NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 6 n.2, 2001

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

SOMMARIO

Cover photo: View of TOSCA showing detector banks for forward and backward scattering.

EDITORIALE

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C. Andreani

RASSEGNA SCIENTIFICA Recent Applications of Small Angle Scattering in Biophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 F. Carsughi, P. Mariani and F. Spinozzi

Structural Studies of Condensed Matter Under Extreme Conditions of High Pressure and Temperature Combining X-ray Absorption Spectroscopy, Powder Diffraction and Temperature Scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

NOTIZIARIO

Il Neutroni e Luce di Sincrotrone è pubblicato a cura del C.N.R. in collaborazione con la Facoltà di Scienze M.F.N. e il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

Vol. 6 n. 2 Dicembre 2001 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96

A. Filipponi

PROGETTO E.S.S.

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DIRETTORE RESPONSABILE:

C. Andreani COMITATO DI DIREZIONE:

DOVE NEUTRONI

M. Apice, P. Bosi

The Final Configuration of TOSCA Neutron Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

COMITATO DI REDAZIONE:

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

M. Celli et al.

SEGRETERIA DI REDAZIONE:

D. Catena HANNO COLLABORATO A QUESTO NUMERO:

VARIE

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F. Aliotta, A. Deriu, S. Mobilio, K.C. Prince GRAFICA E STAMPA:

om grafica via Fabrizio Luscino 73 00174Roma

SCUOLE E CONVEGNI CALENDARIO

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SCADENZE

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FACILITIES

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Finito di stampare nel mese di Dicembre 2001 PER NUMERI ARRETRATI:

Paola Bosi, Tel: +39 6 49932057 Fax: +39 6 49932456 E-mail: p.bosi@dcas.cnr.it.

PER INFORMAZIONI EDITORIALI:

Desy Catena, Università degli Studi di Roma “Tor Vergata”, Presidenza Facoltà di Scienze M.F.N., via della Ricerca Scientifica, 1 00133 Roma Tel: +39 6 72594100 Fax: +39 6 2023507 E-mail: desy.catena@roma2.infn.it

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EDITORIALE

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a Rassegna Scientifica di questo numero si apre con l’articolo di F. Carsughi et al. sulle applicazioni recenti dello Small Angle Scattering di neutroni e raggi X alla biofisica; segue il contributo di A. Filipponi sugli studi strutturali della materia condensata in condizioni estreme di temperature e pressioni. Nella sezione Dove Neutroni l’articolo di M. Celli et al. illustra le prestazioni dello spettrometro TOSCA nella sua configurazione finale ad ISIS. Riceviamo dalla segreteria dell’ESS R&D Council e pubblichiamo la ESS Newsletter, che descrive lo stato di avanzamento del progetto ESS. Nella sezione Varie viene infine pubblicato il bando dell’Accademia Nazionale dei Lincei relativo ad una borsa di perfezionamento con fondi messi a disposizione dalla famiglia Ricci al fine di onorare la memoria del Prof. Francesco Paolo Ricci.

Un’importante data per la comunità scientifica italiana è Marzo del 2002 quando scadrà l’accordo internazionale tra il CNR ed il CCLRC. A questo proposito mi auguro che il CNR continuerà a svolgere l’efficace ruolo di Ente referente per le attività di spettroscopia di neutroni e di muoni, presso la sorgente pulsata ISIS (Rutherford Appleton Laboratory-UK), per conto di tutta la comunità scientifica italiana. Infatti è stato grazie a questo accordo, fin dalla prima stipula tra il CNR ed il SERC che risale al 1985, che la comunità italiana operante in questo settore si è considerevolmente sviluppata. Una comunità di ricercatori, del CNR ed universitari, che ha dimostrato in questi anni un’attiva partecipazione sia alla ricerca ad ISIS sia alla progettazione e realizzazione di strumentazione per neutroni e muoni. Fin dal 1985 il CNR ha destinato a queste attività di ricerca notevoli finanziamenti, garantendo sia l’accesso di tutta la comunità scientifica alla struttura di ricerca inglese sia il personale e le risorse necessarie ai suoi istituti (ISM e IEQ) per la progettazione e costruzione di strumentazione (PRISMA e TOSCA) che oggi è installata ed operante ad ISIS. Questo accordo ha permesso l’instaurarsi di una proficua collaborazione tra ricercatori italiani ed europei e ha avuto un riconoscimento internazionale che si è manifestato attraverso numerosi finanziamenti alla comunità italiana da parte della Comunità Europea (come risulta dai Consuntivi relativi all’attività di ricerca della comunità di italiana ad ISIS e nell’ambito del Progetto ESS pubblicati sull’ultimo numero del Notiziario - Vol 6., N.1). Perciò mi auguro che il CNR possa rinnovare l’accordo con il CCLRC, con l’obiettivo prioritario di continuare a garantire l’accesso della comunità scientifica italiana ad ISIS.

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he section Rassegna Scientifica in this issue has two contributions: the first one by F. Carsughi et al, presenting recent applications of Small Angle neutron and X-ray Scattering on biophysical systems; the second one by A. Filipponi regarding structural studies in condensed matter under extreme pressure and temperature conditions. M. Celli et al., in the section Dove Neutroni, illustrate the performances of the TOSCA spectrometer in its final configuration at the ISIS facility.

We also report the latest ESS Newsletter, as received from the ESS R&D Council, which describes the state of the art of the ESS project. In the section Varie we announce that a fellowship is made available by the Accademia Nazionale dei Lincei, thanks to the generous donation by the family of prof. Francesco Paolo Ricci.

An important date for the Italian scientific community is March 2002 when the international agreement between CNR and CCLRC expires. I wish that CNR will continue to be the reference italian institution for neutron and muon spectroscopy at the ISIS source. Indeed thanks to this agreement, CNR has favoured, since 1985, the remarkable growth of the italian scientific community in this field.

This community, composed of CNR as well as of university researchers, has both performed valuable basic research at ISIS and designed and realised innovative instrumentation for neutron and muon spectroscopy. CNR in the last 16 years has funded the access to ISIS facility for the whole scientific community, provided the personnel funded the CNR Institutes (ISM and IEQ) for the design and realisation of neutron instrumentation (PRISMA and TOSCA), which today are fully operational at ISIS. This agreement has allowed to establish a fruitful collaboration between Italian and European researchers which has also favoured funding for further R&D project on neutron and muon instrumentation by the European Community. For these reasons I wish the CNR will renew the agreement with CCLRC, with the primary objective to guarantee access for the Italian community to ISIS facility.

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Carla Andreani

RASSEGNA SCIENTIFICA

Articolo ricevuto in redazione nel mese di Maggio 2001

RECENT APPLICATIONS OF SMALL ANGLE SCATTERING IN BIOPHYSICS F. Carsughi, P. Mariani and F. Spinozzi INFM Research Unit of Ancona and University of Ancona, Via Brecce Bianche, I-60131 Ancona, Italy

Abstract The analysis of the structural properties of biological macromolecules in solution can be a unique key to understand biomolecular mechanisms. Small angle neutron and X-ray scattering can be very suitable approaches, due to small perturbing effects on the system and the possibility to perform measurements in very different experimental conditions. In this paper, we present some recent results concerning the analysis of size, shape, structure, conformational changes and interactions of proteins in solution. A particular emphasis will be devoted to discuss particle shape reconstruction methods recently developed in our group.

1. Introduction Biological macromolecular systems in aqueous solution are often referred as colloidal systems, being formed by semimicroscopic particles that have a linear dimension between 10 and 1000 Ă… dispersed in a continuous medium. Based on interactions between the colloidal particles and the solvent, biological systems can be generally classified into two main colloid categories: aggregates of amphiphilic molecules (like lipids and detergents) are association colloids, while proteins and biopolymers belong to lyophilic colloids, and in this case the colloid particles are just macromolecules [1]. In this paper, we are mainly focusing on proteins: determination of their size, shape, structure and interactions using solution small-angle scattering of X-rays (SAXS) or neutrons (SANS) will be our major concern. In fact, although the crystal structure of a number of proteins is known, the study of their structural properties in solution and the analysis of the structural and conformational changes involved in dimerization or aggregation processes, in activation or inhibition by effectors or associated to folding/unfolding processes, can be a unique key to understand their biological functions. To derive particle shape and size and conformational changes, solution SAXS or SANS are very suitable approaches, mainly due to the small perturbing effects on the system [1-9]. Moreover, size, shape, molecular weight, compactness degree and aggregation state of a protein in solution can be obtained in very different experimental conditions, e.g. as a function of pH, salt concentration or temperature, pressure presence of cosolvents, ligands or denaturing agents.

Two peculiar points should be also stressed: first, the high beam intensities which can be obtained in synchrotron sources made possible time-resolved SAXS experiments (50-100 msec resolution): therefore, the kinetics of structural changes can be easily studied [10]. Second, SANS can take advantage of the different scattering lengths of hydrogen and deuterium. Using the contrast variation technique, the aggregation state of soluble proteins, the characteristic of the hydration layer and the structural properties of membrane proteins can be derived [11-14].

Even if the analysis of a small angle scattering (SAS) curve can be limited to the determination of a few parameters (like the gyration radius, related to the particle average dimensions, the forward scattering intensity, related to the particle concentration, and the Porod invariant, related to the particle surface), the form factor of the particle can be also derived. This factor gives direct information on the in-solution biomolecule structure. However, because of the low resolution and the loss of information incurred from averaging the scattered intensity over all particle orientations in the usual case of scattering from isotropic solution, the derivation of the particle structure from the experimental SAS curve is a rather tough problem, the solution of which might not be unique. To reconstruct the shape of the scattering particles, different procedures can be used, some of which are direct (such as the well-established methods based on the multipole expansion of the excess scattering length density [6,15,16]), while others usually require the data fitting to a form factor calculated from a model shape, which can range from a simple geometrical structure to a complex heterogeneous particle, or directly evaluated from the crystallographic coordinates [17,18].

It should be observed that SAS experiments are usually performed in a regime in which the scattering particles are completely uncorrelated (large separation or weak interaction force), so that the SAS intensity only depends on the particle form factor. However, when the interactions cannot be disregarded, an additional scattering signal related to the spatial distribution of the particles (i.e., the structure factor) is detected. In this context, an analysis of the structure factor allows the correlation function (which describes the spatial arrangement of the

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particles) and then the direct pair potential, describing the interaction force between two particles, to be derived [4,17,19].

In this paper, we will report some recent results obtained by our group using solution SAS technique for the analysis of protein systems. Since the biological relevance is straightforward, emphasis will be in particular devoted to discuss recent improvements on data analysis for particle shape reconstruction. In fact, the multipole expansion method has been reconsidered to include the advantages of the group theory and maximum entropy to optimize the particle shape recovering [16]. Moreover, original Monte Carlo methods for calculating SAS profiles from different protein domain orientations, from multidomain particles and from proteins subjected to a monomer-dimer equilibrium have been developed [2022,29].

2. Theory 2.1 General Equations The most general expression of the SAS is [17]

re with p components. By introducing the effective form factor Peff(Q) as

(5) being ni the number density of species i, one can also define the effective structure factor Seff(Q) of the system by rewriting Eq. 1 in the form

(6) Here, Peff(Q) accounts for the distribution of scattering matter inside the particles, while Seff(Q) is related to the particle-particle interactions and the resultant equilibrium structure of the system, and it is defined through Eq. 6.

2.2 Isotropy If the particle-particle interactions are spherically symmetric (homogeneous and isotropic systems), then the SCS can be cast into the Fournet-Vrij’s form [24]:

(1) Here [dΣ/dΩ](Q) is the macroscopic differential coherent Scattering Cross Section (SCS) as a function of the exchanged wave vector Q, whose magnitude is defined by (2) where λ is the neutron or X-ray wavelength and 2θ the full scattering angle. In Eq. 1, the integral is extended over the total sample volume V and r is the position vector. The angular brackets 〈....〉 represent an ensemble average over all possible positions, orientations and microstates of the particles in the system. Being ρ(r) the local scattering length density or the electron density (multiplied by the classic electron radius) for neutrons and X-rays, respectively, in absence of long range order, the scattering length density can be thought as having a uniform value ρ0 on which fluctuations δρ(r) are superimposed. The distribution of scattering material inside a particle is then defined by its form factor [23]

(7)

where the brackets 〈....〉ωQ represent angular averages (this formula being still valid in case of non-spherically symmetric particles). The Sij(Q) are the Ashcroft-Langreth partial structure factors [25], defined by

(8) where δij is the Kronecker delta and hij (Q) is the threedimensional Fourier transform of the total correlation function hij(r), which can be obtained by solving the OZ integral equations of the liquid state [26].

The first term on the rhs of Eq. 7 is called the “incoherent” part, and the remainder the “coherent’’ one. The incoherent part vanishes if the distribution of scattering matter is spherosymmetric and in this case ˜

(3) where

(9)

(4) is the scattering amplitude at zero-angle, and VP is the volume of the scattering particles. Let us consider a polydisperse system, which can be regarded as a mixtu-

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2.3 Two-Phase Model Fortunately, for most of biological systems the particles can be considered as formed by a single type of uniform scattering material (first phase = solute) and are embedded in a uniform medium (second phase = solvent

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for a solution). In this case, δρ(r) in Eq. 3 can be replaced by ∆ρ = ρP−ρS, where ρP and ρS are the scattering length density of the scattering particles and the embedding medium, respectively. Thus the zero-angle scattering amplitude (Eq. 4) is given by

(10) and the form factor F(Q) (Eq. 3) becomes

2.5 Distance Distribution Functions The gyration radius can be expressed also by

(18) where p(r) is the pair distance distribution function, which gives the probability to find both the extremes of a segment of length r inside the particle. The p(r) can be calculated from the squared form factor by

(11) In a simple case, for an isotropic, monodisperse and spherosymmetric system, the Eq. 7 takes the following form

(19) which inversely gives

(20)

(12) (13) where P(Q) ≡ < F2(Q) > ωQ and Seff(Q) are the squared form factor and structure factor, respectively, in case of isotropic scattering. Moreover, in case of dilute system and of weak interactions between particles, the Eq. 12 takes a simpler form (14) 2.4 Guinier Approximation Due to the mathematical properties of the form factor, it can be demonstrated that P(Q) can be approximated by the so-called Guiner law (15)

It can be also defined the single distance distribution function, p(1)(r), which gives the probability they find a point inside the particle at distance r from the centre.

The average form factor results

(21)

2.6 Monte Carlo A Monte Carlo method can be used to generate the distance distribution function p(r) and then the P(Q) profile, as shown in [18]. NR random points in real space can be generated and checked the probability to belong to the particle. The p(r) histogram of the particle can be then calculated by taking into account the distances between all possible pairs of NR points,

where Rg is the gyration radius, defined as

(16) which represents the same quantity as found in mechanics, when the scattering length density is constant within the particle. The Guinier approximation is valid upon the condition Rg Q ≤ 1.3 [27]. In this respect, the

(22) being ∆r the grid amplitude in the space of radial distance and rij the distance between the points i and j. H(x) is a step function (H(x)=0 if x <0 and H(x)=1 if x ≥ 0). The acceptance of a random point is related to the position function s(r), defined as

SCS becomes (23) (17) where [dΣ/dΩ](0) = (∆ρ)2 n VP2 (see Eq. 14) and Rg can be obtained by linear fitting the experimental data in the ln{[dΣ/dΩ](Q)} vs Q2 plane.

where F(ωr) is the shape function, which can represent any arbitrary but compact shape [28].

This method can be equally applied to homogeneous particles as well as to protein in solution, even by accounting for a hydration shell, described by a gaussian

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function of width σH [28], with another position function described by

(24) Given the protein model, F(ωr) can be evaluated from the envelope surface of the van der Waals spheres centred in each atom.

This method can be easily extended to describe the scattering volume of particles constituted by several subunits with relative positions and orientations. Note that to simulate the whole particle with uniform scattering density, the number of random points for each subunit k should be proportional to its volume (subvolume). Moreover, the portions of space for overlap common between two or more subunits need to be checked using the method developed by Hansen [18]. This procedure has been also used to describe conformational changes in a protein [21].

2.7 Non-homogenous particles If the homogeneous particle condition is no longer valid, this Monte Carlo method can be applied with a similar formalism [29]. If particles are formed by ND different domains, each of one with a different scattering properties, one can define the cross-linked distance distribution function phk(r), which gives the probability to find a segment with one extrem in the h-th domain and the other one in the k-th domain. In this case the scattering cross section can be expressed by

Fig. 1. Scattering length density as a function of the D2O content in the solvent, RNA, BMV protein, lipid head group and CH2, after [2].

of water is compared with that of CH2, lipid head group, BMV protein and RNA [2]. Since the particle scattering length density ρP(r) changes locally, it is useful to define it as its average value < ρP > and the fluctuations ρF(r) around the mean value, ρP(r) = s(r)<ρP> +ρF(r). In Fig. 1, the average contrast < ∆ρ > represents the difference of scattering length density of the solvent < ρS > and the average value within the particle ( <∆ρ>=<ρS> − <ρS> ).

