD Global Science ForumWorkshop on Strategic Policy Issues (High Intensity Proton Beam Facilities- Paris 25 Settembre 2000), il progetto per una sorgente di neutroni a spallazione del tipo ESS viene inserito tra quelle iniziative che, nei prossimi anni, dovrebbero essere prese in considerazione, per un eventuale investimento dai paesi dell’OECD, tenuto anche conto dell’interesse strategico sia su scala regionale, sia su scala globale.

Prof. Marcello Fontanesi Delegato del CNR nell’ ESS R&D Council Referenze per la stesura della relazione 1. Autrans Report: ‘Impact and Prospects of Neutron Methods and Neutron Sources for European Science and Research’, 1996 2. ESS – Volume II, 1997 3. ESF Assessment Report of the AUSTRON Project and the Revision of the AUSTRON Case, 1997 4. ESF-ENSA technical report: ‘Survey of Existing Neutron Facilities and User Communities, 1998 5. ESF-OECD Technical Report: European and Global Forward Look on Future Neutron Facilities for Science and Research, 1998 6. ESF Review of European Needs for Synchrotron Facilities for Biomedical Research, 1998 7. OECD Megascience Forum Working Group on Neutron Facilities, 1998 8. Report on US SNS project, 1998 9. OECD Global Science ForumWorkshop on Strategic Policy Issues (High Intensity Proton Beam Facilities- Paris 25 Settembre 2000) Referenze addizionali: 10.Access to Large-scale facilities: mid-term review of TMR-LSF activities in 1998

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Survey o the users of large-scale facilities by the European Commission 11.Relazione Progetto Austron per il Presidente CNR Prof. L. Bianco, relativa alla riunione tenutasi al MURST il 22/2/99 con la delegazione austriaca.

Allegato 1. ESS/P3/00 Resolution of the 8th ESS R&D COUNCIL (November 27, 2000, Florence) 1. The ESS R&D COUNCIL, taking note of the progress reported by the Executive Committee on ESS and CONCERT, the Project Director and the Task Group Leaders, and ENSA, adopts the following resolution with respect to the strategy, time frame and the various options it wants to have investigated before coming to a final proposal. In a separate decision it draws the consequences for the organisation for the European Spallation Source in the period up till a decision on ESS is being taken.

2. The ESS R&D COUNCIL aims for a decision in 2003 to build ESS according to a time schedule that results in ESS becoming operational in 2010.

To facilitate discussions and decisionmaking it will ensure that on top of the detailed ESS proposal of 1997 (covering both the science case and the ESS design) by the end of 2001 a mid-term review can be held on the basis of a first revision that will include tentative decisions on the long pulse target station and the accelerator options, before the final proposal will be introduced early 2003. While the ambitions remain that ESS should be a unique third-generation neutron source, the ESS R&D COUNCIL expresses the view that realistic risk levels and a cost range between 1 and 1.5 GEURO should determine the final assessment.

3. The ESS R&D COUNCIL endorses and continues the efforts in CONCERT, established by ESS and CEA, to investigate the feasibility of a siteindependent proton-accelerator based facility with realistic, i.e. afforda-

ble power levels, primarily for condensed matter and R&D on transmutation, with additional use in nuclear physics and astrophysics through rare isotope beams. The ESS R&D COUNCIL will consider final proposals in the light of their realisability and affordability. This option will be compared to a stand-alone option for ESS to decide which will be the option the ESS R&D COUNCIL finally prefers.

More specifically, the ESS R&D COUNCIL will compare the superconducting multipurpose option (also in a variant with an additional long-pulse target station, possibly combined with one for research on transmutation and irradiation) and a superconducting stand-alone version that is derived from it, to the modified 97 normal conducting reference design (for which a superconducting variant also exists). Any other proposal for options to be included in the comparison should pass the scrutiny of the ESS accelerator task leader, in conjunction with the CONCERT project director. The comparison should be based on designs that include the infrastructure as well as safety and environmental aspects. For each design full beam dynamics should have been simulated.

The R&D COUNCIL requests the Executive Committee to ensure that progressively insight in the feasibility and the comparison will be obtained so that a decision can be taken at the earliest possible date. The options considered contain both a short pulse version and one with short and long pulses for condensed matter research. Also in this respect the R&D COUNCIL requests the Executive Committee to propose a decision on the final option to be worked out on the basis of a clear set of criteria and at the earliest possible date.

4. The workplan for the ESS will contain three categories of activities.

Core activities: those that are necessary for completing a final proposal in 07/03.

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Necessary activities: those that deemed necessary to ensure that the ESS that will be completed in 2010 will be built and operated at a reasonable risk and the best performance levels possible and with an optimised costeffectiveness. These activities therefore must be completed not later than approximately 2006.

Relevant activities: those that should be useful for other projects as well, but which might result in later modifications of ESS, i.e. to be introduced after its first commissioning.

The (deputy) Task Leaders, and as far as the accelerator work package is concerned in conjunction with the CONCERT Project Director, will see to it that in the period December 2000-February 2001 the current work packages and contributions are reconfigured in conformity with the above categories. They will work together with the Project Director, who will have final authority. In case of remaining disagreement the Executive Committee should consult with individual laboratories.

5. The organisation to be responsible for proposing ESS will have to meet the characteristics of, or be an independent legal entity. The ESS R&D COUNCIL requests the Executive Committee to investigate first whether ESF could be of assistance in this regard, and second to look into the legal situation in its member organisationsâ&#x20AC;&#x2122; countries in order to minimise cumbersome procedures and avoid delays. The construction chosen in no way prejudices the organisation and its legal basis for the ESS facility on its implementation or operational phase.