So far, the scattering cross section can be defined as

(27) (25) where Phk(Q) represents the partial squared form factor of each domain defined as

where A(Q), B(Q) depend on the shape and on the internal structure of the scattering particles, respectively, and C(Q) is a cross term. On the other hand, the gyration radius depends on the internal structure (see Eq. 16) and it turns out that

(26) and ρh and ρk are related to the scattering properties of the h-th and k-th domain, respectively.

(28) Where

2.8 Contrast variation technique This special application of SAS in the field of soft matter investigation is particularly suitable with neutrons due to the very much different scattering lengths of H (−0.3742·10−12 cm) and D (0.6671·10−12 cm). The two hydrogen isotopes behave in the same way from the chemical point of view, but the scattering properties result very different. In fact, the scattering length density of water varies very rapidly with D2O content in the solvent. In Fig. 1 the variation of scattering length density

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(30)

(31)

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The values of the gyration radius as a function of the average inverse contrast can be approximated by a parabole, whose coefficients are described in Eq. 29-31; Rg,0 is the gyration radius of the scattering particle at infinite contrast. If a particle has a larger scattering length density in the inner or in the outer part, α turns out to be negative or positive, respectively.

2.9 Shape reconstruction Under the assumption of monodisperse, diluted twophase model systems, it is possibile to reconstruct the shape of the scattering particles. This method was originally devoleped by Svergun, Stuhrmann and coworkers [27, 30-32]. In this respect, the form factor (Eq. 11) can be also written as

(32) where s(r) is the position function, as defined in Sec. 2.6.

Both the functions s(r) and F(Q) can be expanded as a series of spherical harmonics Yl,m

(33)

(34) where the value L, theoretically ∞, represents the maximum rank used in the expansion. The F(ωr) shape function and its q-powers can be expanded in series of spherical harmonics

(38)

(39) (40) It is evident that higher is the rank L used in Eq. 34, better is the approximation, but this results in a too long computational time. A compromise must be used in order to select the proper value of L. 2.9.1 Group Theory Integration The particle reconstruction can be more efficient if it is possible to find some properties that actually reduce the number M of independent real quantities in the complex space of parameters fl,m. A first immediate way is to impose that the shape function F(ωr) is real. Thus, being Yl,m*(ωr) ≡ (-1)m Yl,-m(ωr), we obtain (41) Therefore, the number M for a given L reduces from (L+1)(2L+1) to (L+1)2. The main and not quite so intuitive way to reduce the effective number M of parameters describing the shape function is to deal with the symmetry of the particle. In practice, this can be obtained by applying the group theory [33-35]. Considering the point group G of the scattering particle, its shape function should be invariant with respect to any symmetry operator of the group. In other words, the shape function F(ωr) must belong to the total symmetrical representation of the group. Hence, by applying the projection operator, one obtains

(35) (42)

where the expansion coefficients are

(36) The squared form factor P(Q) then becomes

(37)

where the b k are complex functions of the expansion coefficients {fl,m(q)}, given in [16]. Moreover, the set {fl,m(q)} contains all the geometrical information on the particle, like volume, radius of gyration and geometrical centre of the particle, given by

where the symbol S indicates this symmetrization scheme.

In order to simplify the identification of the particle symmetry, according to the Schönflies notation, we define the point group sequence shown in Tab. 1. The first point group is the most symmetric, i.e. the spherical one Kh, defined only by f0,0, followed by the two next symmetric, i.e. D∞h and C∞v, for which fl,m=δm,0fl,0. Then, we consider the other groups containing symmetry axes of order 2 ≤ n ≤ L which are sequenced as follow: Dnh, Cnh, Cnv, Dnd, D , S n, C n. Finally, the less symmetric group C s and the n completely asymmetric one C 1 are processed. At the end, for the sake of completeness, the so-called special groups, with a very strict symmetry, are also considered. Among them, three show the tetrahedrical (T , Th , Td ), two the octahedrical (O , Oh ) and two the icosahedrical symme-

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Table 1. Independent real quantities (M) in the complex space of shape parameters fl,m up to L=6 permitted for each point symmetry group G. Indexes R and I indicate the real and immaginary components, respectively.

try (ℑ , ℑh ). In Tab. 1 the number M and the whole sets of parameters {fl,m}, which survive the symmetrization procedure when L=6 for chosen groups, are shown. For example, while the asymmetric group C1 is described by using all the M = (L+1)2 ≡ 49 parameters, the D2h group is described only by 10.

to a non-physical meaning of the shape function. An elegant way to solve this problem is the use of the Maximum Entropy principle [36]. The best approximation to the true shape function results

2.9.2 Maximum Entropy Shape Function The symmetry properties of the shape function FS(ωr) lead to a reduction of the parameter number M for a given L. Moreover, according to the definition, FS(ωr) is positive, but, by expanding it, it results that this occurs only when L→ ∞ since the spherical harmonics Yl,m(ωr) contain some negative parts, except for the case l=0.

Thus the truncation to a maximum finite rank L can lead

(43) where the new parameter set {al,m} will be determined under the condition that, by integration, they should give the known set {fl,m}, i.e. that

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The mathematical form of the Maximum Entropy shape function (Eq. 43) ensures that it will be positive and that its symmetry properties will remain those of YSl,m(ωr).

Moreover, all the symmetry considerations previously reported for the expansion coefficients fl,m are still valid for the new Maximum Entropy parameters al,m, so that the Tab. 1 can be re-written in terms of the set {al,m}.

2.10 Protein-protein interaction The total h ij(r) and direct c ij(r) correlation functions between two particles of specie i and j are linked together in the Orstein-Zernike (OZ) relation [26]

(45) The solution of the OZ relation is only possible via a closure, i.e. another relation for the same quantities; among which, Perkus-Yevick for hard sphere, Mean Spherical Approximation for charged particles. Once the total correlation function is known, the pair correlation function gij(r) (or the radial distribution function) can be simply defined as (46) and gives the probability to find another scattering particle at distance r. It is also described by

3 Examples 3.1 SANS contrast variation technique: analysis of a natural membrane The considered membrane system was obtained by mechanical rupture of photosynthetic cells of Rhodobacter capsulatus and consists of natural unilamellar vesicles (chromatophores), showing identical composition and function, but inside-out polarity with respect to the membrane of the bacterium [38]. Chromatophores are characterized by a phospholipid-to-protein ratio of 0.8± 0.2 µmol per mg protein and a bacteriochlorophyll-toreaction center molar ratio of about 100 [38]. A minimum model of the light-induced electron transport chain includes two types of light harvesting pigment-protein complexes, two reaction center complexes, an ubiquinolcytochrome c2 oxidoreductase enzyme complex and a soluble cytochrome c2 molecule [38,39]. It can be estimated that 30% of the total protein content of chromatophores is transmembrane. This functional unit is asymmetric and generates a transmembrane electrochemical potential for protons, which drives the cytoplasmic photophosphorylation catalyzed by the membrane-bound ATPase enzyme complex [40]. The ATPase consists of a soluble F1 part and a membrane embedded counterpart F0. The large hydrophilic portion (the average diameter of the overall size of ATPase from Escherichia coli, as detected by cryoelectron microscopy, is about 100 Å [41]) is expected to protrude some ten Å beyond the membrane surface [42].

(47) where Wij(r) is the mean force potential, kB is the Boltzmann constant and T the absolute temperature. The zero-density limit yields to the following approximation

(48) where uij(r) is the direct pair potential between two particles, without taking into account the presence of the others and fij(r) is the Meyer function. One can also expand the mean force potential as a series of density n in order to improve the approximation of gij(r). Using of a perturbative term up to the first order, ωij(1)(r) [37], we obtain (49) The calculation involve a single convolution and turns out to be

(50) being xk is the molar ratio of specie k.

3.1.1 Experimental details SANS contrast variation experiments were performed on native and EDTA-treated chromatophores. In this last population, the soluble, large F1 counterpart of the ATPase complex was extracted following EDTA treatment and sonication [44]. Both preparations were characterized as previously reported and bacteriochlorophyll content and protein and lipid phosphorus determined [38,44]. Their similar morphology was also confirmed by electron microscopy measurements. It should be observed that the ATPase activity of depleted chromatophores was 80% reduced as compared to untreated membranes, indicating that this procedure is able to remove most of F1 from the enzyme complex [44].

Native and EDTA treated chromatophores in the mixtures of D2O and H2O were obtained several days before the experiments by suspending again the dried chromatophores at the estimated concentration of 10 g l−1 (i.e., 1% w/w) in a buffer with the desired D2O content. The measurements were performed at the Laboratoire Leon Brillouin, Saclay (France) using the small-angle diffractometer PAXE. The scattering vector Q range was between about 0.003 and 0.3 Å−1.

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Fig. 2. Guinier plots ln{[dΣ/dΩ](Q)Q2} vs Q2 for natural unilamellar vesicles from Rhodobacter capsulatus chromatophores and after EDTA-treatment at different D2O concentrations (percentages reported in the frames). The characteristic sinusoidal oscillations reflect the membrane curvature [42].

3.1.2 Scattering from a sheet-like structure In the limit of small Q, the SCS from sheet-like structures (like membranes) can be approximated by [2,42]:

(51) where A is the total membrane surface, < ∆ρ > the mean contrast, defined as the difference between the mean scattering length density of the membrane < ρP > and that of the solvent < ρS > , and t and D are the one-dimensional analogues of the volume and of the radius of gyration for a 3D object, respectively. They are defined by Fig. 3. Variation of the square root of the scattered intensity [dΣ/dΩ](Q)Q2 extrapolated at zero angle as a function of the D2O concentration.

Uncertainties are less than the size of the symbols. Solid and broken lines are for EDTA-treated (F1-depleted) and native chromatophores, respectively.

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(52) and

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ATP-ase soluble 60-100Å 10-15Å 40Å

Lipid polar region

Lipid hydrophobic region

Protein outer fraction

ATP-ase transmembrane Protein inner fraction Fig. 5. Representation of a simple model for the chromatophore membrane used to analyse SANS data of Rhodobacter capsulatus samples.

(54) (the function s(z), considered symmetrical, is an adimensional term, ranging between zero and unity, which takes account of the partial exchange of surface hydrogens with the solvent). D is related to the contrast by [2,42]:

(55) where:

(56)

Fig. 4. Variation of the square of the thickness parameter D as a function of the inverse contrast 1 / < ∆ρ > . The lines represent the data fits: solid and broken lines are for EDTA-treated (F1-depleted) and native chromatophores, respectively. Large errors at small contrast reflect the difficulties of measurements in low contrast condition.

(53) being δρ(z) the excess scattering length density along the membrane. According to the contrast variation formalism [2], δρ(z) could be written in term of < ∆ρ > and of ρF(z), the fluctuations of the scattering length density around the mean:

(57)

(58) In these equations, D0 represents the thickness radius of gyration at infinite contrast, α measures the difference between the scattering length density in the region near the boundary and at the centre of the lamellae and β depends on the asymmetry of the membrane described by ρF(z). By performing SANS measurements at different contrast, the variation of the radius of gyration following Eq. (55) can be obtained and the different terms determined.

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Fig. 6. Pseudo-experimental scattering curves [dΣ/dΩ](Q) (red points), fitted P(Q) functions obtained from Maximum Entropy analysis (continous blue lines), shape function Fexp(ωr) calculated from the multipole expansion up to L=6 (red surface) and reconstructed Maximum Entropy shape functions FS(ωr) (blue surface). From the bottom: cylinder; double-cone; ellipsoid; cube.

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their slopes, and estimating the total surface of the membrane from the sample composition, t values of 54.6 ± 2.0 and 51.7 ± 2.5 Å for native and F1-removed chromatophores were calculated, respectively. From the intercepts, which correspond to the D2O concentrations for which the contrast is zero (matching point), membrane average scattering length densities < ρP > of 1.32 ± 0.09 and 1.18 ± 0.09 10−14 cm Å−3 for the native and F1-removed chromatophores were obtained, respectively (see Table 2).

Fig. 7. Multipole expansion analysis of experimental SAXS data from Carcinus Aestuarii 16S hemocyanin in diluted solution. The input data with experimental errors are shown in red and the fitted curve in green.

The PDB (entries 1HC1-6) structure and the reconstructed shape are also shown.

Experimental

F1 depleted

Native

values

chromatophores

chromatophores

< ρP > (10−14 cm Å−3)

1.18 ±0.09

1.32 ±0.09

D20 (Å2)

304.3 ±8.4

537.4±9.8

t (Å)

51.7 ±2.5

54.6±2.0

α (10−14)

1.41 ±0.11

2.21±0.07

β (10−11 Å−2)

−0.1 ±0.5

−5.4±0.6

Model calculation (100% asymmetry)

0.5 ÷2 F0

0.5 ÷2 F0 F1

< ρP > (10−14 cm Å−3)

0.97 ÷1.07

1.10 ÷1.46

hout (Å)

14.5 ÷11.0

14.5 ÷11.0

L (Å)

104.0 ÷58.0

104.0 ÷58.0

t (Å)

49.1 ÷51.2

52.0 ÷62.4

α (10−14)

1.35 ÷1.29

2.61 ÷2.23

β (10−11 Å−2)

−0.36 ÷−1.90

−7.0÷−18.9

Table 2. Results obtained from the experimental SANS data and from a simple model described in Fig. 5 on Rhodobacter capsulatus chromatophores in the native and F1 depleted conditions. The average scattering length density, < ρP > , the thickness radius of gyration at infinite contrast, D0, the thickness, t, the difference between the scattering length density in the region near the boundary and at the centre of the lamellae, α, the asymmetry of the membrane, β, the height of outwardfacing parts, hout, and the height of the F1 part, L, are reported.

Fig. 8. Comparison between the form factor P(Q) of an ellipsoid calculated from the analytic expression [47] and the Monte Carlo method.

The Monte Carlo partial form factor Phk(Q) are also reported.

3.1.3 SANS results Guinier plots [ln(dΣ/dΩ(Q)Q2)vsQ2] for F1-depleted chromatophores at different D2O concentrations are reported in Fig. 2. According to eq. (51), (A < ∆ρ >2 t2) and D have been obtained by fitting the experimental data. The variation of the observed (A < ∆ρ >2 t2)0.5 with the D2O concentration for both series of samples is shown in Fig. 3. Direct structural information can be derived from the two straight lines fitting the experimenatl points: from

The derived average scattering length densities have been used to determine the contrast < ∆ρ > in each measurement. The dependence of D2 on < ∆ρ >−1 is shown in Fig. 4: by least-squares quadratic fit to the data, the three terms in eq. (55) have been obtained and are reported in Table 2. Qualitative information about the structural properties of the photosyntetic membrane can be directly derived from these parameters. First, the D0 is larger for native than for the F1-removed chromatophores, suggesting that the F1 fragment projects appreciably out from the membrane. Second, in both series, the thickness parameters increase with < ∆ρ >−1, showing that the components of lower scattering length densities are preferentially located in the inner part of the membrane. Third, the D2 vs < ∆ρ >−1 dependence is different for the two series. A linear dependence is observed in the case of F1-removed chromatophores, indicating that in eq. (55) β is negligible: within the experimental limits, the membrane scattering length density is about centro-symmetric.

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Fig. 9. Form factors P(Q) of a prolate ellipsoid (PE), a spherocylinder (SC), an oblate ellipsoid (OE), a disk-like shape (PL) and a biaxial ellipsoid (BE) calculated by the Monte Carlo method.

Fig. 10. Experimental scattering intensity [dΣ/dΩ](Q) of the system SLS/water (with 1 % w/w of SLS) and the fitting curves obtained by using polydisperse two-domain spherical (SP), biaxial ellipsoidal (BE) (bottom plot), prolate ellipsoidal (PE) and spherocylindrical (SC) (top plot) models. The upper curves are shifted by 0.5 for clarity.

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In order to account for the experimental parameters, a simple model for the chromatophore membrane was considerd (see Fig. 5). According to biochemical results, the model contained 15 functional units and from 13 to 20 F1F0 complexes [38,40,44], i.e., from 0.5 to 2 ATPase complexes per functional photosynthetic unit. The lipid bilayer consists of a paraffinic layer 30 Å thick of scattering length density of −0.29 10−14 cm Å−3 between two 5 Å thick layers for the lipid headgroups of scattering length density of 2.4 10−14 cm Å−3 [42]. All proteins were assumed cylindrical in shape and according to Rost and coworkers [43], were separated in a transmembrane fraction and in two outward-facing parts of height hout. The F1 part, of height L, was considered to protrude on both sides of the membrane. Considering the aminoacid composition and scattering data reported by Jacrot and Zaccai [45], mean scattering length densities of 2.5 and 3.1 10−14 cm Å−3 for the transmembrane and for the outer and F1 sectors were calculated, respectively. These values were determined assuming that 80% of the labile hydrogens exchange with the solvent. Moreover, to simulate the effect of EDTA washing, in the model membrane of the F1-removed chromatophores, the content in F1 was 80% reduced, according to the measured residual ATPase activity. The model is shown in Fig. 5: it should be however clear that the membrane models for the native and the F1-removed chromatophores only differ in the F1 content.

A trial-and-error procedure was used to determine the h out and L model parameters from the experimental thickness D0, see eq. (56). The parameters α and β were then obtained solving Eqs 57 and 58. The results are reported in Table 2: the ATPase F1 fragment projects out from the membrane from 60 up to 100 Å, while the outer portion of the other protein components protrudes 10- 15 Å on both sides of the lipid bilayer. Calculation shows that the asymmetry experimentally detected for native chromatophores is significant and that the observed value of β could be accounted for the presence of the F1 sector protruding on one side only of the membrane.

As a conclusion, these results strongly support the notion that energy coupling membranes are asymmetrical and that this is mainly due to the presence of the F1 sector of the ATPase complex.

3.2 Shape reconstruction of model particle In order to illustrate the capability of the multipole expansion method (sec. 2.9) to recover the particle shape and size, we have generated scattering data of four regular solids, such as cylinder, double-cone, ellipsoid and cube (Fig. 6), all with a gyration radius R g=30 Å, as shown in [16]. The number of parameters, M, together with the volume VP and surface SP of the particle are reported in Tab. 3 for all the four solids. Pseudo-experi-

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Fig. 11. SANS profiles for the TG-ase in different experimental conditions. The curves are scaled for clarity. From the bottom: nTG-ase without ligands; with 1.8 mM of CaCl2 (scaled by 5); with 0.4 mM GTP (scaled by 25). The solid blue lines correspond to fits SANS calculated by using the Monte Carlo method described in the text. Left bottom shape: three-dimensional representations of the structure of tissue-type TG-ase. The four domains assembled into two peptides p56 and p31 of the enzyme are shown in different colours. The N-terminal p56 peptide includes the domains #1 (orange) and #2 (yellow), while the C-terminal p31 peptide #3 (cyan) and #4 (blue). The active site, hidden in a narrow cleft between these two regions, is shown in red. The exposed loop, which is the preferred site of cleavage by proteinases and connects the domains #2 and #3, is shown in black. Right side of the curve: shapes of possible structures of TG-ase reconstructed from SANS profiles using a Monte Carlo method and starting from the computer designed model and molecular dynamics results. Note also that the high resolution of the structural representation is not derived from SANS data.

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Fig. 12. Probability distribution of the TG-ase conformational angle ψ obtained from the Monte Carlo analysis of the SANS data (ψ is the angle between the longest axes of the two peptides p31 and p56). The distributions were calculated by sampling the Euler angles space Ω2 and considering acceptable only conformations giving a goodness of fit χ2 ≤ 2.

mental scattering curves [dΣ/dΩ](Q) have been then calculated by the expansion coefficients {fl,m} using Eq. 44 in NQ = 80 points until Qmax = 0.5 Å-1 (i.e. QmaxRg = 15). The maximum rank used was L=6. For each point, we have chosen an error σi=10-2 {[dΣ/dΩ](Qi)}1/2, assuming a poissonian statistics. The reconstruction of the shape functions FS(ωr) has been then performed by finding the best {al,m} set which minimizes the reduced χ2

(59) where P(Q) is determined from Eq. 37; κ and B are a scaling factor and a flat background, respectively.

Cylinder Doublecone Ellipsoid Cube

2

χ

G exp/fit

0.218

D∞h

4

4

29.96 ±0.04

194.5 195.3±0.5

0.151 0.220 0.181

D∞h D2h Oh

4 10 5

3 6 5

29.88 ±0.03 30.07 ±0.08 29.88 ±0.04

226.4 225.8±0.6 154.9 154.4±0.8 221.3 220.2±0.7

M exp fit

Rg ( Å) fit

VP exp

3

3

(10 Å ) fit

Table 3. Pseudo-experimental (exp) and recovered (fit) main particle parameters. For each particle shapes, the χ2 functional, the reproduced point group symmetry G, the number of Maximum Entropy parameters M, the gyration radius Rg and the volume VP are reported. The radius of gyration of the input particle was 30 Å.

The numerical procedure starts from M = 1 optimazing one parameter a 0,0 only. Subsequentely, further steps

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with higher number M were considered. It is important to point out that by increasing M, the group degeneration gradually vanishes. After having considered all the groups, we select that one with the best χ2.

The pseudo-experimental scattering and the recovered scattering curves [dΣ/dΩ](Q) are shown in Fig. 6 for all the solids considered. It is noticeable that, in all cases, the point group recovered from the best fit coincides with the original one. Moreover, the agreement between experimental and fitted data is found to be excellent. The reconstructed shape functions FS(ωr) are also reported in Fig. 6, showing a very good agreement with the original ones. In summary, the results obtained are in excellent agreement with the experimental data for all the considered shapes. It is important to point out that the determination of the shape functions was in some cases possible even with a lower number of parameters than those used during the simulation, without loosing particle structural details.

The procedure has been then applied to reconstruct the shape of the 16S haemocyanin protein in solution from SAXS data measured at the ELETTRA synchrotron facility, Trieste (Italy). 16S haemocyanin is an oxygen transport protein in arthropods with molecular weight of 750 kDa, constituted by six subunits arranged as two trimers superimposed each on the top of the other, staggered of 60o. The results are showed in Fig. 7. The correct symmetry D3d has been rendered using only M = 4 parameters and the recovered shape is in excellent agreement with the Protein Data Bank [46] structure also shown in the figure.

In conclusion, the low resolution feature intrinsic in the SAS technique reflects in the proposed method: by decreasing the maximum experimental Q value, the procedure renders automatically particle shapes with increasing symmetry, i.e. poorly defined.

3.3 Monte Carlo methods 3.3.1 Micelle shape The case of micellar systems constitutes a good example of multi-domain particles. Direct micelles are usually formed by amphiphilic molecules (lipids, detergents) in water: the inner micellar region is filled by the hydrocarbon chains of the amphiphilic molecules, and a polar shell, where their hydrophilic heads lie, separates the paraffinic core from the water medium. In the case of X-ray scattering, the core has a lower electron density than the solvent, but the interface has a higher electron density. A micelle is seen in this case as a hollow shell and the dominant peak comes from intra-particle interference. In this section we will first check and then apply the Monte Carlo method to reconstruct micelle shapes by analysing SAS curve. We neglect any interference effect (Seff(Q) = 1). For a detailed discussion see [29].

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The analytical form factor of an ellipsoidal micelle with inner half-axes (c,c,a) and polar head group thickness, δ, was reported by Chen and coworkers [47]. First, we try the Monte Carlo method for reproducing the analytical expression. The electron densities for the micellar model were fixed to typical surfactant solution values: ρpar=0.28 e−/ Å3, ρpol=0.42 e−/ Å3, ρS = 0.33 e−/ Å3 [29]. The geometrical parameters were δ = 4.6 Å, c=16.7 Å, and a=41.8 Å. The results of the Monte Carlo calculations performed on a prolate ellipsoid are reported in Fig. 8 togheter with the analytical expression of the form factor. The comparison between the two curves indicates that the agreement is excellent. In Fig. 9 the Monte Carlo form factors for other two-domain particle shapes, such as oblate ellipsoid, spherocylinder, biaxial ellipsoid with c=1.2b, and disk-like forms are reported. For all shapes, the semi-axis b was fixed to 16.7 Å. Taking as reference the paraffinic volume of a spherocylinder with a=2b, the semi-axis a for all the other forms was chosen such as to reproduce the reference paraffinic volume. The first peak of the form factors of different shape does not present any change in position (Q ≈ 0.15 Å−1) but the heights are very much depending on the particle shape; on the other hand, the second peak position and height depend on the particle shape.

The Monte Carlo method presented in section 2.6 was used to fit data obtained at the LNLS synchrotron (Brazil) on the micellar system Sodium Dodecyl (Lauryl) Sulfate (SLS) in water. We report the fitting obtained with 1 wt % concentration (which required synchrotron intensities), in the Q range free from influence of the direct beam (Fig. 10). Monodisperse and polydisperse systems of particles of different shapes, i.e. spheres, oblate, prolate and biaxial ellipsoids, spherocylinder and disks, were used for the fit. The results of the fits for the four best models are given in Fig. 10 and Tab. 4. It is important to point out that the fit with a polydisperse model gave better agreement with the experimental data than the monodisperse one, probably due to the absence of the second peak in the SCS which is related to the particle shape. In all cases best fits were obtained using ρpol=0.42 e−/ Å3. Moreover, attempts to fit the curve with oblate ellipsoids and disk-like shapes gave poor results.

The similarity of the prolate ellipsoid and the spherocylinder fits can be appreciated in Fig. 10. Both forms give the best fits, with similar values of χ2. However, the following differencies are to be pointed out: the a axis is smaller for the spherocylinder and the different κ values refer to different number densities n and volume of the scattering particles.

In conclusion, the results show that our approach can be successfully used to obtain information about the shape of multi-domain scattering particles. In the case of the binary SLS/water system only spherocylinder and prolate

ellipsoids provide possible models with only slightly differencies in dimensions and polydispersity. This information can be very useful for further investigations changes in shape and dimensions of micelles, particularly as predicted to occur in mixed micelles [48] and other systems of biochemical interest.

SP PE BE SC

a (Å) 18.4 27.6 27.8 24.9

ξa 0.11 0.15 0.17 0.14

b (Å) − − 16.7 −

c (Å) − 16.7 16.6 16.7

ξc

χ2

− 0.07 0.13 0.04

0.565 0.107 0.107 0.110

κ (a.u.)

2.11(3) 2.30(4) 2.38(4) 2.20(4)

B (a.u.)

1.92(1) 1.89(1) 1.88(1) 1.92(1)

Table 4. Fitting parameters (semi-axes a, b, c and dispersions ξa, ξc) obtained from the Monte Carlo analysis of the system SLS/water. The fixed parameters are in bold. Shape models are: sphere (SP), prolate ellipsoid (PE), biaxial ellipsoid (BE) and spherocylinder (SC). χ2 is the merit functional, κ the scaling factor and B the flat background.

3.3.2 Ligand-Induced Conformational Changes in Tissue Transglutaminas Tissular transglutaminases (TG-ase) are monomeric proteins of molecular mass of about 80 kDa, which act as bifunctional enzymes to catalyze either the post-translational modification of proteins at glutamine residues [49] or the transduction of extracellular hormonal signals [50].

Apparently, this dual role is carried out by distinct conformations of the protein, stabilized by the interaction with the ligands Ca2+ and guanosine triphosphate (GTP), in relation to different cellular physiologic processes. At the normal physiologic concentration of calcium (the essential activator) and GTP (an allosteric inhibitor), the enzyme is kept inactive; under the condition where the Ca2+ concentration is raised and the cell GTP declines, such as in an irreversibly damaged cells, the enzyme becomes active.

The crystallographic structure of TG-ase is not known, but much has been learned about possible structural changes in the secondary and tertiary structures. In particular, Ca2+ binds to relatively high-affinity binding sites (up to six), activating the enzyme through conformational changes which allow exposure of the active site to the incoming protein substrate [51]. On the contrary, GTP binds to a single site, hampering Ca2+ binding and related structural modifications. Recently, a computer designed model for the structure of the tissue-type 2 TGase has been proposed and validated by means of SANS [20]. The data indicate that the protein can be approximated by a prolate ellipsoidal shape with axis lengths of 62, 42 and 110 Å and consists of four domains, assembled in two pairs which can be separated into N- and Cterminal regions by limited proteolysis (Fig. 11), i.e. into the p31 and p56 peptides. The active site is buried in a cleft between the two regions, hidden from the contact

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Fig. 13. SAXS data for βLG at pH=2.3 and concentration 10 gl−1. The ionic strength of the solution is written above each curve. The continuous green and blue curves have been obtained by the global fitting procedure using the 0th- and the 1st-order approximation, respectively. Bottom and top shapes: threedimensional representation of the monomer and dimer of βLG, respectively.

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In the case of the nTG-ase, because the SAS data indicate a particle with radius of gyration larger than that one calculated from the crystallographic structure the scattering intensity was calculated from the atomic model taking into account the presence of a border shell attributed to the mobility of the protein surface [28]. According to Eq. 24, only one free parameter was adjusted in the fitting procedure, the width of the gaussian (σH) used to describe the particle at the border [28]. The good fit shown in the Fig. 11 and in Tab. 6 was σH = 4.40 ±0.05 Å.

Rg

[dΣ/dΩ](0)

n

(Å)

(cm−1)

(1016 cm−3)

% diff.

nTG-ase

31.9 ±1.4

0.108 ±0.008

1.07±0.08

27

nTG-ase/Ca2+

38.3 ±1.3

0.375 ±0.013

3.71 ±0.13

5

nTG-ase/GTP

29.9 ±0.7

0.140 ±0.010

1.36 ±0.10

7

Table 5. SANS experimental results on TG-ase samples in different experimental conditions. Rg is the gyration radius, [dΣ/dΩ](0) the forward cross sections, calculated by the Guinier approximation. The number density of scattering particle, n, was calculated using Eq. 17. The difference between the nominal and the experimental concentration is also shown.

Fig. 14. Radial distribution functions between particles of species i and j (i,j=1,1 red; i,j=1,2 green, i,j=2,2 blue) obtained by the global fitting procedure of the SAXS data for βLG at pH=2.3 and concentration 10 gl−1. The ionic strength of the solution is written above each set of curves. Left and right plates: results of the 0th- and the 1st-order approximation, respectively.

with the solvent or with the macromolecular substrates.

The model presents an interesting feature: 50 ps protein dynamics show that the two protein regions move apart upon addition of Ca2+, thus disclosing the active site for catalysis [20]. These observations suggested that conformational changes induced by calcium regulate the enzyme activity.

In this section we summarise SANS experiments performed on TG-ase in solution and analyzed using the Monte Carlo simulation technique in order to obtain molecular details of the conformational changes [21]. Native TG-ase samples at concentration 5 g l −1 were then analysed pure and after addition of CaCl2 2.5 mM and of GTP 0.5 mM. SANS experiments were performed at room temperature using two different instruments, namely the V4 diffractometer of the Hahn-Meitner Institut (HMI) in Berlin (Germany) and the KWS II diffractometer of the ForschungsZentrum Julich (FZJ) in Julich (Germany) (Fig. 11). Gyration radii and [dΣ/dΩ](0) values were derived using Eq. 17 and the results are listed in Tab. 5. In particular, while GTP (the inhibitor) only slightly reduces Rg of the nTG-ase, Ca2+ (the activator) increases it from 31 to about 38 Å.

To fit the experimental scattering data observed in the presence of GTP, Ca2+, we simulated the scattering volume of TG-ase by moving different regions of the protein from the position occupied in the computer-designed model. However, in order to reduce the number of possibility, two relevant results were considered. First, 50 ps protein dynamics realized in the presence of Ca 2+ showed that after activation, the p31 peptide moves away from the p56 peptide [20]. Second, the present SAS data show that the cleavage of the peptide chain at the exposed loop interferes with the conformational changes. Therefore, we analyzed all possible conformations obtained by rotating around the flexible loop and in all directions the peptide p31. Hence, the free parameters in the protein model were only the three Euler angles Ω2, which describe the position of the p31 peptide with respect to the p56. In order to obtain a more accessible parameter to describe the protein conformation, we resort the angle ψ between the longest axes of the p31 and p56 peptides. In the computer-designed model, the ψ angle is found to be 34.6o. The Euler angles have been sampled using NR=20000 points by a Monte Carlo method in the three-dimensional space α , cosβ , γ . According to the standard numerical methods, a solution (i.e. a set of angles Ω2) is considered to be acceptable when the corresponding χ2 is lower than 2. The final result is then described by the distribution function H(ψ,χ2 ≤ 2) (Fig. 12) giving the probability to obtain an accettable conformation with an angle ψ.

To check the sensitivity of the method, we first re-analyzed the data obtained from the pure nTG-ase fixing

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σH=4.40 Å and sampling the Ω2 angle set. The resulting distribution, shown in Fig. 12, presents a maximum around ψ = 45o. However (in connection to the low resolution of the scattering data) all the other conformations appear populated as well. In the case of the nTGase/GTP sample, the distribution (see again Fig. 12) is rather similar to that one obtained for the pure nTG-ase but a significative difference appears in the increasing population of conformations with ψ lower than 40o. As an example, the fitting curve relative to one of these solutions (see. Tab. 6) and a view of the corresponding TGase conformation is reported in Fig. 11. σH (Å) nTG-ase 4.40 nTG-ase/GTP 4.40 nTG-ase/Ca2+ 4.40

Ω2 (deg) (0,0,0) (267,13,107) (245,29,69 )

ψ (deg) 34.6 20 64

χ2

κ (cm−1) 0.212 0.196 0.258

1.02 0.82 1.61

B (cm−1) 0.0448 0.0429 0.0456

Table 6. Results of the analysis of SANS data on TG-ase samples in different experimental conditions by using the Monte Carlo method described in the text. σH is the width of the gaussian describing the particle border; Ω2 the Euler angles describing the position of the p31 peptide with respect to the p56; ψ the angle between the longest axes of the p31 peptide with respect to the p56. χ2 is the merit functional, κ the scaling factor and B the flat background. The fixed parameters are in bold.