6. The ESS R&D COUNCIL decides to ask the ESF to take responsibility for organising in the Autumn of 2001 a review of the science case for ESS on the basis of the outcome of the ESS-SAC/ENSA workshop and the ensuing decisions by the COUNCIL. It notes with satisfaction the willingness of ESF to assist in alerting its Member Organisations and other re-

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levant Parties in Europe to the science case of ESS. It also invites ESF to continue giving consideration to the role it could play in establishing clear decision mechanisms for ESS and large research facilities in general in Europe.

7. The ESS R&D COUNCIL stresses the importance of as transparent a procedure as possible for arriving at a choice of the site of the ESS. It will therefore establish a set of criteria, including financial indications. It requests its Chairman to start consultations with relevant Parties to contribute to such a transparent procedure on the basis of these criteria and to invite them to come forward with options for sites.

Resolution on the organisation and decisions of the ESS project 1. General nature of the organisation.

The organisation to be responsible for proposing ESS will have to meet the characteristics of, or be an independent legal entity. The ESS R&D COUNCIL requests the Executive Committee to investigate first whether ESF could be of assistance in this regard, and second to look into the legal situation in its member organisations’ countries in order to minimise cumbersome procedures and avoid delays. The entity will be established and governed by a consortium of laboratories or organisations who will be either Partners or Associate Members. Partnership to the consortium is open to every laboratory or organisation that is contributing a significant (the level is to be determined later) amount to the Core and/or Necessary activities. Associate Membership is open to all laboratories or organisations that support the strategy and objectives of ESS and contribute to the activities.

The structure though not its more formal mode of operation will be modeled rather like the present organisation. While governed by the main stakeholders, i.e. laboratories and organisations, its executive

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functions will be concentrated largely with an Executive Director who is in charge of the design team. The importance of having close relations to the scientific community will be underlined by maintaining the Scientific Advisory Committee and the role of its Chairperson.

The construction described above in no way prejudices the organisation and its legal basis for the ESS facility on its implementation or operational phase.

2. Means and voting rights.

The organisation will have its own means to which Participants and Associate Members will contribute according to a scale which reflects their interest, the size of the national community of neutron users and GNP, the latter two on a national and not individual laboratory/organisation basis. The commitments will be in the form of guaranteed contributions of the Participants to the Central Design Team that is the operating arm of ESS. Members of the ESS R&D COUNCIL will provide the Executive Committee and the Central Team with the means to implement this resolution and prepare the new organisation efficiently.

While the intention is to decide by unanimity qualified votes based on the scale of contributions will be introduced 3. Procedure to establish the organisation.

On the basis of the categorisation of the work in Core and Necessary activities the present signatories to the MoU for the ESS R&D Phase and any other Parties that to the satisfaction of a majority of the present signatories comply with the above conditions for Partnership or Associate Membership, will decide, after appropriate national consultations, if and in what capacity they will constitute the ESS organisation. The organisation will be established on the basis of a MoU and statutes that

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should take effect as of the COUNCIL meeting of June 2001. The ESS organisation then can take responsibility for all the steps that subsequently are necessary in order to arrive at a final design in 2003. These steps are summarized below.

4. Milestone decisions to be taken by the ESS COUNCIL. a) Fixing the neutron performance parameters for the ESS on the basis of the science-instrument confrontation of May 2001 which includes an assessment of the target requirements and performance. b) Budget for the Core and Necessary activities, and the costs of the organisation itself. c) Preliminary review of the feasibility of the multipurpose option versus a stand-alone version. d) Decision on which accelerator option to choose. e) Decision on the target stations to include in the final ESS design. f) Decision on the final scope and cost level for ESS as a basis for the final detailed design and costing work. g) Monitoring of the evolution of discussions and consultations to arrive at a decision on the site. Passing of any recommendations that may deemed necessary.

Dr Uschi Steigenberger Secretary to the ESS Council ISIS Facility, Rutherford Appleton Laboratory Chilton, Didcot, Oxon OX11 0QX,UK e-mail: U.Steigenberger@rl.ac.uk Prof Kurt Clausen Project Director ESS – Central Project Team c/o Forschungszentrum Jülich D – 52425 Jülich, Germany e-mail: k.clausen@fz-juelich.de

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NEUTRONS FOR THE 21st CENTURY The European Spallation Source - Newsletter May 2001

Introduction.

The purpose of this newsletter is to inform everyone who is interested in ESS (or we feel should be interested), about the important events in the project.

ESS has been working steadily since the 1996/1997 reference design and science case, published as ESS Vols.

I-III. The time has now come to show the scientists, the engineers, the instrument builders, research councils, governments and other funding agencies what we are doing and how we are progressing towards our goals: a final design for ESS by mid-2003, a decision late 2003/early 2004, and operations starting in 2010.

The year 2000 was used to tidy up the structure of the ESS project: the project director Kurt Clausen is now based at the European headquarters in Jülich, and together with the task leaders and their deputies he leads the work on the accelerator, target station(s) and instruments that is carried out in various laboratories in Europe. Dieter Richter as the chair of the Science Advisory Committee has already put in motion a major effort to base the final neutron parameters for ESS on the prospects for exciting new vistas in science. Working with Professor Clausen and Professor Richter on the Council’s Executive Committee are Peter Tindemans as chair, and Uschi Steigenberger as secretary of the Council.

The scientific case for the ESS is very strong, not least because the industrial relevance of research with neutrons is increasing all the time. The case should rest on the conviction of European scientists, not only neutron users, that we need the ESS: •because neutrons provide unique insights into scientific problems;

•because Europe has built up the strongest neutron research community in the world; •because we must ensure the supply of neutrons for future generations of researchers.