For the Ca2+-activated structure the distribution shows a clear shift towards larger ψ angles (see Fig. 12). In particular, conformations with ψ lower than 50o are not compatible with the experimental data. Even if the distribution shows a maximum at about 130o, to show the smallest difference with the native conformation, we describe one of the solutions belonging to the first significatively populated range (ψ = 60 ÷70o). In Fig. 11 the protein conformation with ψ = 64o is shown: the corresponding fitting parameters are reported in Tab. 6 and the fitting curves in Fig. 11. In Fig. 11 the widening of the cleft which makes the active site available can be clearly appreciated. It should be noticed that this conformation is already enough to accommodate macromolecular substrates: conformations with larger ψ are more and more favourable.

It is also important that these results confirm recent conclusions obtained by immunoreactions with antibodies and site directed mutagenesis [50]: in particular, our data support the idea that the role of GTP in TG-ase activity is also related to the integrity of the C-terminal region. According to the deduced conformational change, any modification in the C-terminal sequence might also result in structural and functional differences which would affect the GTP binding.

3.4 Protein-protein interaction: the βLG case The study of protein-protein interactions in solution and

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the determination of both the physical origin of long range interactions and the binding geometry can provide the most effective way of correlating structure and biological functions of proteins. A detailed analysis of the long-range interactions has been so far limited to few colloidal systems and has usually been based on light scattering or osmotic stress methods. However, SAS is certainly the most appropriate tool to study the whole structure of protein solutions.

A recent paper of some of us [22] reported a photon correlation spectroscopy and SAXS analysis of the structural properties of a protein, the β-LactoGlobulin (βLG), in acidic solutions (pH 2.3) at several values of ionic strength in the range 7-507 mM. Both experiments gave a clear evidence of a monomer-dimer equilibrium affected by the ionic strength. Here, we consider a set of SAXS data on βLG under the same experimental conditions but for an enlarged Q range. As the ionic strength decreases and the electrostatic repulsion between charged macroions become stronger, a lowering in the scattering intensity with a progressive development of an interference peak is clearly observed at small angles.

The monomeric unit of βLG is composed by 162 amminoacid residues and has a molecular weight of 18400 Da. By considering the basicity of the amino acids, at pH=2.3 the monomer charge would be about 20e . The crystallographic structures of βLG both in monomeric and in dimeric form found in the Protein Data Bank [46] are ketched in Fig. 13. It can be observed that all 20 basic amino acids are on the protein surface, but two of them are at the monomer-monomer interface; therefore at pH=2.3 the ratio Z2/Z1 between the dimer and monomer charges could be about 1.8.

3.4.1 Experimental details A bovin milk βLG B stock solution (C=40 g l−1) was obtained by ionic exchange of protein samples against a 12 mM phosphate buffer (ionic strength I S = 7 mM and pH=2.3) [22]. Nine samples at ionic strength from 7 to 507 mM were then prepared by adding appropriate amounts of NaCl. The final protein concentrations were about 10 g l−1. SAXS measurements were collected at the Physik Department of the Technische Universitat Munchen (Germany) using a rotating-anode generator. The radiation wavelength was λ = 0.71 Å and the temperature 20 ° C . The Q range was from 0.035−0.1 Å−1.

The single and pair distribution functions, p(1)i(r) and pi(r), respectively, of both monomers and dimers have been calculated from the crystallographic structure using a Monte Carlo method, outlined in section 2.6. Then the form factors < Fi(Q) >ωQ and < Fi2(Q) >ωQ have been obtained through Eqs. 19-21.

The partial structure factors have been calculated from

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the simplest possible model potential which is still capable of capturing the essential features of the system. It will be the sum of two repulsive contributions:

(60) Where (61) is a hard-sphere (HS) term which accounts for the excluded-volume effects (Ri being the radius of species i), and

(62) represents a screened Coulomb repulsion between the macroion charges, which are of the same sign. Here, e is the elementary charge, ε the dielectric constant of the solvent, and the effective valency of species i, Zi, may depend on the pH. The inverse Debye screening length κD depends on the ionic strength IS.

3.4.2 Data analysis According to the dissociation free energy model described in Ref. [22], the monomer molar fraction a is a function of the ionic strength IS. This suggests the possibility of a simultaneous fit for all SAXS intensities curves, using just few parameters, all independent of IS. In particular, the following parameters have been fixed: the monomer and dimer ``bare’’ radii, R1=19.15 Å and R2=21/3 R1; the dielectric constant of the solvent, ε = 78.5; the experimental temperature, T=293 K; the ratio between the effective charges of dimer and monomer, Z2/Z1=1.8 [22].

In the global fit the only free parameters are therefore Z1 and ∆Gnel, the non-electrostatic free energy. The merit functional to be minimized was defined as

(63) where N S is the number of scattering curves under analysis, NQ,h is the number of experimental points in the h-th curve, and σh,i is the experimental uncertainty on the intensity value at the Qi. [dΣ/dΩ]hfit(Qi) is the corresponding SCS predicted by the model and calculated by Eq. 7; for each experiment, the calibration factor κh and

the flat background Bh have been adjusted from a linear least-squares fit of [dΣ/dΩ]hexp(Q).

3.4.3 SAXS results Fig. 13 depicts the experimental results for the X-ray intensity [dΣ/dΩ](Q) as a function of the transferred momentum Q at different ionic strengths. The appeareance of an interference peak as IS decreases, is evident and it is a consequence of the increasing effective protein-protein interaction. In the same figure, the comparison between the 0th- and the 1st-order approximation of the pair correlation functions is also displayed. The quality of the 1st-order approximation fit indicates that our approach does reproduce the main features of the proteinprotein interactions. A more transparent comparison between the two approximations, can be carried out at the level of RDFs. This is done in Fig. 14, where the gij(r), (i,j=1,2) are computed using the 0th-order and 1st-order approximations, respectively. We emphasize that in the 1st-order approximation, the fact that gij(r) > 1 in some regions (mainly for IS ≤ 27 mM), indicates the presence of an induced attractive protein-protein interaction due to the osmotic ``depletion’’ effect exerted on two given proteins by the remaining ones. This effect is clearly lacking in the 0th-order approximation as it can be clearly seen in Fig.14. The parameters resulting from a global best-fit procedure, are Z1=16 ±1 and ∆Gnel/kBT=13.1 ±0.6, which produce a merit functional χ2=0.92. These values are in good agreement with the corresponding ones obtained in a previous work [22].

Two features are particularly noteworthy. First the proposed procedure is able to take into account both the structure factor and the form factor for highly asymmetric particles - such as the case of a protein - suspended in a weakly-diluted solution, in a rather simple but physically sound way. Second, the global fit is able to allow for relevant parameters describing both the equilibrium properties (non-electrostatic dissociation free energy) and the solution protein-protein interactions (monomer charge Zi, etc) which are independent of the solution ionic strength.

Acknowledgement We are indebted to all the colleagues who contributed to our research activity. The long list of their names can be easily found in the referenced common works.

The SAS group at the University of Ancona is directed by Franco Rustichelli. The group consists of 6 scientists, 4 of them mainly devoted to the analysis of biological systems. Recently, the research activity was focused on protein folding/unfolding processes, on protein-protein interactions and aggregations and on protein hydration.

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Articolo ricevuto in redazione nel mese di Novembre 2001

STRUCTURAL STUDIES OF CONDENSED MATTER UNDER EXTREME CONDITIONS OF HIGH PRESSURE AND TEMPERATURE COMBINING X-RAY ABSORPTION SPECTROSCOPY, POWDER DIFFRACTION AND TEMPERATURE SCANS. A. Filipponi Unità di Ricerca INFM and Dipartimento di Fisica, Università di L’Aquila, 67010 Coppito, L’Aquila, Italy

Abstract The experimental devices and scanning modes available at the BM29 beamline of the European Synchrotron Radiation Facility are illustrated, including the latest advances developed within the HS1197 long term project and the INFM project PURS008. Examples of applications of the energy scanning xray diffraction and x-ray absorption temperature scans techniques are presented. The potential of this setup for investigations on condensed matter under extreme conditions, which is of interest in a wide scientific context, is emphasized.

Introduction The availability of third generation synchrotron radiation sources, such as the European Synchrotron Radiation Facility (ESRF), characterised by small source dimensions (with a typical size of 150x300 micrometers VxH) [1] and small vertical divergence (1/γ ~ 0.0001 radiants), has stimulated the development of advanced experimental setups for studies of condensed matter under extreme conditions of high pressure and temperature.

The interest in experimental setups which combine x-ray absorption spectroscopy (XAS) with other techniques covers a wide range of scientific disciplines ranging from physics, chemistry, biology and earth sciences. The major achievements and results in the physics of liquid matter have been the subject of a recent review article [2].

The electron beam characteristics of third generation sources are valuable not only for insertion devices but also for bending magnet (BM) sources which provide an intense continuous spectrum of x-ray radiation suitable for energy scanning (spectroscopic) experiments and usable up to even 100 keV (ESRF). In this article I will review the experimental station developed at the BM29 beamline of the ESRF [3] which combines the possibility to perform high-quality x-ray absorption spectroscopy (XAS) with temperature scanning techniques [4] and xray powder diffraction detection using a novel energy scanning method (ESXD) [3]. This latter detection option has been recently enhanced in the framework of the PURS008 INFM project with the development and construction of a multi channel detector assembly, which is

going to be made available to public users in the framework of an agreement between the INFM and the ESRF. The BM29 experimental setup is equipped with advanced devices able to generate non standard sample environment conditions of high-pressure and temperature up to 10 GPa and 3000 K [3]. Typical examples of the applications of the above mentioned techniques to research topics in the field of condensed matter will be illustrated.

The BM29 Experimental Station In its simplest configuration BM29 [5,3] exploits a hard BM source of the ESRF through a double flat crystal monochromator, typically a Si (311) (updated information on BM29 can be found at http:// www.esrf.fr/exp_facilities/BM29/BM29new/bm29control.html). The beam size is usually defined by the vertical primary slits (set typically to 0.2 mm) and by horizontal secondary slits (closer to the sample), up to 10

Fig. 1. Pressure (P) and temperature (T) ranges of operation for the major experimental devices available at the ESRF-BM29 beamline.

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mm wide. The horizontal primary slits are piloted through a background process to maintain constant the heat load on the primary crystal during the beam decay [3]. Even in this very simple configuration a flux of the order of 109 photons/s is obtained on the sample, which is fully sufficient to collect high quality x-ray absorption spectra with noise level of 10-4 or better in the timescale of one hour/spectrum. While a suitable mirror/bender system could have increased the photon density on the sample by even two orders of magnitude, the advantage of a mirror-less setup is in the possibility to operate the beamline also at high energies and in the simplified alignment procedures.

The BM29 experimental setup offers various sample environments to perform experiments under non standard pressure and temperature conditions. The operation regions of the major experimental devices in the PT plane are illustrated in Fig. 1, and include: a) The L’ Aquila Camerino oven [6] (red). b) The Paris-Edinburgh press [7] (purple). c) A Displex cryostat 20-300 K, heatable to ~400 K (blue). d) A tubular oven (built by the BCD company), for gas phase samples sealed in Pyrex or quartz cells [8] (green). e) A pressure cell system for water solution measurements [9] (light blue). f) A low pressure autoclave for fluid iodine samples [8].

This setup offers a wide variety of scanning modes which can be combined in sequence according to the experimental needs, in particular: 1) Fast x-ray absorption scans in the so called QuickEXAFS mode (10-30 s) 2) High quality x-ray absorption scans (for standard XAS experiments, typical acquisition time 0.5-2.0 s per point) 3) Sequences of identical rapid scans of type 1) or 2) to monitor sample transformations (for instance while changing the temperature) 4) Absorption detection at a fixed energy point (or through a sequence of points) while scanning the sample temperature [4].

5) X-ray powder diffraction detection using the energy scanning mode (ESXD) [3].

The excellent performances of the beamline monochromator and the quality of the x-ray absorption scans has been recently exploited to apply a core-hole width deconvolution procedure to enhance hidden spectral features [10]. In addition to XAS, scanning modes of type 4) and 5) are (at present) uniquely available at BM29.

As it will be shown later, they are extremely powerful methods to improve the sample characterization and are often essential for a correct management of the experiment.

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The Multi-Channel ESXD Detector The Energy Scanning X-ray Diffraction (ESXD) method was introduced in previous publications [3] as well as the principle of a multi-channel detector system [11]. In the transmission setup the monochromatic direct beam will be partly absorbed by the sample, but also, in a small part, diffracted. The crystalline powder components of the sample will generate Debye-Scherrer rings which carry fundamental structural information complementary to the one measurable by XAS, (see Fig. 2). Various ways can be in principle used to detect a powder diffraction pattern, including a standard angular scanning setup or a position sensitive detector. High-pressure x-ray diffraction setups such as those available at the ESRF ID9 and ID30 beamlines are based on the usage of an image plate detector system. At BM29 we developed a novel concept based on the usage of a two slit collimator system which is aligned with respect to the sample at a fixed 2θ angle. Photon counting detectors are placed behind the rear slit. The diffraction scan is performed using a high precision goniometer already available at the beamline: i.e. the monochromator, by per-

Fig. 2. Schematic representation of the transmission geometry for absorption and diffraction detection. The arrow on the cones represents the diffraction angle shrinkage upon increasing the energy of the incident beam.

forming a monochromator energy scan. When the monochromatic beam energy is increased the radius of the Debye-Scherrer rings will shrink and eventually will intercept the collimator/detector acceptance fan. The diffraction patterns are therefore recorded as a function of energy, similarly to an energy dispersive diffraction setup (where the diffraction of a white beam is recorded simultaneously as a function of energy, using an energy sensitive solid state Ge detector), but not simultaneously. This collection strategy is not efficient, actually only a small portion of the Debye-Scherrer ring is intercepted by the rectangular slit, which approximates a small arc with a cord, but the resolution can be quite good since the collimator length can be greater than 1 meter (peak widths of DE/E=0.001 have been achieved). Moreover the absence of any moving part guarantees high stability and reproducibility. In conclusion, this strategy turned out to be

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the most efficient when x-ray diffraction detection is to be combined with XAS on a standard energy scanning beamline.

The q-range accessible with a single collimator/detector channel is defined by the angle and by the extension of the energy scan. The usable energy range is limited by the detector sensitivity, source/monochromator characteristics, but also by the sample nature. A reasonable scanning range at BM29 is between 15 and 30-45 keV giving a range of 2-3 between the minimum and maximum q. The multi channel collimator project was undertaken with the idea to extend the q-range of detection placing detectors at a set of different 2θ angles and to improve the overall efficiency. For example detectors can be placed at fixed angles in such a way that Bragg peaks of cubic systems appear in various detectors in the same energy range using scans only a few 100 eV wide. If the lattice spacing changes, the peaks will continue to appear simultaneously in a slightly shifted energy range.

After the approval of the project PURS008 and funds availability in Jan 2001 a six channel detector system was designed and built in the record time of 5 months with the technical help from personnel of the University of L’Aquila (see acknowledgments). The system was assembled using commercial products including Slits from Huber gmbh, miniature translation stages from Microcontrole, and aluminum profiles from Item Italia s.r.l.. From the detector side various kinds of x-ray detectors for photon counting can be hosted behind the rear slit of the collimator. In particular, standard NaI(Tl) scintillation detectors can be integrated in our setup.

The cost of a typical 38-50 mm NaI(Tl) with standard NIM electronics and HV power supply ranges between 3000 and 8000 euros.

We have, however, explored and adopted a different detector strategy which was never implemented in a largescale synchrotron radiation application. We have used CdZnTe (CZT) solid state detectors from the eV-products company (model eV-2801) [12]. These detectors are equipped with a typical 2 mm thick, 1.8x32 mm wide crystals which are suitable for photon counting at room temperature for E>10-15 keV. The detectors include all the electronic chain and generate TTL pulses suitable for counting purposes at a total cost much lower than scintillation detectors. The size of the detectors matches the typical slit aperture simplifying the screening requirement with respect to standard scintillation detectors and resulting in a reduction of background counts. The HV required for the detector bias is in the 200-250 V range which is much cheaper than the one required for PMT’s.

The final multi-channel detector assembly results in a versatile and highly configurable setup which exploits the flexibility in the aluminum profile fixation system.

Typical arrangements showing a 4-channel configuration

setup collecting diffraction from a sample in the hightemperature oven on the vertical plane is shown in Fig. 3. A 6-channel configuration around a sample confined in the high-pressure Paris-Edinburgh press setup is shown in Fig. 4. The diffraction is collected on the horizontal plane because the vertical fan is obstructed by the anvils, and on both sides with respect to the transmitted beam. In the Figs. 3 and 4 both the motorized front slits and the rear detector/slits units are clearly visible.

The alignment procedure is simple since it requires only one-dimensional scans. In 2-3 hours all of the 6 channels can be aligned to a single scattering point on a reference sample. This is sufficient for all subsequent experiments until a new angle setting is required.

ESXD scans An example of the performance of the 6-channel collimator developed within the PURS008 project is given in Fig. 5. This diffraction pattern was collected using the 6channel high-pressure setup of Fig. 4 and refers to an Ag powder sample. Many Bragg reflections of the f.c.c. lattice are clearly visible. The resolution and the consistency among the various channels can be appreciated.

This ESXD detection is a quite useful complementary technique for measurement and sample characterization on a standard XAS setup, and, in particular, it is able to provide the following information: 1) “In situ’’ check of all crystalline phases/components present in the sample, useful also to reveal/exclude contamination or decomposition [11].

2) Verify the absence of crystallization in a sample composed of undercooled liquid droplets [13].

3) Determination of the temperature and pressure on a sample using established equation of states of reference crystalline calibrants introduced in the sample [14].

4) Determination of lattice spacing with high-resolution Da/a<10-4 and of the corresponding equation of state of crystalline solids as a function of T [15] and P. An intrinsic limitation of this technique is the very poor powder statistics of the detection. This is the consequence of the high collimation of the synchrotron radiation beam and small detector acceptance, and it is the price to pay for the excellent resolution. In fact, with a randomly oriented powder only a small fraction of crystalline grains will be in the Bragg condition for a given geometry. For a coarsely grained sample the DebyeScherrer rings will be composed of discrete spots and only a few of them will be intercepted by the detector slits. Even in the case of a finely grained sample (typically of 1 micron size powder) the intensity of the diffraction peaks are not reliable and the main information provided by this ESXD detection is limited to peak positions. The requirement for a finely grained sample is also

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able for this setup. The size of the crystalline grains is sometimes subject to modification during the experiment. A typical case is that of a metal foil subject to an heat treatment. The growth of the single crystal domains at high temperature may result in the apparent disappearance of Bragg peaks. In the limit of a single crystal the probability to see the diffraction peaks vanishes. In spite of this limitation in many experiments the diffraction detection remains essential to characterise important sample properties. The limited powder statistics can be partly compensated for by using an oscillating sample setup.

Fig. 5. Powder diffraction pattern from an Ag sample confined the large volume high-pressure setup of Fig. 4 collected using the 6-channel ESXD detector.

essential for the x-ray absorption detection to achieve the required thickness homogeneity to avoid non-linear effects, so, coarsely grained sample are definitely not suit-

TSCAN Another auxiliary scanning mode which is of fundamental importance in several experiments is the possibility to perform controlled scans of the sample temperature while monitoring some physical property sensitive to phase transitions. The x-ray absorption coefficient at a selected energy provides a very useful probe of this kind. The main characteristics is that, at variance with

Fig. 3. A 4-channel collimator/detector for ESXD collecting scattered radiation on the vertical plane. The sample is inside the high-temperature Lâ&#x20AC;&#x2122;AquilaCamerino oven.

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the diffraction detection, all sample particles contribute to the absorption signal and the sensitivity to changes in the absorption coefficient can be as high as 1 part in 104.

Far from absorption edges the absorption coefficient is sensitive to the sample density which undergoes discontinuous changes, usually of a few percent, at first order phase transitions. This applies only to bulk samples confined in a fixed gap cell, and not to powder samples dispersed on a substrate or a matrix. The x-ray scanning capability can be however exploited to tune the photon energy to some feature at, or above, the absorption edges of some element present in the sample. The absorption cross-section itself is, in this case, sensitive to phase transitions since it also undergoes a discontinuous change at a first-order phase transition associated with transformations in structural and electronic properties. For instance, changes in the oxidation state or insulator to metal transitions induce changes or shifts in the edge shape. Structural changes are seen also in the x-ray absorption fine structure region.

Temperature scanning techniques have been early intro-

duced at BM29 [4] and have been widely exploited for several scientific purposes, including: 1) Detection of first order phase transitions (i.e. solidsolid, melting undercooling) [4,13] 2) Check of the temperature calibration and homogeneity on the sample.

3) Preparation of the sample in a well defined state, (i.e. metastable) [13].

4) Determination of the eutectic temperature and liquidus curves in binary eutectic alloys [4,15].

5) Measurements of the nucleation rate of crystallization in undercooled liquids as a function of temperature [16,17].

An example of the application of this technique is illustrated in Fig. 6, in the case of an AgGe mixture. Looking at the AgGe alloy phase diagram, at low T the sample is a two-phase mixture of, practically, pure Ge and the Ag:Ge terminal solid solution. Upon increasing temperature the solubility of Ge in Ag increases slightly in the solid phase until the eutectic temperature is reached.

Then a finite sample fraction melts including all Ag and

Fig. 4. A 6-channel collimator/detector for ESXD collecting scattered radiation on the horizontal plane around the transmitted beam. The sample is mounted on the Paris-Edinburgh press.

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field of investigation. Clearly this scans can be used also to prepare the sample in a well defined state, by stopping the cycle at the desired temperature, to perform successively XAS or ESXD. The possibility to use the crystalline nucleation region of these hysteresis curves to determine the nucleation rate of undercooled liquids has been widely discussed elsewhere [17].

Fig. 6. X-ray absorption temperature scan at the Ge K-edge of a AgGe alloy with Ge molar fraction (x~0.7). Upon increasing temperature the absorption increases reflecting progressive Ge melting at and above the eutectic temperature. After melting the homogeneous liquid alloy can be undercooled by over 100 K.

the appropriate Ge quantity to reach the eutectic composition, (the Ge fraction is above the eutectic composition). A fraction of Ge still remains crystalline. Upon increasing temperature the molten fraction of sample enriches in Ge following the liquidus curve of the alloy phase diagram, until all the sample is melted at the temperature of the liquidus curve corresponding to the macroscopic sample concentration. This progressive melting of the sample can be monitored in situ using the x-ray absorption technique. A particularly sensitive energy point is, in this case, at the Ge K-edge due to the edge shift associated with the semiconductor to metal transition occurring in Ge when it is dissolved in solid or liquid Ag. As the Ge component progressively dissolves in solid Ag or melts, the absorption coefficient at the Ge Kedge increases dramatically.

All these melting features are clearly visible in Fig. 6. In addition when the temperature is decreased from the molten state the absorption coefficient remains high indicating that the sample remains in a molten undercooled (metastable) state, till crystal nucleation occurs.

The hysteresis in the x-ray absorption temperature scan is a direct proof of the occurrence of metastable states.

From the experiment shown in Fig. 6 quantitative information on the AgGe alloy can be obtained. In addition to the determination of the eutectic temperature, the fraction of molten sample as a function of T can be estimated, which allows one to determine precisely a large portion of the liquidus curve (in a single experiment) [4].

The main interest in this particular application stems from the fact that it can be applied equivalently to samples under high pressure. This opens a completely new

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Ackonwledgments The ESRF BM29 beamline was originally designed by J. Goulon [5] and successively developed by several generations of scientists including N. Brooks, P. W. Loeffen, A. Filipponi, D. T. Bowron, S. Ansell, and M. Borowski (presently in charge) with the invaluable help of the ESRF technical staff S. Feite, L. Leclerc, J. Jensen, R. Consentino, S. Pasternak, A. Beteva, and R. Weigel. Other research staff (students and Post-Docs) include T. Ressler, S. Muellender, S. De Panfilis, F. Sperandini, G. Subias-Peruga.

The tubular oven was built by the company B.C.D. Sistemi S.r.l. and the autoclave was designed by prof. U. Buontempo (L’Aquila). The high pressure cell for aqueous solutions was designed and built by R. Weigel and D. T. Bowron [9].

The latest developments of the BM29 experimental setup were performed in the framework of the HS1197 long term project, in collaboration with M. Borowski, A. Di Cicco, S. De Panfilis and J.P. Itiè.

The multi-channel collimator for ESXD was built in the framework of the PURS008 INFM project by the workshops of the Dipartimento di Fisica, Universita` dell’ Aquila. The mechanical interfaces were built by the mechanical workshop head by O. Consorte, with the invaluable technical help of F. Del Grande. The power supply for the CZT detectors was assembled at the electronic workshop with the invaluable help of L. Ciuca.

References [1] P. Ellaume, “Parameters of Bending Magnet’’, ESRF internal note 91EL139, (1991).

[2] A. Filipponi, “EXAFS for liquids.”, J. Phys.: Condensed Matter 13, R23-R60 (2001).

[3] A. Filipponi, M. Borowski, D.T. Bowron, S. Ansell, S. De Panfilis, A. Di Cicco, and J.P. Itiè, “An experimental station for advanced research on condensed matter under extreme conditions at the ESRF - BM29 beamline.”, Rev. Sci. Instr. 71, 2422-2432 (2000).

[4] A. Filipponi, M. Borowski, P.W. Loeffen, S. De Panfilis, A. Di Cicco, F. Sperandini, M. Minicucci, and M. Giorgetti, “Single energy x-ray absorption detection: a combined electronic and structural local probe for phase transitions in condensed matter.”, J. Phys.: Condens. Matter 10, 235-253 (1998).

[5] J. Goulon, N. B. Brookes, C. Gauthier, J. Goedkoop, C. Goulon-Ginet,

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M. Hagelstein, and A. Rogalev, Physica B 208 & 209, 199 (1995).

[6] A. Filipponi and A. Di Cicco, “Development of an oven for x-ray absorption measurements under extremely high temperature conditions.”, Nucl. Inst. & Methods in Phys. Res. B 93, 302-310 (1994).

[7] J. M. Besson, R. J. Nelmes, G. Hamel, J. S. Loveday,G. Weill,and S. Hull, Physica B180 & 181, 907 (1992).

[8] U. Buontempo, A. Filipponi, P. Postorino, and R. Zaccari, “Density effect on molecular bond length in fluid I2 by x-ray absorption spectroscopy.”, J. Chem. Phys. 108, 4131-4137 (1998).

[9] D. T. Bowron, R. Weigel, A. Filipponi, M. A. Roberts, and J. L. Finney, “X-ray absorption spectroscopy investigations of the hydrophobic hydration of krypton at high pressure.”, Mol. Phys. 99, 761-765 (2001).

[10] A. Filipponi, “Deconvolution of the lifetime broadening from x-ray absorption spectra of atomic and molecular species.” J. Phys. B: At. Mol. Opt. Phys. 33, 2835-2846 (2000).

[11] A. Filipponi, A. Di Cicco, S. De Panfilis, A. Trapananti, J. P. Itiè, M. Borowski, and S. Ansell, “Investigation of undercooled liquid metals

using XAFS, temperature scans and diffraction.”, J. Synchr. Rad. 8, 81-86 (2001).

[12] eV-products is represented by Tecnologie Avanzate T.A. S.r.l.

[13] A. Filipponi, A. Di Cicco, and S. De Panfilis, “Structure of undercooled liquid Pd probed by x-ray absorption spectroscopy.”, Phys.

Rev. Lett. 83, 560-563 (1999).

[14] U. Buontempo, A. Filipponi, D.Martìnez-Garcìa, P. Postorino, M. Mezouar, and J. P. Itiè, “Anomalous bond length expansion in liquid iodine at high pressure.”, Phys. Rev. Lett. 80, 1912-1915 (1998).

[15] A. Filipponi, S. De Panfilis, and A. Di Cicco, “High-temperature xray absorption and diffraction investigations of Pd and Pd-C saturated solid solutions.”, Physica Status Solidi (b) 219, 267-277 (2000).

[16] S. De Panfilis, A. Filipponi, and C. Meneghini, “Local structure in crystalline and liquid Te probed by XAS.”, J. Synchrotron Rad. 6, 549551 (1999).

[17] S. De Panfilis and A. Filipponi, “Nucleation rate of solidification probed by x-ray absorption temperature scans in undercooled liquid metals“, J. Appl. Phys. 88, 562-570 (2000).

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NEWS FROM THE ESS CENTRAL TEAM Outcome of the 7th ESS General Meeting held in Seggau, Austria, September 2001 The European Spallation Source (ESS) project aims for a decision in 2003 to build a unique third generation neutron source, according to a schedule which will allow start of operation in 2010. That the ESS is in a very active state became evident at the 7 th ESS General Meeting held at Seggau Castle in Styria, Austria, on September 26- 29, 2001. About 130 participants and more than 50 oral presentations dealt with the design and technical aspects of the facility. The meeting focused on the progress of the ESS project and the efforts required to achieve the milestones of the project.

The first day was devoted to the presentation of the American and Japanese projects which are already under construction, and to an extensive overview of the status of the ESS project. The second day concentrated on detailed technical presentations and discussions of the ESS study. It was arranged in three parallel sessions – accelerator and ring, target stations and instrumentation.

Leading experts in the fields chaired these sessions, and presented summaries of them on the last day of the meeting. Science and science-political issues were also discussed. The main goal of the meeting was to progress towards a new ESS reference design.

Concerning the technical aspects of the project there are four different topics: the 10 MW accelerator, the compressor rings, the two target stations and the instrumentation. At Seggau it became clear that both a normal conducting and a superconducting linear accelerator could fulfil the specifications for the ESS. The normal conducting reference design has been worked on extensively; it is robust and feasible. The superconducting option has the potential to be a better solution but will require more detailed development work. As to the compressor ring, the meeting confirmed that the modified 1996 design was clearly the preferred solution. The ESS will have two individually optimised 5 MW target stations: (1) short pulsed at 50 Hz and (2) long pulsed at 16.7 Hz target. With this overall design the ESS will be unique and will to excel over the other projects in progress.

Latest news from the COUNCIL Chairman The Council has worked hard since the Abingdon meeting in June to progress the ESS project. We are now all set to work towards the ESS presentation next May 2002 (see elsewhere in the newsletter) and the work programmes for 2002 and 2003. A special council meeting took place at Schiphol Airport in October this year.

To do justice to the major tasks that are on our table and to the contributions of major partners, we have created a

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– December 2001

Project-Directorate of four directors (see elsewhere in the newsletter).

The accelerator part of ESS is an area where thanks to the work of the ESS team under Ian Gardner and the CONCERT team under Jean-Louis Laclare, we have a number of options; a sort of embarrassment of riches, to quote Jonathan Israel’s history of the Dutch Republic.

Where there are options there is debate! Therefore the council has indicated a procedure for arriving at a decision on what to present in May. We have a normal conducting solution that will do the job for us. But inside and outside the council there are many people who believe that a superconducting option might be superior.

The Seggau meeting, however, clearly pointed out that on current evidence the differences are not that clear cut.

So the council wants to see from the Project-Director, in collaboration with his colleagues in the Directorate, more evidence on the advantages of a superconducting solution. This will define the work for the next few months. The newly appointed Technical Advisory Committee that will meet at the beginning of January 2002 will certainly give critical input to the Project Directorate and the council on the various options. The council will then decide early next year on what to present.

This already introduces a third important element: the council decided on the composition of the TAC. You will find more details elsewhere in this newsletter.

There has also been a lot of discussion on the political position of ESS. There was an OECD Global Science Forum meeting in Copenhagen that was not only focused on neutron sources; there have been discussions on AUSTRON; ENSA has continued its work on the neutron landscape; in Germany the Wissenschaftsrat is gearing up for its assessment of some nine large projects in which German labs are involved, including the ESS project. In a later issue more space can be devoted to these important discussions. Two things stand out, however, very clearly. ESS is not only among the big projects under consideration in Europe, and the only truly European one, it is also a project that is grounded firmly in a widely accepted global strategy.

As the Copenhagen meeting agreed, the number of neutron sources has already declined faster than anticipated in 1990 when OECD ministers endorsed a global neutron strategy. This underlines the necessity for building the ESS now, to supply Europe’s community of neutron researchers in what will then be the top facility in the world. The hope that some people understandably express, that for example SNS can for a long time and in large measure accommodate European needs, was firmly contradicted by representatives from the SNS and the

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US DoE at the Copenhagen meeting. This points to a second issue: European users and facilities must accept that we have to focus our resources in order to develop and realise front ranking neutron facilities. This sometimes amounts to a hard message, but in the reality of governments and science agencies it is one that counts.