It is therefore very encouraging to note that ENSA has clearly stated the need for the ESS. It is also gratifying to see that various national meetings are taking place to discuss the need for ESS and its possibilities. The ESS Executive and the ESS Council members will try to support you – the Community - in every possible way.

Of course, there is much more information on our website, but we hope that this regular newsletter still serves a useful purpose.

The Florence Council Resolution The resolution that the ESS R&D Council agreed upon unanimously in Florence in late November 2000 marked a turning point in the ESS project. Efforts will now be devoted to completing a final design by the middle of 2003, and getting a decision by the end of 2003 or early 2004.

That would make it possible to have ESS operational in 2010.

The reference design of 1996/1997, the additional R&D that has been carried out since then and the efforts in the CONCERT collaboration form a solid basis for arriving at a final design.

But some major decisions with respect to the scope of the facility still need to be made. The Council expressed the wish that these issued should be resolved in the course of 2001.

First of all there is the question of which target station or stations will be used. Next to the 50Hz and 10Hz short pulse stations the Council wants to look into a 16Hz long pulse station. That decision is up for di-

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scussion at the next Council in Abingdon in June 2001. Then there is the question of the accelerator, where the choice is between superconducting or normal conducting. Several options were singled out by the Council, a number of which are investigated as part of CONCERT. This is the third major decision which the Council wanted to make as soon as possible: will ESS be proposed as a stand alone facility, or as part of a multipurpose facility where the proton accelerator would serve research with neutrons, neutrinos, muons, research on transmutation, irradiation, etc. The work in CONCERT is well under way to provide the clues to that decision.

The Council felt that ESS should not cost more than 1.5B¤.

The consequence of all this is that the work packages in the three areas, i.e. the accelerator, the target(s) and the instruments, have to be redesigned to reflect the focus on producing a final design. The activities that are vital for producing this design are called core activities. Other R & D work, sufficiently promising for enhancing performance or reducing costs, which would lead to results say in 2005 or 2006 and therefore could be built into the ESS during construction, are dubbed necessary.

Free-floating R&D will no longer be part of ESS. The ESF has in the meantime agreed to another request from the Council, namely to organise a final review of the science case. In consultation with ESS this is now scheduled for spring or summer 2002.

Finally, the June Council will decide on changes in the structure, funding and decision-making in ESS that are necessary to meet these ambitious targets.

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The European Spallation Source (ESS) Scientific Advisory Committee (SAC) The ESS will be the front rank European neutron source for the first half of the 21 st century. As the central facility for neutron research in Europe it will have to serve the needs of users in all fields of science relevant for neutrons in the most effective way possible. In order to achieve this goal, at its Berlin meeting in May 2000 the ESS Council created a Scientific Advisory Committee (SAC-ESS). In close consultation with the European Neutron Scattering Association (ENSA) leading scientists from different European countries were appointed to serve in the SAC. The first meeting, which took place in Jülich in September 2000, focussed on the first milestone of the Memorandum of Extension concluded in Berlin, which asks for the complete specification of the neutron parameters of ESS to be fixed by July 2001.

This final definition of the ESS layout needs to be based on an analysis of future trends in science and a projection on instrumentation opportunities. In particular, the different target options for ESS need scrutiny. At present three options are being discussed: (i) a short pulse (1µs) 50Hz target with 5MW beam power, (ii) a 1MW short pulse 10Hz target and (iii) a 5MW long pulse (~2.5ms) 16 2/3 Hz target.

In order to investigate the scientific opportunities derived from these options, the SAC has initiated eight science working groups with the mission: (i) to explore the likely lines of development in neutron related scientific fields, (ii) on the basis of these projections into the future, to define the science demands on ESS; and (iii) to derive the implications for the ESS layout. This relates to the accelerator (pulse structure, power, etc.), the target stations and the moderators which pro-

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vide different opportunities for instrumentation.

At the same time, the ESS instrument task group has set up ten expert instrumentation groups, who are exploring the performance of key instruments at different target moderator combinations. They base their work on predictions of moderator performance delivered by the target group, and consider generic scientific problems specific to key instrument classes.

At the second SAC meeting in San Sebastian (March 2001) a first exchange of information between the science and instrumentation group conveners took place. These activities will culminate in an ESS/ENSA workshop at the beginning of May. This workshop on ‘Scientific Trends in Condensed Matter Research and Instrumentation Opportunities at the ESS’ is sponsored by the European Neutron Round Table, the European Science Foundation, Members of the ESS Council, the Forschungszentrum Jülich, the PSI Villigen and the Swiss National Fund.

The workshop’s goal is to set priorities for ESS design options based on scientific requirements, and to facilitate a decision on the ESS design parameters by time the next ESS Council meeting takes place in June 2001.

Status of the ESS Technical Study At the June 2001 ESS council meeting a decision on the key ‘neutron parameters’ will be taken. The council will select two target stations, and decide on their power levels and frequencies, and also on whether the facility will be based on a stand-alone or a multi-purpose accelerator.

The target stations under consideration are 1) a 50 Hz short pulse 5 MW target station, 2) a 10 Hz short pulse 1 MW target station, and 3) a 16 Hz 4-5 MW long pulse (2 - 2.5 ms) target station.

These three target station options ha-

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ve been selected because they present the expected technical limits within the current design.

The accelerator groups have followed two paths: a) an update of the normal conducting ESS reference design from 1996/1997 (headed by the accelerator group at ISIS) and b) a scaleable superconducting accelerator concept, which can be used for either a stand- alone ESS or for a multi-purpose facility (headed by the CEA CONCERT team).

The findings are that from a technical point of view a multi-purpose accelerator is feasible, and both normal and superconducting designs of the high energy linac will be able to deliver intermittent long and short pulses. The results also show that the construction and running costs for a facility of this power level only increase rather slowly in relation to the source power.

The target and instrumentation groups have concentrated on providing the Science Advisory Group with performance estimates for each of the three target stations. Using Monte Carlo codes and the target station geometry from the ESS reference design of 1996/97, the neutron pulse (peak intensity, pulse width and pulse shape) have been estimated for coupled, de-coupled and de-coupled/poisoned, cold (H2 – 20K) and thermal (H 2O – 300K) moderators on each of the three target stations.

Another major task for the technical groups has been to look into the costing of the facility. This has been approached in two ways. The first is to update the figures from the ESS feasibility study of 1996/97, and the second is to compare them with the cost of the SNS project. This is a rather complicated process, but a first rough indication of the cost of the proposed options will be ready for the June council meeting.

During the last year an impressive amount of work has been done by

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the best European experts within all relevant areas, and we are in the happy situation of being able to let the science case determine the design. Technically, all options are open and none of the proposed possibilities seems to be excluded for economical reasons.

Instrumentation Task Group Report The instrumentation Task Group has focussed its attention on assessing the performance of a generic instrument suite in order to inform the decision about the optimum target station configuration for the ESS. Teams were formed to address the specific instrument areas, according to the table below, involving instrumentation experts throughout Europe. The teams met for the first time in October at HMI to discuss their terms of reference and mode of operation.

Intermediate reports were presented at a meeting in February, which also gave participants the opportunity to address unresolved issues before compiling final reports prior to the SAC workshop in Engelberg. All the instrument reports can be found on the instrumentation task group’s website at http://www.Hmi .de/ bereiche/SF/ess/ess_02_en.html.

The teams have worked hard to produce a comprehensive appraisal of the possible performance of instruments on the ESS. Attention will now focus on working closely with the Target Station Task Group to optimise the moderator configuration and on developing the instrument designs, implementing the recommendations of the SAC workshop.

Powder Diffraction Paolo Radaelli,* ISIS Steve Hull, ISIS Hans-Jürgen Bleif, HMI Emmanuelle Suard, ILL Juan Rodriguez Carvajal, LLB

Reflectometry Helmut Fritzsche,* HMI John Webster, ISIS Claude Fermon, LLB

Presenting Peter Tindemans Q. You took over the chair of the ESS R&D Council at the Berlin Council in May 2000. Where did you come from?

I was trained as a theoretical physicist at Leyden University, but apart from science per se the relationship of science to society has always interested me. Science policy was one way of working on those issues; so in 1975 I approached the new Dutch ministry for science policy.

And that is what I have been doing since: science, technology, innovation policy, in the Netherlands but in a large number of international responsibilities as well. I have seen the development of virtually all of the Framework Programmes.

I was involved in EUREKA from the beginning in 1985. I was chairman of the COSINE Policy Group, which as some readers will remember was a project of 19 governments and the Commission to establish computer connectivity Europe-wide and to the outside of Europe, based on the national research networks such as JANET, DFN, Surfnet etc.

As of 1991 I was responsible for all of the research and science policy in the Netherlands, which meant dealing with natural sciences, but also humanities and social sciences, and research at universities, the research council NOW, and applied research organisations such as TNO or the

Direct Geometry Spectrometers Roger Eccleston,* ISIS Rob Bewley, ISIS Feri Mezei, HMI Hannu Mutka, ILL Ruep Lechner, HMI Single Crystal Diffraction and Protein Crystallography Chick Wilson,* ISIS Wolfgang Jauch, HMI Gary McIntyre, ILL Dean Myles, EMBL Indirect Geometry Spectrometers Ken Andersen,* ISIS Björn Fak, ISIS Peter Allenspach, PSI Marco Zoppi, CNR Oliver Kirstein, FZJ S(Q) determination Alan Soper,* ISIS Robert McGreevy, Studsvik Neutron Spin Echo Michael Monkenbusch,* FZJ Katia Pappas, HMI Bela Farago, ILL Engineering Philip Withers,* Manchester Mark Daymond, ISIS Walter Reimers, HMI Torben Lorentzen, Risø Small Angle Scattering Richard Heenan,* ISIS Albrecht Wiedenmann, HMI Bob Cubitt, ILL Kjell Mortensen, Risø Dietmar Schwahn, FZJ Monte Carlo Geza Zsigmond, HMI Klaus Lieutenant, HMI Kim Lefmann, Risø

Aerospace Laboratory. And while I strongly believe in better European cooperation, the world is of course

* Team leaders

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much larger. So I did quite a bit to establish connections to a number of Asian countries, and then at the beginning of the OECD Megascience Forum in 1992 I became its chairman, a post I held until it changed its name in 1999 to become the Global Science Forum. But by then I had left the Dutch government, and now I am working independently.

Q. So what are you doing now? Just ESS? No, my idea was that there are three keys to present and future developments: knowledge, looking for partners, and positioning organisations strategically on a global basis; and that I had substantially linked these things together in what people call strategic alliances.

So I decided to try to make a living out of that, and I started my company Global Knowledge Strategies & Partnerships. Just to give you some idea, apart from my ESS work I have been rather busy in getting the first transnational university in Europe started, between the University of Maastricht and the Limburgs Uni-

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ver-sitair Centrum in Hasselt/Diepenbeek, Flanders.

And I am now working with national authorities and the support of UNESCO to renew and revitalise science and technology policy in the Lebanon.

Q. So why ESS, why neutrons?

Sure, it could have been any other large international project. I believe that neutrons remain vital for science and that their application is unique, and that Europe must improve the way it is organising and funding its science base. With Pierre Papon, a former Director-General of CNRS, I coordinated a project, EUROPOLIS, that has resulted in some far-reaching recommendations for change in this area.