Peter Tindemans ESS Organisation From the organisational point of view, many things are changing within the ESS project management, which is now formed by the following committees: • Council • Executive Committee (EC) • Project Directorate (PD) • Science Advisory Committee (SAC) • Technical Advisory Committee (TAC) • Technical Management Team (TMT) - the former Core Group The most important change is the establishment of the Project Directorate, formed by four Directors: Kurt Clausen, Risø, Denmark; Ian Gardner, ISIS, UK; Jean Louis Laclare, CEA, France; Dieter Richter, FZ Jülich, Germany Jean Louis Laclare is the Project Director and has responsibility for the superconducting linac and infrastructures.

Kurt Clausen has overall responsibility for the targets and the instrumentation and for the ESS presentation in May 2002 in Bonn (see below). Ian Gardner has responsibility for the normal conducting linac and the overall costing of the project. Dieter Richter is the Science Director, with the role of promoting the ESS in the scientific community and continuing to develop the scientific case. The Directors manage all technical and science work for ESS as well as the support for publicity, external presentation and other areas relevant to the project.

The ESS Council is formed by representatives of the laboratories of the ESS collaboration of 11 different European countries.

ESS ORGANISATION MoU Partners 11 Europ. Countries

Technical Advisory Committee

INSTRUMENTATION

ESS Council

Executive Committee

Directorate

Central Project Team

Scientific Advisory Committee

TARGET SYSTEMS

RING & ACHROMAT

LINAC

BEAM TRANSPORT

CONVENTIONAL FACILITIES

Members A. Belushkin JINR Dubna, Russia; F. Barocchi INFM, Italy; R. Cywinski ENSA; R. Eichler PSI, Switzerland; M. Fontanesi CNR, Italy; F. Gounand CEA, France; R. Feidenhans’l Risø, Denmark; H. Klein University of Frankfurt, Germany; E. Koptelov Troitsk, Russia; H. Rauch Atominstitut Wien, Austria; D. Reistad University of Uppsala, Sweden; D. Schildt EPSRC, UK; U. Steigenberger Secretary; M. Steiner HMI, Germany; A. Taylor CLRC, UK; P. Tindemans Chairman; A. Verkooijen IRI, Netherlands; R. Wagner FZ Jülich, Germany; F. Yndurain CIEMAT, Spain The ESS Council includes a number of official observers: Observers T. Mason SNS Project, USA; S. Nagamiya JSNS Project, Japan; N. Williams ESF; NN EU. The Project Directorate also attends the council meetings.

The Executive Committee comprises the Council Chairman, the Council Secretary and the four Directors and forms the day-to-day link between the Council and the Directorate.

Technical Advisory Committee The Technical Advisory Committee contains five groups with a total of 17 scientists involved. Their role is to advise the ESS Council on technical aspects of the machine.

• Chairman: G. Lander ITU, Germany • Conventional facilities: J. Lawson SNS, USA; J.P. Magnien ESRF, France • Instruments: W. Press ILL, France; P. Böni FRM II, Germany; M. Arai JNS, Japan; D. Myles EMBL, France • Linac, beam transport: R. Garoby CERN, Switzerland; D. Proch DESY, Germany; J. Stovall LANL, USA; Y. Yamazaki KEK, Japan • Rings, painting and beam transport: H. Schönauer CERN, Switzerland; B. Weng BNL, USA • Target J. Carpenter ANL, USA; M. Furusaka KEK, Japan; K. Jones LANL, USA; J. Knebel KFK, Germany Scientific Advisory Committee The Scientific Advisory Committee consists of the representatives of each Science Group – and has 11 members. The role of the SAC is to advise the ESS Council on scientific aspects of the ESS applications.

• Chairman: D. Richter FZ Jülich, Germany • Biology and Biotechnology: J.R. Helliwell Univ. Manchester, UK • Chemical Structure, Kinetics and Dynamics: W.I.F. David ISIS, UK H. Jobic Univ. Lyon, France • Earth Science, Environmental Science and Cultural Heritage: R. Rinaldi Univ. Perugia, Italy • Fundamental Physics: H. Rauch Atominstitut Wien, Austria • Liquids and Glasses: R.L. McGreevy Univ. Uppsala, Sweden; F. Mulder IRI Delft, The Netherlands • Materials Science and Engineering: H. Zabel Univ.

Bochum, Germany; T. Lorentzen, Danish Stir Welding Tech. Denmark • Soft Condensed Matter: J. Colmenero Univ. San Sebastian, Spain • Solid State Physics: C. Vettier ILL, France; A. Furrer PSI,

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Switzerland; B. Cywinski Leeds, UK; C. Fermon CEA, France; B. Stirling ESRF, France • SNS Project: T. Mason SNS, USA • JSNS Project: S. Ikeda KEK, Japan Task Groups The ESS work has been divided into six individual tasks headed by the following task leaders: Instrumentation: Leader F. Mezei (HMI, Germany); Deputy R. Eccleston (ISIS, UK) Target systems: Leader G. Bauer (FZJ, Germany); Deputy T. Broome (ISIS, UK) Ring and achromat: C. Prior (ISIS, UK) Linac: A. Mosnier (CEA, France) Beam transport: R. Maier (FZJ, Germany) Conventional facilities: P. Giovannoni (CEA, France).

Public Launch of the ESS Project The ESS Project will be presented to the scientific community, politicians and decision makers at a European ESS Meeting next May 16-17, 2002, at the former German Parliament in Bonn. Neutron users from all over Europe will be invited to support the project. The key event will be in the afternoon of May 16 with high level presentations on the scientific, technical and political status of the ESS as well as on the site proposals for hosting the ESS. On Friday 17 May there will be more detailed and more specific scientific presentations on the opportunities that the ESS will open up in different scientific areas. A number of satellite activities are planned on Wednesday 15 th May and on the afternoon of Friday 17 th May: poster sessions, meetings of the national neutron scattering association, networking meetings and a meeting of the Neutron Round Table.

This meeting is a key event in the ESS Project. Mark this date in your diary! For further information please consult http://www.ess-europe.de News from the Scientific Advisory Committee (SAC): Next steps to the Final Science Case and the choice of Day One Instruments for the ESS As reported in the last ESS newsletter, the SAC workshop in conjunction with the 3rd SAC Meeting in May 2001, in Engelberg (Switzerland) ended with the recommendation of SAC to build the ESS facility with a 50Hz short pulse target station and a 162/3Hz long pulse target station, both at a level of 5MW proton beam energy with equal priority. In its meeting on June 15, 2001, in Abingdon, the ESS Council followed this recommendation. The workshop led to a new approach to the science case by focussing on threshhold areas and experiments in the different science fields which are accessible by ESS. The reports on the disciplinary science case were published a few weeks after the workshop in a Progress Report on ‘Scientific Trends in Condensed Matter Research and Instrumentation Opportunities at ESS’ which is available on the ESS web site (http:\\www.ess-europe.de under ‘Documentation’).

In November 2001 the 4 th SAC Meeting took place at the ILL in Grenoble, France.

There further steps were taken towards the final science

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case and the instrumental layout of the ESS. The SAC concluded that the ESS science case should also address contributions of the ESS to priority research missions and thus demonstrate how the ESS will contribute to the societal needs in Europe. Nine research missions were identified, which are widely supported by the European Union as well as by many European countries. It will be the task of the upcoming SAC workshop in March 2002 to finalize the science case and to position ESS in the context of science, technology and society. The second major goal of the 4 th SAC meeting in Grenoble was the selection of ‘Day One Instruments’ for the ESS. Since the ESS will be a science driven facility, input from the different science groups was important. To define the science demands on the ESS instrumentation each science group provided a priority list of instruments based on the flagship areas and experiments which are described in the Engelberg reports. The evaluation of the priority lists from the different science groups is illustrated by the blue bars in Fig. 1 for instruments at the long pulse target station.

Fig. 1. Instrument priorities at the long pulse target station with (white) and without (blue) weighting factors.

Selecting the instruments with a score higher than the mean value, represented by the blue horizontal line in Fig. 1, one obtains a proposal for an instrument suite with five instruments at the long pulse and eight instruments at the short pulse station. Since in this evaluation procedure the demands of each science group were given equal importance, a different approach was made by applying weighting factors for the demands of each science group. These weighting factors were obtained by evaluating the statistics of requested beam times at the ILL in Grenoble in April 2001.

The red bars in Fig. 1 illustrate the results of this procedure for the instruments at the long pulse target station. Now, only three instruments score higher than the average (red horizontal line). However, the list of six instruments with the highest scores, extracted from Fig. 1, again contains the five instruments one obtains without weighting factors, plus the Diffuse Scattering instrument.

This weighting experiment shows that the selection of instruments is more or less independent of the weighting factors. The same holds for the instruments at the short pulse target station. The list of instruments derived from the science demands of the different science groups was therefore taken as a sound basis for discussion.

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DOVE NEUTRONI Articolo ricevuto in redazione nel mese di Novembre 2001

THE FINAL CONFIGURATION OF TOSCA NEUTRON SPECTROMETER M. Celli(1), F. Cilloco(2), D. Colognesi(1,3), R.J. Newport(4), S.F. Parker(3), V. Rossi-Albertini(2), F. Sacchetti(5), J. Tomkinson(3) and M. Zoppi(1) (1) Consiglio Nazionale delle Ricerche, Istituto di Elettronica Quantistica, Via Panciatichi 56/30, 50127, Florence, Italy.

(2) Consiglio Nazionale delle Ricerche, Istituto di Struttura della Materia, Via Fosso del Cavaliere, 00133, Rome, Italy.

(3) ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom.

(4) Physics Laboratory, University of Kent at Canterbury, Canterbury, Kent, CT2 7NR, United Kingdom.

(5) Dipartimento di Fisica and UdR Istituto Nazionale per la Fisica della Materia, Università di Perugia, Via G. Pascoli, 06100, Perugia, Italy.

Abstract

placed at 12 m from the neutron moderator, included two and ten back-scattering detector modules, respectively, and showed a ∆ω/ω of 2.5÷3.5% and 2÷3.5%, respectively (as reported in Fig. 1). Resolution calculations have been experimentally tested [4].

TOSCA is now located at 17 m from the water moderator and makes use of ten detector banks: five in forward (≅47.500) and five backward (≅132.300) scattering (see Fig. 2 for a schematic view of TOSCA). Each bank contains thirteen 3He squashed tubes, the same thickness as in TOSCA-I (2.5 mm), but much thinner than on TFXA (6.0 mm). However they are quite longer than on TOSCA-I (from 100 mm to 250 mm) in order to compensate for the larger distance from the neutron source but, at the same time, to maintain the spectrometer sensitivity. Other changes have been made to the TOSCA beam-line in order to enhance the neutron flux and to reduce the instrumental background, which on TOSCA is flat and structureless, but presently slightly larger than on TFXA. The beam cross-section is now 40 times 40 mm2 (was 20 times 50 mm2 in the phase I) and a “Nimonic” chopper has been installed to avoid frame overlap at the sample

The forward and backward scattering parts of a new neutron spectrometer, TOSCA-II, replacing the old TFXA and TOSCA-I, have been successfully installed at the ISIS pulsed neutron source. The results show a significant enhancement in the counting rate due to the larger detector area. The improved resolution (to 1.5÷3% of the energy transfer) as compared with the previous instruments (TXFA 2.5÷3.5%, TOSCA-I 2÷3.5%) has been achieved by increasing the primary flight path from 12 m to 17 m. A chopper has been added in order to avoid neutron frame overlap and to reduce the fast neutron background.

Additional diffraction capability will be installed in the near future.

The second and final version of a high-resolution crystal analyser spectrometer, TOSCA, has been built, installed and operated at the ISIS pulsed neutron source (Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK) as a joint project of Consiglio Nazionale delle Ricerche (Italy), University of Kent at Canterbury (UK) and Rutherford Appleton Laboratory (UK). TOSCA is mainly intended for molecular vibrational spectroscopy but will have also some general diffraction capability with elastic detectors in forward and backward scattering in order to provide a good coverage of the momentum transfer, Q, space (5÷350 nm-1). This is equivalent to the range (0.02÷2 nm) in d-plane spacing.

The instrument makes it possible to measure inelastic neutron scattering in a large energy transfer range -hω=(5÷1000) meV, with a relative energy-resolution of about ∆ω/ω=1.5÷3% in the whole interval of h-ω. It has replaced both the old TFXA [1], removed in spring 1998, and subsequently, a prototypical version of TOSCA [2,3] (TOSCA-I) which was incorporated into the final version of TOSCA in spring 2000. All the three aforementioned spectrometers viewed the same water moderator, which is kept at room temperature and is poisoned at a depth of 20 mm by a thin Gd foil. The advantage over its two predecessors is both in detected flux and energy resolution. As far as the intensity is concerned TOSCA-II exhibits an overall gain factor of 6.3 over TFXA and 1.9 over TOSCA-I, mainly obtained through a larger detector bank area. These two earlier spectrometers, which were

Fig. 1. Comparison of the energy resolution functions for TFXA (dotted line), TOSCA-I (dashed line) and TOSCA-II (full line).

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position and to remove fast neutrons normally associated with open-geometry machines. The whole instrument is buried in shielding composed of borated plastic of various sorts, B4C, steel and cadmium plates. A threehead closed-cycle refrigerator, embedded in the spectrometer, allows measurements in the range between 10 K and room temperature. A computer-controlled 24position sample changer, based on a robust conveyor belt system, is currently available for solid and liquid systems.

The spectrometer is of the inverted-geometry type [5] and in this respect is similar to older instruments operating at various neutron sources: ZING-P (Argonne, USA) [6], WNR at LANSCE (Los Alamos, USA) [7], CAT

[8] and LAM-D [9] at KENS (Ibaraki, Japan), NERA-PR at FNLP (Dubna, Russia) [10]. Scattered neutrons are detected at a fixed energy (between 4.1 meV and 3.5 meV) by a set of pyrolytic graphite analysers with a thickness of 2.0 mm and a mosaic spread of 2.50. They are oriented in such a way as to make use of the (002) Bragg plane, which implies an interplanar distance of 0.3354 nm. The incident neutron energy is determined from the measured total time of flight, t, through the following kinematic relation:

Fig. 2.View of TOSCA showing detector banks for forward and backward scattering.

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â&#x20AC;˘

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t=

L0 L1 + v0 v1

DOVE NEUTRONI

where L0, L1, v0 and v1 are incident (0) and scattered (1) flight-path lengths and neutron velocities, respectively.

On TOSCA typical values for the neutron flight-paths are: L0=17.00 m, L1=0.55÷0.70 m. Before neutron detection, 75 mm-thick beryllium filters, cooled to below 35 K by single-stage closed-cycle refrigerators, are used to suppress high-order harmonics reflected by the graphite analysers. Since the uncertainty of L1 dominates the instrument energy resolution [5], a two dimensional focusing of t has been arranged for the detectors, in order to compensate the errors on L1 with the opposite errors on v1 [5]. This configuration has been obtained on TOSCA by arranging the detector banks in a circular geometry around the beam axis and aligning the sample and each detector tube in two parallel planes, again parallel to the plane of the

References [1] J. Penfold, J. Tomkinson: The ISIS Time Focused Crystal Analyser Spectrometer TFXA, RAL-86-019, (1986).

[2] S.F. Parker et al.: Physica B, 241-243, 154 (1998).

[3] Z.A. Bowden et al.: Physica B, 276-278, 98 (2000).

[4] V. Rossi-Albertini, D. Colognesi and J. Tomkinson: Journal of Neutron Research, 8, 245 (2000).

[5] C.G. Windsor: Pulsed Neutron Scattering, (Taylor and Francis Ltd, London, 1981).

[6] K. Sköld, K. Crawford and H. Chen: Nucl. Instr. and Meth., 145, 117 (1977).

[7] J. Eckert et al.: Proc. ICANS-IV, 434 (1981).

[8] S. Ikeda, N. Watanabe, K. Kai: Physica B, 120B, 131 (1983).

[9] K. Inoue et al.: Nucl. Instr. and Meth. A, 327, 433 (1993).

[10] I. Natkaniec, S.I. Bragin, J. Brankowski, J. Mayer: Proc. ICANS-XII, 1, 89 (1994).

[11] J. Tomkinson: J. Serb. Chem. Soc., 6, 729 (1996) and references therein.

[12] D.K. Ross, V.E. Antonov, E.L. Bokhenkov, A.I. Kolesnikov, E.G. Ponyatovsky and J. Tomkinson: Phys. Rev. B, 58, 2591 (1998).

[13] A.I. Kolesnikov, J.C. Li, S.F. Parker, R.S. Eccleston and C.K. Loong: Phys. Rev. B, 59, 3569 (1999).

Fig. 3. Momentum transfer accessible with TOSCA in forward (dashed line) and backward (dotted line) scattering as a function of the energy transfer.

respective analyser (see Fig. 2). In addition, to further improve the energy resolution, the “Marx principle” [1] has also been applied by positioning the 3He tubes perpendicular to the scattering plane.