A devolution of the science infrastructure from national control is an important element of this. Better decision procedures for large scale facilities are badly needed, and I believe that trying to get projects like ESS built will not only benefit science, but also decision-making in Europe.

And then, I must confess that when I

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was chairing the OECD Megascience Forum, I always had a weak spot for the neutron working party.

This was one of the best examples of how such an intergovernmental forum, working in close association with the scientific community, could produce practical results for the long-term future of science, and therefore also of society.

Editors: ESS Executive For further information please contact: Dr Uschi Steigenberger Secretary to the ESS Council ISIS Facility Rutherford Appleton Laboratory Chilton, Didcot, Oxon OX11 OQX e-mail: U.Steigenberger@rl.ac.uk Prof. Kurt Clausen Project Director ESS Central Project Team C/o Foschungszentrum Jülich D-52425 Jülich, Germany http://www.ess-europe.de/

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SCUOLE E CONVEGNI

CONGRESSO ANNUALE DELL’I.N.F.M. Quest'anno, uno dei simposi parallell che si terranno durante il Congresso Annuale dell'INFM (INFMeeting, Roma, 18-22 giugno 2001) è di particolare interesse per i lettori del Notiziario Neutroni-LdS. Infatti, il giorno 21 giugno, dalle ore 16.00 alle ore 18.00, è stato organizzato in collaborazione tra la sezione B (biofisica) e la sezione C (materia disordinata) un simposio dal titolo "Neutron and x-ray scattering in biomolecules and glasses". I coordinatori sono Paolo Mariani (Unità INFM di Anco-

na, sezione B) e Giancarlo Ruocco (Unità INFM dell'Aquila, sezione C).

Nella relazione su invito e nei 4 interventi previsti durante il simposio verranno illustrati alcuni risultati particolarmente interessanti ottenuti applicando tecniche di scattering dei raggi X e dei neutroni allo studio di sistemi biologici e di sistemi parzialmente disordinati. E' interessante osservare che verranno descritte tecniche molto diverse, alcune particolarmente efficaci e di recente introduzione, che vanno dallo scattering

inelastico dei raggi X e dei neutroni a misure di assorbimento dei raggi X in riflessione totale (REFLEXAS) e a tecniche di scattering neutronico in variazione del contrasto. Verranno inoltre discussi alcuni aspetti particolarmente importanti per chi si occupa di sistemi biologici in soluzione: lo studio dei moti dinamici collettivi e la determinazione dei potenziali di interazione in soluzione. La scaletta degli interventi del simposio è la seguente:

21 giugno 16.00 - 16.40 Relazione su invito: Sow-Hsin Chen (Department of Nuclear Engineering, MIT, Cambridge, USA) "Colletive dynamics in lipid bilayers and globular proteins studied by Inelastic x-ray scattering" (40 min)

16.40 – 18.00 Interventi selezionati: - F. Bruni (sez C, Roma Tre) "Structural characterization of NaOH aqueous solution in the glass and the liquid state" (20 minuti) - F. Carsughi (sez B, Ancona) "Interaction of proteins in solution from small angle scattering: a perturbative approximation to the correlation functions" (20 minuti) - F. d'Acapito (ESRF) "Myelin basic protein-zinc-lipid interaction: a total reflection X-ray absorption study" (20 min) - L. Cristofolini (sez.C, Parma) "Light induced effects in neutron scattering spectra of photosensitive side chain polymers" (20 minuti)

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CALENDARIO

9-13 settembre 2001

MUNCHEN, GERMANY

27 agosto - 6 settembre 2001

International Conference on Neutron Scattering 2001 (ICNS 2001) Physik Dept. E13, Technische Univ. München , D-85747 Garching, Germany Tel: +49 89 28912452; Fax: +49 89 289 12473 e-mail: info@icns2001.de - http://www.icns2001.de 18-22 giugno 2001

7th Oxford School on Neutron Scattering Oxford, UK http://www.isis.rl.ac.uk/conferences/osns2001/ 2-6 settembre 2001 Polymers in the third millenium

GAITHERSBURG, USA

5-7 settembre 2001

Summer School on Methods and Applications of Neutron Spectroscopy NIST, Gaithersburg, USA http://www.ncnr.nist.gov/staff/john/ss01.html 10-11 luglio 2001

MELBOURNE, AUSTRALIA

5-14 settembre 2001

PRAGUE, CZECH REPUBLIC

9-13 settembre 2001

American Crystallographic Association Annual Meeting Los Angeles, USA http://nexus.hwi.buffalo.edu/ACA/ACA-Annual/ LosAngeles/LosAngeles.html

9-14 settembre 2001

USA

26-29 settembre 2001

SAN DIEGO, USA

MICHIGAN, USA

SCES 2001 Strongly Correlated Electron Systems Ann Arbor, Michigan, USA http://research.physics.lsa.umich.edu/sces2001/

STYRIA, AUSTRIA

7th ESS General Meeting Schloss Seggau, Styria, Austria http://ess.tu-graz.ac.at/ 2-6 dicembre 2001

SPIE Neutron and Hard X-ray Optics and Applications San Diego, USA http://spie.org/conferences/calls/01/am/ 6-10 agosto 2001

OXFORD, UK

Gordon Research Conference on Superconductivity Oxford, UK http://www.grc.uri.edu/programs/2001/supercon.htm

Real-Space Pair Distribution Function Methods Workshop ACA Annual Meeting, USA 1-3 agosto 2001

MÜNCHEN, GERMANY

International Conference on Neutron Scattering ICNS 2001 München, Germany http://www.icns2001.de/