In TOSCA, owing to the low values of v1 (and hence of final neutron momentum), Q rapidly increases with ω along a narrow stripe in the (Q, ω) kinematic space (see Fig. 3). It starts at ω=0 from Q equal to 15-21 nm-1, then increases approximately as (2mnω/h-)1/2. However this spectrometer will be used mainly for studying undispersed modes in solids (molecular vibrations [11], proton local modes in metal hydrides [12], etc.), or phonon densities of states [13], where the Q-dependence is not usually crucial. On TOSCA, the presence of forward and backward scattering detector banks will allow, for low values of energy transfer, the exploration of two close, but still different, (Q, ω) trajectories at the same time (as shown Fig. 3).

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Attività della SILS Nel corso del 2001 due sono state le attività principali della SILS, l’organizzazione del Convengo Annuale e della Scuola di Luce di Sincrotrone.

Nei giorni 5-7 luglio 2001 si è tenuto a Firenze il IX Convegno della Società Italiana di Luce di Sincrotrone, organizzato dal prof. V. Lombardi. Il Convegno, articolato in quattro sessioni orali e una sessione poster, ha coperto numerosi campi disciplinari di interesse per la comunità di luce di sincrotrone. Sono state presentate cinque “invited lectures” precisamente sul meccanismo della contrazione muscolare da M. Irving, sulle proprietà strutturali di semiconduttori in condizioni estreme di pressione e temperatura da A. Filipponi, sulla microscopia e microspettrocopia ad alta risoluzione nei raggi X da J. Susini, su studi ad alta risoluzione di specie reattive da S. Stranges, su sistemi correlati da M. Grioni. Le relazioni orali ed i poster hanno consentito di fare un quadro delle ricerche attualmente svolte dalla Comunità. Lo stato delle facility e le prospettive future sono state presentate e diffusamente discusse. In particolare e’ stato illustrato lo stato di ESRF da parte di F. Comin, che ha sottolineato il significativo incremento di proposals italiani accettati passati da 140 nel 1998 a 216 nel 2001. Lo stato attuale di ELETTRA è stato illustrato da M. Altarelli, che ha sottolineato anche le prospettive di miglioramento con la prevista aggiunta di un booster al sistema di iniezione. Sono stati presentati i due progetti italiani sul Free Electron Laser, con gli interventi di P. Perfetti, sul progetto SPARC del CNR, ENEA, INFN ed INFM, e di F. Parmigiani, sul progetto VUV-Soft Xrays FEL dell’ INFM ed ELETTRA. Nell’ ambito del Convegno si è tenuta anche l’Assemblea annuale dei Soci che ha discusso lo stato delle facilities di luce di sincrotrone ELET-

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TRA ed ESRF e le linee di attività della Società anche in vista dei progetti di Free Electron Laser in via di definizione. In Assemblea sono state presentate le candidature per le nuove cariche sociali (Presidenza e Giunta). Conformemente al nuovo statuto della SILS, le elezioni sono avvenute per posta. S. Mobilio è stato confermato come Presidente per il prossimo biennio durante il quale la Giunta sarà così formata: R. Felici, G. Iucci, V. Lombardi, E. Paris, M. Pedio, G. Ruocco.

La nuova Giunta della SILS ha recentemente deliberato di affidare al prof. G. Stefani dell’Università “Roma Tre” il compito di organizzare il X Convegno tra la fine di giugno e l’inizio di luglio del 2002.

Durante le ultime due settimane di settembre si è tenuta la VI edizione della tradizionale Scuola Nazionale di Luce di Sincrotrone della SILS. Organizzata, come di consueto, da S. Mobilio e G. Vlaic nello splendido panorama di S. Margherita di Pula (Ca), la Scuola ha avuto un notevole successo con la partecipazione di ben 50 giovani, massimo numero possibile, distribuiti principalmente tra le aree di Fisica (≈50%), Chimica (≈25%), e Scienza della Terra (≈25%).

La Scuola, come di consueto, ha offerto una panoramica attuale delle caratteristiche delle sorgenti e delle potenzialità di ricerca connesse con l’ utilizzazione della Luce di Sincrotrone, affrontando sia da un punto di vista teorico che sperimentale le varie metodologie in uso, viste nella loro connessione alle differenti discipline (chimica, fisica, biologia, scienze della terra, scienza dei materiali, medicina). Questo programma è stato svolto in circa 70 ore di lezioni tenute da una trentina di docenti diversi, ed in esercitazioni pratiche facoltative tenute la sera dopo cena.

L’elevato livello delle lezioni, la qualità dei lucidi elaborati dai docenti e delle dispense distribuite rendono oggi questa Scuola un importante momento formativo non solo per la

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comunità degli addetti ai lavori ma in generale per tutti gli studenti di dottorato. La SILS ha deliberato la stampa di un volume di proceedings, che sarà pronto nei primi mesi del 2002. Importante per il successo della scuola è stato certamente il clima umano che si è instaurato tra i partecipanti che si sono immediatamente sentiti un gruppo unito, e tra questi ed i docenti, che non si sono limitati a tenere le lezioni e poi scappare, ma si sono fermati per qualche giorno, consentendo uno scambio scientifico e didattico con gli studenti e partecipando alle attività di tempo libero di Sincrotrone organizzate dagli studenti e dai direttori.

S. Mobilio Presidente SILS

Bando di concorso alla Borsa di Perfezionamento negli studi della Spettroscopia Neutronica “Francesco Paolo Ricci” per il 2002 Con i fondi messi a disposizione dalla signora Silvana Piermattei Ricci e dai figli al fine di onorare la memoria del Prof. Francesco Paolo Ricci, l’Accademia Nazionale dei Lincei bandisce una Borsa di perfezionamento di EURO 15.000 destinata a un giovane studioso che intenda svolgere un progetto di ricerca nel campo della spettroscopia neutronica. La Borsa, della durata di un anno, non è rinnovabile.

Possono concorrere studiosi Italiani che non abbiano supeato i 30 anni alla data di scadenza del termine di presentazione delle domande.

La borsa non è cumulabile con borse attribuite da altri Enti né con assegni

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o sovvenzioni di analoga natura. Coloro che intendono concorrere alla Borsa debbono inviare alla Segreteria dell’Accademia Nazionale dei Lincei (Via della Lungara, 10 - 00165 Roma) entro il 31 dicembre 2001 (data del timbro postale) la domanda di ammissione al concorso in carta libera diretta al Presidente dell’Accademia. Su tali domande i concorrenti dovranno dichiarare, sotto la propria responsabilità, i propri dati anagrafici. I concorrenti dovranno allegare alla domanda la seguente documentazione: a) curriculum di tutti gli studi e degli esami universitari sostenuti con i relativi voti; b) una copia della tesi di laurea; c) programma della ricerca con l’approvazione del Direttore del laboratorio presso cui sarà eseguita la ricerca stessa; d) dichiarazione da cui risulti che il candidato ha assolto gli obblighi di leva, oppure che è in grado di ottenere il rinvio del servizio militare per tutto il periodo di fruizione della borsa; e) ogni altro eventuale lavoro o titolo; f) elenco degli eventuali lavori presentati.

Non è ammessa la presentazione delle domande o dei titoli fatta personalmente negli Uffici dell'Accademia. La Commissione giudicatrice, nominata dalla Classe di Scienze Fisiche, Matematiche e Naturali, è composta da tre Soci dell’Accademia, da un esperto di spettroscopia neutronica e da un membro designato dalla Famiglia Ricci. Il giudizio della Commissione è inappellabile.

Il periodo di fruizione della borsa non può essere differito per più di tre mesi. Il borsista fruirà per tutta la durata della borsa di una assicurazione per infortuni di laboratorio.

La somma corrispondente alla borsa sarà versata in rate quadrimestrali anticipate. Per il pagamento delle rate successive alla prima, il borsista deve inviare una dichiarazione di frequenza e di profitto firmata dal

Direttore dell'lstituto. E' in facoltà insindacabile dell'Accademia dei Lincei, sentito il parere del Direttore dell'lstituto presso cui l’assegnatario della borsa lavora, di interrompere il pagamento delle rate, con preavviso di almeno un mese, qualora non giudicasse soddisfacente il profitto degli studi e del lavoro compiuto. Entro tre mesi dalla fine del periodo di fruizione della borsa, il borsista è tenuto a trasmettere al Presidente dell'Accademia una relazione sul lavoro compiuto (approvata dal Direttore dell'lstituto) e una copia delle eventuali pubblicazioni.

Il vincitore della borsa dovrà far per-

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venire alla Segreteria dell'Accademia, entro 30 giorni dalla data del conferimento, una lettera di accettazione della borsa nonché la seguente documentazione: a) certificato di nascita; b) certificati di cittadinanza italiana e penale rilasciati in data non anteriore a quella del presente bando; c) dichiarazione comprovante il possesso dei requisiti relativi alla non cumulabilità della borsa; d) certificato di laurea; e) certificato da cui risulti la posizione relativa agli obblighi di leva.

Roma, 15 novembre 2001

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SCUOLE E CONVEGNI

XIII Conferenza – Trieste, 23-27 Luglio 2001

Vacuum Ultraviolet (VUV) Radiation Physics Una delle più grandi, se non la più grande , conferenza nel campo della scienza fatta con la radiazione VUV e raggi X molli si è svolta in Luglio a Trieste. Circa 500 ricercatori attivi non solo nel campo della radiazione di sincrotrone, ma anche teorici e utilizzatori di altri tipi di sorgenti, come per esempio laser, hanno partecipato alla conferenza. Questa edizione della conferenza, che ha cadenza triennale, è stata organizzata da Elettra in collaborazione con il Consiglio Nazionale delle Ricerche (CNR) e l’Istituto Nazionale di Fisica della Materia (INFM).

La conferenza si è svolta al Centro Conferenze della Stazione Marittima, nel centro della città di Trieste e di fronte al mare. Questa costruzione era il punto di imbarco per le navi che lasciavano Trieste per destinazioni lontane, come per esempio l’Australia.

Nella sessione di apertura, Massimo Altarelli, chairman della conferenza, ha salutato i partecipanti, ricordando che questa edizione continua la lunga serie di queste conferenze che si sono tenute a San Francisco, Tokyo e Parigi per ricordare solo le più recenti. Altre personalità, fra cui il sindaco di Trieste, i rappresentanti della Regione e degli enti che hanno sponsorizzato l’evento hanno partecipato a questa sessione con un saluto ai partecipanti. La prima relazione plenaria dedicata all’uso dell’eccitazione risonante di core di molecole e cluster per comprendere la dinamica al femtosecondo è stata tenuta Olle Björneholm (Uppsala).

La breve vita media delle vacanze di core, tipicamente qualche femtosecondo, permette di studiare i processi di dissociazione che avvengono su questa scala di tempi, attraverso la variazione dell’energia incidente ed

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altre tecniche. Inoltre il relatore ha illustrato la recente osservazione dell’effetto doppler nell’emissione Auger da parte di frammenti cinetici.

Questo inizio nel campo della fisica atomica e molecolare è stato seguito da un contributo di Kiyoshi Ueda (Tohoku University, Sendai) che ha descritto l’attività svolta a Spring-8 per studiare la dinamica nucleare di molecole poliatomiche. La dinamica della dissociazione di queste molecole, può essere seguita misurando con tecniche di ion imaging tutti i frammenti ionici dopo la fotoeccitazione. Uwe Hergenhahn (Berlino) ha discusso risultati di studi recenti sulla molecola di azoto, in cui gli stati di core 1s “gerade” e “ungerade” dell’N 1s sono stati risolti in fotoemissione. Ha descritto quindi risultati di esperimenti di fotoemisisone che mostrano fenomeni di dicroismo circolare su stati di core di molecole in fase gassosa.

Un ampio spettro di altri esperimenti in vari settori della fisica sono stati discussi; per esempio Jan-Erik Rubensson (Uppsala) ha presentato risultati di misure di fluorescenza di stati doppiamente eccitati dell’atomo di elio, mentre M. Krisch (ESRF, Grenoble) ha mostrato come lo scattering Raman di raggi X da parte di materiali a basso Z possa fornire gli stessi risultati di misure NEXAFS con raggi X molli su livelli di core senza la necessità di lavorare in vuoto, e possa fornire anche più informazioni permettendo di accedere a transizioni proibite di dipolo.

Chuck Fadley (Università della California a Davis) ha parlato di spettroscopia di livelli di core e di diffrazione, Enzo Di Fabrizio (INFM, Trieste) ha descritto alcuni eccitanti sviluppi nel settore dell’”imaging con zone plate” usando il contrasto di fa-

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se. Una notevole presentazione in un settore correlato è stata fatta dal Prof. R. Wiesendanger che ha parlato di spettromicroscopia risolta in spin con risoluzione atomica.

La microscopia con raggi X molli basata su tecniche di contrasto in fotoemissione e fotoassorbimemnto e i suoi recenti sviluppi e applicazioni in diversi campi dalla ricerca sui materiali, in biologia ecc. è stata oggetto di grande interesse con una relazione plenaria di C. Larabell (Università della California a San Francisco), e una serie di comunicazioni fra cui quelle di F. Nolting (PSI, Svizzera e ALS Berkeley), J. Susini (ESRF Grenoble), Adam Hitchcock (Università McMaster, Hamilton, Canada), C.A. Larabell (Università della California a San Francisco) e Chris Jacobsen (SUNY, Stony Brook, USA). Contributi a studi connessi con la catalisi sono stati presentati da Ronald Imbihl (Hannover), Falko Netzer (Graz) e Giorgio Paolucci (Elettra). Questo è solo un estratto del vasto programma che ha incluso anche risultati recenti su materiali superconduttori, magnetismo ed altri “hot topics”.

A causa dell’alto livello di cultura gastronomica dell’Italia, la cena ufficiale della conferenza è stato forse l’evento che ha occupato il maggior tempo e attenzione da parte degli organizzatori. La cena si è tenuta a Muggia, una piccola città dall’altro lato della baia di Trieste, che i partecipanti hanno raggiunto per mezzo di due grandi battelli scoperti. La cena si è svolta in una piacevole serata estiva italiana con una magnifica vista sul mare Adriatico verso Trieste.

Gli sforzi del comitato organizzatore sono stati premiati da un banchetto risultato memorabile. Nel discorso dopo la cena Giovanni Comelli ha ri-

SCUOLE E CONVEGNI

I membri uscenti ed entranti

cordato alcune difficoltà incontrate nell’organizzazione.

La gran parte dell’organizzazione è stata fatta attraverso Internet, ma non tutti hanno risposto adeguatamente. Alcuni hanno accidentalmente inviato asbtract che erano documenti vuoti, cioè completamente bianchi, ma uno è riuscito ad inviare un documento completamente nero!

Il format richiesto era postscript, ma alcuni hanno inviato documenti Words. Quando richiesti di fornire un documento postscript, alcuni hanno solamente cambiato l’estensione del file in .ps e sottomesso lo stesso file con il nuovo nome.

Infine Giovanni ha ringraziato tutti coloro che hanno pagato l’iscrizione per mezzo carte di credito via internet. Comunque ha ringraziato ancora di più chi ha pagato due volte. Il suo ringraziamento più caloroso è andato comunque alla persona che ha pagato quattro volte! La sua interpretazione personale è che questo era una specie di dono, una elargizione per sostenere la conferenza, e potrebbe essere offensivo restituire un tale dono offerto liberamente. In ogni caso persone a livello più alto del suo hanno deciso di correre questo considerevole rischio e restituire il danaro.

L’ultima sessione del programma scientifico nel giorno finale è stata iniziata Ernst Bauer (Università dell’Arizona) che ha presentato recentissimi risultati della beamline per Nanospettroscopie di Elettra. Sono state mostrate una serie di immagini di microscopia in fotoemissione con contrasto magnetico di parecchie strutture, prese usando la nuova sorgente basata su un ondulatore. Lo ha seguito Michel Van Hove (LBNL, Berkeley) che ha svolto una review degli sviluppi recenti nel settore del-

la diffrazione di elettroni e dell’olografia. L’ultimo contributo è stato quello di T. Takahashi (Tohoku University, Sendai) che ha illustrato come la fotoemissione ad alta risoluzione può contribuire alla comprensione di sistemi altamente correlati come i materiali superconduttori.

Dopo la Conferenza, si è svolto una visita molto partecipata alla vicina sorgente di radiazione di sincrotrone, Elettra. I partecipanti hanno avuto la possibilità di vedere le beamline e discutere con lo staff e i gruppi partner di questa sorgente di terza generazione.

Nella sessione conclusiva è stato annunciato il sito della prossima conferenza, la VUV-XIV: sarà Cairns, Australia, nell’anno 2004.

La proposta di questo sito è stata presentata da Brenton Lewis della Australian National University, Canberra, che ha illustrato le “facilities” disponibili localmente, il clima temperato di Cairns in Luglio, e la bellissima area circostante. Inoltre ha informato l’assemblea dei progressi del progetto di un sincrotrone in Australia.