LOS ANGELES, USA

21 luglio 2001

HAMBURG, GERMANY

EMBO Practical Course on Solution Scattering from Biological Macromolecules EMBL, Hamburg Outstation, Germany http://www.embl-hamburg.de/ExternalInfo/ workshops/2001/EMBO/index.html

Scattering methods for the investigation of polymers Prague, Czech Republic http://www.imc.cas.cz/sympo/20discon/index.htm 21-26 luglio 2001

BERLIN, GERMANY

ESF Exploratory Workshop on Time Resolved Investigations by Neutrons and X-rays of Structural Changes in Soft and Solid Matter HMI, BESSY II, Berlin, Germany

Neutrons for Biology Symposium and Workshop University of Melbourne, Australia http://www.ansto.gov.au/ansto/neut/ 001symposium01.html 9-12 luglio 2001

OXFORD, UK

TRENTO, ITALY

Analysis of microstructure and residual stress by diffraction Methods Trento, Italy - http://bragg.ing.unitn.it/sizestrain/ maggio 2002

NIST, USA

American Conference on Neutron Scattering NIST, USA http://cmp-o1.ameslab.gov/nssa/ ACNS_announmt.htm

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SCADENZE

Scadenze per richieste di tempo macchina presso alcuni laboratori di Neutroni

Scadenze per richieste di tempo macchina presso alcuni laboratori di Luce di Sincrotrone

ISIS

ALS

La scadenza per il prossimo call for proposals è il 16 ottobre 2001 e il 16 aprile 2002

Le prossime scadenze sono il 15 marzo 2002 (cristallografia macromolecolare) e il 1 giugno 2002 (fisica)

ILL

BESSY

La scadenza per il prossimo call for proposals è il 15 agosto 2001 e il 15 febbraio 2002

Le prossime scadenze sono il 4 agosto 2001 e il 15 febbraio 2002

LLB-ORPHEE-SACLAY

DARESBURY

La scadenza per il prossimo call for proposals è il 1 ottobre 2001 per informazioni: Secrétariat Scientifique du Laboratoire Léon Brillouin, TMR programme, Attn. Mme C. Abraham, Laboratoire Léon Brillouin, CEA/SACLAY, F-91191 Gif-sur-Yvette, France.

Tel: 33(0)169086038; Fax: 33(0)169088261 e-mail: abraham@bali.saclay.cea.fr http://www-llb.cea.fr

La prossima scadenza è il 31 ottobre 2001 e il 30 aprile 20002

ELETTRA Le prossime scadenze sono il 31 agosto 2001 e il 28 febbraio 2002

ESRF Le prossime scadenze sono il 1 settembre 2001 e il 1 marzo 2002

BENSC La scadenza è il 15 settembre 2001 e il 15 marzo 2002

GILDA (quota italiana) Le prossime scadenze sono il 1 maggio 2000 e il 1 novembre 2000

RISØ E NFL La scadenza per il prossimo call for proposals è il 1 aprile 2002

HASYLAB SINQ La scadenza è il 15 novembre 2001 e il 15 maggio 2002

(nuovi progetti) Le prossime scadenze sono il 1 settembre 2001, il 1 dicembre 2001 e il 1 marzo 2002

LURE La prossima scadenza è il 30 ottobre 2000

MAX-LAB La scadenza è approssimativamente febbraio 20002

NSLS Le prossime scadenze sono il 30 settembre 2001, il 31 gennaio 2002, e il 31 maggio 2002

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FACILITIES

LUCE DI SINCROTRONE SYNCHROTRON SOURCES WWW SERVERS IN THE WORLD (http://www.esrf.fr/navigate/synchrotrons.html)

DAFNE INFN Laboratori Nazionali di Frascati, P.O. Box 13, I-00044 Frascati (Rome), Italy tel: +39 6 9403 1 fax: +39 6 9403304 http://www.lnf.infn.it/ Tipo:P Status: C

ALS Advanced Light Source MS46-161, 1 Cyclotron Rd Berkeley, CA 94720, USA tel:+1 510 486 4257 fax:+1 510 486 4873 http://www-als.lbl.gov/ Tipo: D Status: O AmPS Amsterdam Pulse Stretcher NIKEF-K, P.O. Box 41882, 1009 DB Amsterdam, NL tel: +31 20 5925000 fax: +31 20 5922165 Tipo: P Status: C

DELTA Universität Dortmund,Emil Figge Str 74b, 44221 Dortmund, Germany tel: +49 231 7555383 fax: +49 231 7555398 http://prian.physik.uni-dortmund.de/ Tipo: P Status: C

APS Advanced Photon Source Bldg 360, Argonne Nat. Lab. 9700 S. Cass Avenue, Argonne, Il 60439, USA tel:+1 708 252 5089 fax: +1 708 252 3222 http://epics.aps.anl.gov/welcome.html Tipo: D Status: C

ELETTRA Sincrotrone Trieste SCpA, S.S. 14, km 163,5 in AREA Science Park, 34012 Basovizza, Trieste, Italy tel: +39 40 37581 fax: +39 40 9380902 http://www.elettra.trieste.it Tipo: D Status: O

ASTRID ISA, Univ. of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark tel: +45 61 28899 fax: +45 61 20740 Tipo: PD Status: O

ELSA Electron Stretcher and Accelerator Nußalle 12, D-5300 Bonn-1, Germany tel:+49 288 732796 fax: +49 288 737869 http://elsar1.physik.uni-bonn.de/elsahome.html Tipo: PD Status: O