Ingolf Lindau, Chairman dell’International Advisory Board, ha riassunto infine la Conferenza. Con tristezza ha osservato che dall’ultima conferenza VUV un certo numero di colleghi ci hanno lasciato (Ugo Fano, Yvette Cauchois, Gerhard Herzberg.). L’International Advisory Board ha il compito di seguire l’organizzazione della Conferenza e i suoi membri sono eletti per un periodo di nove anni, che copre tre conferenze.

La conferenza continua ad essere interessante e vivace e ad attrarre un ampio spettro di partecipanti da tutto il mondo. Il numero di partecipanti è rimasto infatti costante nelle

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Uscenti M.L. Cohen (USA) K. Codling (Great Britain) D. L. Ederer (USA) B. Lewis (Australia) I. Lindau (Sweden) I. Nenner (France) E. Verkhovtseva (Ukraine) Entranti E. Shirley (USA) D.P. Woodruff (Great Britain) A.P. Hitchcock (Canada) F. Larkins (Australia) J. Nordgren (Sweden) P. Morin (France) G. Margaritondo (Switzerland)

I nuovi membri onorari K. Codling L. Hedin T. Ishii D. Shirley

ultime tre conferenze, ma lo spettro delle attività scientifiche si è allargato. Ingolf ha ringraziato i membri del comitato e tutti coloro che hanno contribuito all’organizzazione, in particolare Maya Kiskinova del comitato di programma e Giovanni Comelli, il responsabile del comitato organizzatore locale.

Gli abstract della conferenza sono disponibili sul sito Internet di Elettra (www.elettra.trieste.it): selezionate “VUV XIII” nella parte News and Events. Quando leggerete queste note anche le fotografie della Conferenza saranno già disponibili sul sito.

Infine gli atti della conferenza saranno pubblicati in un numero speciale di Surface Review and Letters, con editori Kevin Prince, Maya Kiskinova e Giovanni Comelli.

K.C. Prince

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SCUOLE E CONVEGNI

Workshop

Decay Processes in Core-Excited Species Il Workshop “Decay Processes in Core-Excited Species”, meeting satellite della conferenza VUV-XIII, si è tenuto nella Villa Mondragone, una costruzione del sedicesimo secolo sui colli romani, nei giorni dal 30 Luglio al 2 Agosto. In questo luogo papa Gregorio Magno firmò la bolla di riforma del calendario, istituendo l’attuale calendario gregoriano.

Data la connessione storica del luogo con la misura del tempo, è stato più che appropriato che nell’intervento iniziale della prima giornata Wilfried Wurth (Hamburg) descrivesse gli esperimenti recenti che utilizzano il processo di scattering Raman risonante come “orologio atomico”. In questi esperimenti il rapporto delle intensità dei canali Auger risonante e Raman di atomi di argon adsorbiti su una superficie fornisce informazioni dirette sul tempo impiegato dall’elettrone eccitato per transire dall’atomo adsorbito al substrato.

Sia Phil Woodruff (Warwick) che Gunnar Öhrwall (recentemente all’Università del Nevada ) hanno poi presentato risultati relativi ad effetti non dipolari nella fotoemissione da molecole, usando comunque metodi alquanto diversi. Phil Woodruff ha mostrato come la tecnica delle Xray Standing Waves a incidenza normale può essere usata per estrarre il parametro di asimmetria (avanti/indietro) al primo ordine nell’intervallo energetico di qualche KeV. I livelli interni 1s di tutti gli elementi tra il carbonio e il cloro sono stati studiati ed i risultati sono in accordo con la teoria. L’intervento di Gunnar Öhrwall ha riguardato invece la validità dell’approssimazione di dipolo per energie inferiori al KeV. Con molti esempi è stata illustrata la deviazione dalle predizioni teoriche

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più semplici in molecole sia nella regione delle soglie interne che ad energie più elevate.

Questi notevoli contributi sperimentali del primo giorno sono stati bilanciati da altrettanto stimolanti contributi di teorici. Lorenz Cederbaum (Heidelberg) ha sottolineato come atomi e molecole ionizzati nella regione della valenza interna non possono decadere attraverso decadimenti Auger quando l’energia di eccitazione è minore del potenziale di doppia ionizzazione. Ha comunque predetto che in cluster debolmente legati, il processo diventa permesso, con le vacanze situate in differenti membri del cluster. A causa del debole legame e dello scarso overlap delle funzioni d’onda , ci si potrebbe aspettare che l’effetto sia molto debole, mentre i calcoli indicano invece che è piuttosto forte : pertanto l’osservazione dell’effetto è ora una sfida per gli sperimentali. Faris Gel’mukhanov (Stoccolma) ha presentato una review della dinamica dello scattering Auger Raman risonante e ha predetto fenomeni che dipendono dalla struttura temporale degli impulsi di luce del sincrotrone. La giornata è stata completata da Robert Lucchese (Texas A & M) che ha discusso l’influenza delle correlazioni elettroniche nelle ionizzazioni di core di molecole.

Il programma abbastanza intenso del giorno seguente ha compreso interventi relativi alle eccitazioni di core di atomi, molecole, cluster e adsorbati. Fra gli highlight sono da includere l’osservazione fatta da Uwe Becker (Fritz-Haber-Institute, Berlino) di doppi e tripli decadimenti Auger e l’analisi dell’allineamento dei frammenti della molecola di HCl a seguito di una eccitazione di core, usando la spettroscopia ottica di

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fluorescenza, presentata da Michael Meyer (LURE, Orsay).

Gli interventi dell’ultimo giorno sono cominciati con la discussione del ruolo del moto nucleare nei processi di decadimento da shell interne. R. Dörner (Frankfurt), E. Shigemasa (IMS, Okazaki, Japan) N. Saito (AIST, Tsukuba) e Paul Morin (LURE, Orsay) hanno presentato dei contributi sperimentali, mentre Reiner Fink (Uppsala) ha fornito un punto di vista teorico. Questo è un settore in rapida evoluzione in cui le tecniche di ion-imaging e di coincidenza elettrone-ione giocano un ruolo chiave. Alexander Föhlisch ha invece presentato dei risultati relativi ad azoto molecolare su nickel e stimolato una vivace discussione se questi risultati costituiscano un esempio di Multi Atom Resonant Photo Emission (MARPE).

L’ottimo banchetto ufficiale della conferenza si è tenuto in un ristorante sulla via Appia antica. Qui Nils Mårtensson (Max Lab) ha riassunto le attività del meeting e ringraziato gli organizzatori a nome dei partecipanti.

K.C. Prince

SCUOLE E CONVEGNI

Congresso Annuale della SISN Nei Giorni 8, 9 e 10 Novembre 2001, si è tenuto a Milazzo il XII convegno della Società Italiana di Spettroscopia Neutronica. Nonostante la data del Convegno venisse a sovrapporsi con altri impegni (congressi) dei Soci, sono state registrate più di cinquanta presenze, un numero certamente non inferiore a quello delle passate edizioni, organizzate per consuetudine generalmente all’inizio dell’estate. Ciò può essere ritenuto con soddisfazione un indice della buona vitalità della Società, e dell’interesse dei Soci per le attività della SISN. Il successo dell’attività della Società nel propagandare i risultati e le opportunità scientifiche offerte dalla spettroscopia neutronica è anche testimoniato dall’elevato numero di nuovi soci (dieci, tra cui molti giovanissimi) che quest’anno hanno chiesto, con successo, di essere ammessi nella Società.

Tra gli sponsor dell’iniziativa si ricordano l’Istituto di Tecniche Spettroscopiche del CNR, l’Università degli Studi di Messina, l’Accademia Peloritana dei Pericolanti e l’INFM. Grazie ai supporti economici ottenuti dagli sponsor ed ai proventi dalle quote di iscrizione al congresso è stato possibile sovvenzionare (almeno in parte) la partecipazione al congresso di numerosi giovani soci.

I lavori sono iniziati con due relazioni tenute da due rappresentanti della spettroscopia neutronica europea: H.D. Middendorf (Clarendon Laboratori, University of Oxford, GB), che ha illustrato uno studio delle proprietà dinamiche di bio-molecole, e G. J. Kearley (TU Delft Interfacultair Reactor Instituut, Mekelweg, NH) il quale ha focalizzato l’attenzione sul fondamentale ruolo di feedback rappresentato dai risultati pro-

venienti dalla spettroscopia neutronica nella modellizzazione delle relazioni tra struttura molecolare e la corrispondente dinamica vibrazionale.

A. Deriu (Dipartimento di Fisica ed INFM, Parma) ha tenuto una relazione sui risultati derivanti dal primo triennio di attività del Collaborative Research Group, avviato nel 1998 ed operante sullo spettrometro IN13 all’ILL (Grenobel). La relazione è stata soprattutto volta ad illustrare le opportunità offerte dall’iniziativa per chi sia interessato all’indagine spettroscopica della materia biologica.

M. Zoppi (CNR, Istituto di Fisica Applicata, Firenze) ha presentato una relazione sia sulle attiviata’ di ricerca svolte dalla comunita’ italiana ad ISIS sia su quelle di sviluppo di strumentazione. G. Fragneto (ILL, Grenoble, FR) ha presentato uno studio di superfici ed interfacce dal riflettometro D17, installato ad ILL. Ha inoltre illustrato lo stato attuale del progetto di un riflettometro per liquidi che dovrebbe essere installato presso la stessa facility europea.

D. Colognesi (CNR, IEQ, Firenze) ha invece relazionato sullo stato attuale della nuova versione dello spettrometro TOSCA, installato al ISIS, mettendo in luce i miglioramenti ottenuti dal nuovo strumento rispetto al più vecchio TFXA ed alla prima configurazione dello stesso TOSCA. Sono stati presentati alcuni risultati sperimentali illustranti le potenzialità della macchina.

A. Triolo ha descritto le potenzialità della spettroscopia neutronica nello studio delle proprietà dinamiche si sistemi polimerici. In particolare l’attenzione è stata posta sulle tecniche più idonee alla caratterizzazione della dinamica segmentale in polimeri amorfi, mettendo in evidenza l’im-

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portanza di collezionare dati da più strumenti tra loro complementari.

Accanto a queste relazioni su invito, tutte come ormai tradizione di carattere generale, essendo soprattutto rivolte ai numerosi giovani partecipanti al convegno, sono stati presentati numerosi lavori originali, tutti di ottimo livello e testimonianti la vitalità e la produttività della comunità scientifica italiana.

Durante il secondo giorno del convegno ha avuto luogo anche una sessione poster, nella quale sono stati presentati alcuni tra i risultati significativi piu’ recenti. All’interno di questa sessione era stato bandito, come di consueto, un concorso per il miglior poster, aperto ai più giovani ricercatori La commissione della SISN per l’assegnazione del premio per i migliori poster, composta da A. Albinati e A. Deriu si è riunita il giorno 9 novembre 2001 alle ore 18 nella sede del congresso della SISN. La scelta non è stata facile, essendo tutti i risultati presentati di ottimo livello, ma alla fine è stato scelto il poster presentato da Caterina Branca del Dipartimento di Fisica dell’Università di Messina, dal titolo: Vibrational versus relaxational contribution for disaccharide/water glass formers: a neutron scattering evidence. In questo lavoro, Caterina Branca presentava un interessante studio sulla dinamica vibrazionale di vetri di trialosio e sucrosio. Il confronto tra i due disaccaridi e la sostituzione isotopica del solvente (H/D) hanno consentito di mettere in evidenza i dettagli della transizione vetrosa.

I lavori congressuali si sono conclusi la mattina del 10 Novembre con la riunione dell’Assemblea dei soci della SISN. Franco Aliotta

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SCUOLE E CONVEGNI

International Conference on

Neutron Scattering (ICNS 2001) La settima edizione dell’International Conference on Neutron Scattering (ICNS 2001) si è tenuta a Monaco dal 9 al 13 Settembre.

La Conferenza, che segue le precedenti edizioni di Toronto,(’97), Sendai (’94), Oxford (’91), Grenoble (’88), Santa fe (’85) ed Hakone (’82), è stata organizzata dalla Technische Universität di Monaco in collaborazione con l’ENSA (European Neutron Scattering Association).

Le sessioni si sono svolte negli edifici dell’Università, ad eccezione della giornata di Mercoledì 12 che è stata, in parte, dedicata alla visita del nuovo reattore FRM-II. che è stato realizzato all’interno del Campus Universitario di Garching. La costruzione del reattore è orami ultimata e si at-

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tendono ormai le ultime licenze per l’entrata in servizio.

Gli argomenti trattati nel corso della Conferenza hanno spaziato su tutti i settori della Fisica della Materia e della Scienza dei Materiali: dal magnetismo, ai vetri e polimeri, ai fluidi complessi, alle biomolecole.

Ampio spazio è stato dato anche alla presentazione delle nuove facility, sia quelle in costruzione (SNS negli Stati Uniti), che quelle ancora in fase di progetto come L’European Spallation Source (ESS) e la nuova sorgente Giapponese a spallazione.

Oltre alle tradizionali e molto numerose (forse troppo) sessioni poster, una serie di Minisimposi su argomenti specializzati ha permesso di ampliare gli spazi dedicati alle co-

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municazioni orali a favore soprattutto dei ricercatori più giovani.

Tra gli eventi particolari del Congresso va ricordata l’assegnazione del premio Walter Hälg alla Prof.ssa Jane Brown per la sua lunga e brillante attività di studio delle proprietà magnetiche dei solidi utilizzando la diffrazione dei neutroni polarizzati.

La partecipazione italiana è stata consistente: oltre settanta italiani figurano tra gli autori dei lavori presentati al Congresso, a riprova della notevole vitalità della comunità neutronica nel nostro paese.

A. Deriu

CALENDARIO

4-6 febbraio 2002

GRENOBLE, SWITZERLAND

Accelerator Reliability Workshop http://www.esrf.fr/conferences/ARW/index.htm

6-8 febbraio 2002

GENOVA, ITALY 24-28 giugno 2002

GRENOBLE, SWITZERLAND SPIE Annual Meeting (International Society for Optical Engineering) http://spie.org/Conferences/Calls/02/am/home.html

19-29 agosto 2002

KYOTO, JAPAN

International Seminar on Photoionization Institute for Chemical research, Kyoto University, Kyoto 611-0011, Japan e-mail: Photoionization@elec11.kuicr.kyoto-u.ac.jp

22-26 agosto 2002

HYOGO, JAPAN

International Workshop on Photoionization (IWP) , Spring-8 http://www.spring8.or.jp/iwp2002

SALERNO, ITALY

SATT 11 - Congresso Nazionale di Superconduttivita' http://satt11.sa.infn.it/

maggio 2002

SEATTLE, USA

NIZHNY NOVGOROD, RUSSIA

6th International Workshop on Scanning Probe Microscopy http://www.ipm.sci-nnov.ru/eng/

19-22 marzo 2002

7-11 luglio 2002

TEHERAN IRAN

International Workshop on Physics and Technology of Thin Films Tehran, Sharif University of Technology (Iran)

3-6 marzo 2002

ROMA, ITALY

19th Conference on X-ray and Inner shell processes http://www.roma1.infn.it/stripes/webxray/

Twelfth ESRF Users' Meeting and Associated Workshops http://www.esrf.fr/conferences/usersmeeting/2002/

16-28 febbraio 2002

BOLOGNA, ITALY

EDS 2002 - International Conference on Extended Defects in Semiconductors http://www.df.unibo.it/eds2002/pagina1.htm

Silicon Workshop 2002 http://simulation.polito.it/~si-workshop/

11-15 febbraio 2002

1-6 giugno 2002

GAITHERSBURG, MD, USA

First American Conference on Neutron Scattering stassis@ameslab.gov

27-29 agosto 2002

HIROSHIMA, JAPAN

International Workshop on Dynamics in Core-Excited Molecules Hiroshima University, Higashi-hiroshima, Japan e-mail: Hiraya@sci.hiroshima-u.ac.jp

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â&#x20AC;˘

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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 aprile 2002 e il 16 ottobre 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 24 febbraio 2002 e il 22 settembre 2002

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

LLB-ORPHEE-SACLAY

DARESBURY

La scadenza per il prossimo call for proposals è il 1 ottobre 2002 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

Le prossime scadenze sono il 30 aprile 2002 e il 31 ottobre 2002

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

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

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

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

(quota italiana) Le prossime scadenze sono il 1 maggio 2002 e il 1 novembre 2002

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

LURE La prossima scadenza è il 30 ottobre 2002

MAX-LAB La scadenza è approssimativamente febbraio 2002

NSLS Le prossime scadenze sono il 31 gennaio 2002, il 31 maggio 2002 e il 30 settembre 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, Padriciano 99, 34012 Trieste, Italy tel: +39 40 37581 fax: +39 40 226338 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

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

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 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 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

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 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 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

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 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 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

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

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 Tipo: D Status: C

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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. 2 Dicembre 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/

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

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

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

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 Herma Büttner; tel: +33 76207179 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

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 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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