BESSY Berliner Elektronen-speicherring Gessell.für Synchrotron-strahlung mbH Lentzealle 100, D-1000 Berlin 33, Germany tel: +49 30 820040 fax: +49 30 82004103 http://www.bessy.de Tipo: D Status: O BSRL Beijing Synchrotron Radiation Lab. Inst. of High Energy Physics, 19 Yucuan Rd.PO Box 918, Beijing 100039, PR China tel: +86 1 8213344 fax: +86 1 8213374 http://solar.rtd.utk.edu/~china/ins/IHEP/bsrf/bsrf.html Tipo: PD Status: O

ESRF European Synchrotron Radiation Lab. BP 220, F-38043 Grenoble, France tel: +33 476 882000 fax: +33 476 882020 http://www.esrf.fr/ Tipo: D Status: O EUTERPE Cyclotron Lab.,Eindhoven Univ. of Technol, P.O.Box 513, 5600 MB Eindhoven, The Netherlands tel: +31 40 474048 fax: +31 40 438060 Tipo: PD Status: C

CAMD Center Advanced Microstructures & Devices Lousiana State Univ., 3990 W Lakeshore, Baton Rouge, LA 70803, USA tel:+1 504 3888887 fax: +1 504 3888887 http://www.camd/lsu.edu/ Tipo: D Status: O

HASYLAB Notkestrasse 85, D-2000, Hamburg 52, Germany tel: +49 40 89982304 fax: +49 40 89982787 http://www.desy.de/pub/hasylab/hasylab.html Tipo: D Status: O

CHESS Cornell High Energy Synchr. Radiation Source Wilson Lab., Cornell University Ithaca, NY 14853, USA tel: +1 607 255 7163 fax: +1 607 255 9001 http://www.tn.cornell.edu/ Tipo: PD Status: O

INDUS Center for Advanced Technology, Rajendra Nagar, Indore 452012, India tel: +91 731 64626 Tipo: D Status: C

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FACILITIES

KEK Photon Factory Nat. Lab. for High Energy Physics, 1-1, Oho, Tsukuba-shi Ibaraki-ken, 305 Japan tel: +81 298 641171 fax: +81 298 642801 http://www.kek.jp/ Tipo: D Status: O Kurchatov Kurchatov Inst. of Atomic Energy, SR Center, Kurchatov Square, Moscow 123182, Russia tel: +7 95 1964546 Tipo: D Status:O/C

LURE Bât 209-D, 91405 Orsay ,France tel: +33 1 64468014; fax: +33 1 64464148 E-mail: lemonze@lure.u-psud.fr http://www.lure.u-psud.fr Tipo: D Status: O

SSRL Stanford SR Laboratory MS 69, PO Box 4349 Stanford, CA 94309-0210, USA tel: +1 415 926 4000 fax: +1 415 926 4100 http://www-ssrl.slac.stanford.edu/welcome.html Tipo: D Status: O

MAX-Lab Box 118, University of Lund, S-22100 Lund, Sweden tel: +46 46 109697 fax: +46 46 104710 http://www.maxlab.lu.se/ Tipo: D Status: O NSLS National Synchrotron Light Source Bldg. 725, Brookhaven Nat. Lab., Upton, NY 11973, USA tel: +1 516 282 2297 fax: +1 516 282 4745 http://www.nsls.bnl.gov/ Tipo: D Status: O NSRL National Synchrotron Radiation Lab. USTC, Hefei, Anhui 230029, PR China tel:+86 551 3601989 fax:+86 551 5561078 Tipo: D Status: O Pohang Pohang Inst. for Science & Technol., P.O. Box 125 Pohang, Korea 790600 tel: +82 562 792696 f +82 562 794499 Tipo: D Status: C Siberian SR Center Lavrentyev Ave 11, 630090 Novosibirsk, Russia tel: +7 383 2 356031 fax: +7 383 2 352163 Tipo: D Status: O SPring-8 2-28-8 Hon-komagome, Bunkyo-ku ,Tokyo 113, Japan tel: +81 03 9411140 fax: +81 03 9413169

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SRC Synchrotron Rad. Center Univ.of Wisconsin at Madison, 3731 Schneider DriveStoughton, WI 53589-3097 USA tel: +1 608 8737722 fax: +1 608 8737192 http://www.src.wisc.edu Tipo: D Status: O SRRC SR Research Center 1, R&D Road VI, Hsinchu Science, Industrial Parc, Hsinchu 30077 Taiwan, Republic of China tel: +886 35 780281 fax: +886 35 781881 http://www.srrc.gov.tw/ Tipo: D Status: O

LNLS Laboratorio Nacional Luz Sincrotron CP 6192, 13081 Campinas, SP Brazil tel: +55 192 542624 fax: +55 192 360202 Tipo: D Status: C

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

Tipo: D Status: C SOR-RING Inst. Solid State Physics S.R. Lab, Univ. of Tokyo, 3-2-1 Midori-cho Tanashi-shi, Tokyo 188, Japan tel: +81 424614131 ext 346 fax: +81 424615401 Tipo: D Status: O

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SRS Daresbury SR Source SERC, Daresbury Lab, Warrington WA4 4AD, U.K. tel: +44 925 603000 fax: +44 925 603174 E-mail: srs-ulo@dl.ac.uk http://www.dl.ac.uk/home.html Tipo: D Status: O SURF B119, NIST, Gaithersburg, MD 20859, USA tel: +1 301 9753726 fax: +1 301 8697628 http://physics.nist.gov/MajResFac/surf/surf.html Tipo: D Status: O TERAS ElectroTechnical Lab. 1-1-4 Umezono, Tsukuba Ibaraki 305, Japan tel: 81 298 54 5541 fax: 81 298 55 6608 Tipo: D Status: O UVSOR Inst. for Molecular ScienceMyodaiji, Okazaki 444, Japan tel: +81 564 526101 fax: +81 564 547079 Tipo: D Status: O

D = macchina dedicata; PD = parzialmente dedicata; P = in parassitaggio.

O= macchina funzionante; C=macchina in costruzione.

D = dedicated machine; PD = partially dedicated; P = parassitic.

O= operating machine; C= machine under construction.

Vol. 6 n. 1 Giugno 2001

FACILITIES

NEUTRONI NEUTRON SCATTERING WWW SERVERS IN THE WORLD (http://www.isis.rl.ac.uk)

BENSC Berlin Neutron Scattering Center, Hahn-Meitner-Institut, Glienicker Str. 100, D- 14109 Berlin-Wannsee, Germany Rainer Michaelsen; tel: +49 30 8062 3043 fax: +49 30 8062 2523 E - Mail: michaelsen@hmi.de http://www.hmi.de BNL Brookhaven National Laboratory, Biology Department, Upton, NY 11973, USA Dieter Schneider; General Information: Rae Greenberg; tel: +1 516 282 5564 fax: +1 516 282 5888 http://neutron.chm.bnl.gov/HFBR/ GKSS Forschungszentrum Geesthacht, P.O.1160, W-2054 Geesthacht, Germany Reinhard Kampmann; tel: +49 4152 87 1316 fax: +49 4152 87 1338 E-mail: PWKAMPM@DGHGKSS4 Heinrich B. Stuhrmann; tel: +49 4152 87 1290 fax: +49 4152 87 2534 E-mail: WSSTUHR@DGHGKSS4 IFE Institut for Energiteknikk, P.O. Box40, N-2007 Kjeller, Norway Jon Samseth; tel: +47 6 806080 fax: +47 6 810920 telex: 74 573 energ n E-mail: Internet JON@BARNEY.IFE.NO ILL Institute Laue Langevin, BP 156, F-38042, Grenoble Cedex 9,France Giovanna Cicognani; tel: +33 476207179 E-mail: sco@ill.fr fax: +33 76 48 39 06 http://www.ill.fr IPNS Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4814, USA P.Thiyagarajan,Bldg.200,RM. D125; tel :+1 708 9723593 E-mail: THIYAGA@ANLPNS Ernest Epperson, Bldg. 212; tel: +1 708 972 5701

fax: +1 708 972 4163 or + 1 708 972 4470 (Chemistry Div.) http://pnsjph.pns.anl.gov/ipns.html ISIS The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxfordshire OX11 0QX, UK Richard Heenan; tel +44 235 446744 E-mail: RKH@UK.AC.RUTHERFORD.DEC-E Steve King; tel: +44 235 446437 fax: +44 235 445720; Telex: 83 159 ruthlb g E-mail: SMK@UK.AC.RUTHERFORD.DEC-E http://www.isis.rl.ac.uk JAERI Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan.

Jun-ichi Suzuki (JAERI); Yuji Ito (ISSP, Univ. of Tokyo); fax: +81 292 82 59227 telex: JAERIJ24596 http:// neutron-www.kekjpl JINR Joint Institute for Nuclear Research, Laboratory for Neutron Physics, Head P.O.Box 79 Moscow, 141 980 Dubna, USSR A.M. Balagurov; E-mail: BALA@LNP04.JINR.DUBNA.SU Yurii M. Ostaneivich; E-mail: SANS@LNP07.JINR.DUBNA.SU fax: +7 095 200 22 83 telex: 911 621 DUBNA SU http://www.jinr.dubna.su KFA Forschungszentrum Jülich, Institut für Festkörperforschung, Postfach 1913, W-517 Jülich, Germany Dietmar Schwahn; tel: +49 2461 61 6661; E-mail: SCHWAHN@DJUKFA54.BITNET Gerd Maier; tel: +49 2461 61 3567; E-mail: MEIER@DJUKFA54.BITNET fax: +49 2461 61 2610 telex: 833556-0 kf d

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FACILITIES

LLB Laboratoire Léon Brillouin, Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur-Yvette Cédex France J.P Cotton (LLB); tel: +33 1 69086460 fax: +33 1 69088261 telex: energ 690641 F LBS+ E-mail: COTTON@BALI.CEA.FR http://bali.saclay.cea.fr/bali.html NFL Studsvik Neutron Research Laboratory, Uppsala University S-611 82 Nyk=F6ping, Sweden R. McGreevy; tel: + 46 155 221831 fax: +46 155 263001 E-mail: mcgreevy@studsvik.uu.se http://www.studsvik.uu.se NIST National Institute of Standards and TechnologyGaithersburg, Maryland 20899 USA C.J. Glinka; tel: + 301 975 6242 fax: +1 301 921 9847 E-mail: Bitnet: GLINKA@NBSENTH Internet: GLIMKA@ENH.NIST.GOV http://rrdjazz.nist.gov ORNL Oak Ridge National Laboratory Neutron Scattering Facilities, P.O. Box 2008, Oak Ridge TN 37831-6393 USA George D. Wignall, Small Angle Scattering Group Leader; tel: +1 423 574 5237 fax: +1 423 574 6268 E-mail: wignallgd@ornl.gov http://neutrons.ornl.gov PSI Paul Scherrer Institut Wurenlingen und Villingen CH-5232 Villingen PSI tel: +41 56 992111 fax: +41 56 982327 RISØ EC-Large Facility Programme, Physics Department, Risø National Lab.P.O. Box 49, DK-4000 Roskilde, Denmark K. Mortenses; tel: +45 4237 1212 fax: +45 42370115 E-mail: CLAUSEN@RISOE.DK or SANS@RISOE.DK NFL-Studsvik in Sweden E-mail: mcgreevy@studsvik.uu.se

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