NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 5 n.2, 2000

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

SOMMARIO

Cover photo: The study of protein-membrane interactions

EDITORIALE

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C. Andreani

RASSEGNA SCIENTIFICA The Tomography Experiment at the SYRMEP Beamline at ELETTRA . . . . . . . . . . . . . . . . . . . . . . . . . 3 S. Pani et al.

Neutron Reflectivity Studies of Immiscible Polymer/Polymer Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Il

NOTIZIARIO Neutroni e Luce di Sincrotrone

è pubblicato a

M. Sferrazza

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

Neutron-Electron Spectroscopy

Vol. 5 n. 2 Dicembre 2000 Autorizzazione del Tribunale di Roma n. 124/96 del 22-03-96

Chemical-Shift Normal Incidence X-Ray Standing Wave Determination of Adsorbate Structures . . . . . . . . . . . . 25

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E. Balcar and W. Lovesey

V. Formoso

DIRETTORE RESPONSABILE:

C. Andreani COMITATO DI DIREZIONE:

M. Apice, P. Bosi

DOVE NEUTRONI

COMITATO DI REDAZIONE:

A Second Target Station at ISIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

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

J. Penfold

SEGRETERIA DI REDAZIONE:

D. Catena

VARIE

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HANNO COLLABORATO A QUESTO NUMERO:

G. Cicognani, M. Zoppi, P. Bosi GRAFICA E STAMPA:

om grafica via Fabrizio Luscino 73 00174 Roma

CALENDARIO

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SCADENZE

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FACILITIES

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Finito di stampare nel mese di Dicembre 2000 PER NUMERI ARRETRATI:

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

PER INFORMAZIONI EDITORIALI:

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

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EDITORIALE

partire da questo numero il prof. Giancarlo Ruocco entra a far parte del Comitato di Redazione in sostituzione del prof. Federico Boscherini. Ringrazio il collega Boscherini per l’attività svolta in questi anni e mi auguro che la sua collaborazione con il Notiziario continui, sia con articoli scientifici che di rassegna e riflessione sulla politica italiana in questi settori. Per quanto riguarda le iniziative internazionali di spettroscopia neutronica vorrei ricordare il progetto che prevede la costruzione di una stazione per una seconda targhetta, che affiancherà quella già operante ad ISIS (sito web http://www.isis.rl.ac.uk/targetstation2/).

La targhetta permetterà la produzione di intensi fasci di neutroni “freddi”, con l’obiettivo di effettuare studi complementari a quelli attualmente possibili ad ISIS; in particolare si prevede un programma di ricerca specificatamente dedicato allo studio della materia soffice, la scienza della vita e i materiali biomolecolari. Questo progetto rappresenta un importante occasione, in vista dell’auspicabile rinnovo dell’accordo tra CNR e CLRC previsto nell’anno 2001, per la definizione di nuove collaborazioni scientifiche tra la comunità italiana e quella inglese, sia nell’ambito della ricerca di base sia nello sviluppo di strumentazione e di formazione del personale.

Ricordiamo che dal 23 Settembre al 3 Ottobre si è svolta, in Sardegna presso l'Hotel Capo D'Orso (Palau, SS) la V edizione della Scuola di Spettroscopia Neutronica intitolata da quest’anno alla memoria di Francesco Paolo Ricci, che della Scuola era stato il sostenitore ed il primo Direttore.

Questa edizione ha avuto come tema la Diffusione Anelastica dei Neutroni, ed è stata diretta dal Prof. A. Deriu (Università di Parma) e dal Dr. M. Zoppi (Istituto di Elettronica Quantistica del CNR Firenze). Alla Scuola hanno partecipato docenti sia italiani che stranieri, questi ultimi provenienti dall’ILL e da Los Alamos, e diciotto studenti che si sono impegnati sia nel seguire le lezioni teoriche, che nelle attività seminariali e nelle esercitazioni pratiche che hanno costituito una parte molto rilevante delle attivita svolte. È stato assegnato un premio al gruppo che aveva svolto la migliore esercitazione, tutti i docenti hanno particolarmente apprezzato il grande impegno e l’entusiasmo dimostrato dai partecipanti.

Infine, per quanto riguarda il progetto TOSCA, negli ultimi mesi è stata completata l’istallazione ed il commissioning di TOSCA II presso ISIS. Lo spettrometro, realizzato presso l’Istituto di Elettronica Quantistica del CNR di Firenze, è ora disponibile per la comunità internazionale di utenti della spettroscopia neutronica, ed arricchisce il parco strumenti della sorgente ISIS.

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tarting from this issue the Editorial committee will benefit from the scientific expertise of Prof.

Giancarlo Ruocco, who substitutes Prof.

F. Boscherini. I would like to thank Federico Boscherini for his work in these years and I hope that his collaboration with Notiziario will continue, both with scientific papers and with contributions on the organisation aspects of Italian Research. As far as the international initiatives in neutron scattering are concerned I want to recall the project of a second target station at ISIS (web site http://www.isis.rl.ac.uk/targetstation2/), which aims to double the number of neutron instruments and will be optimised for neutron techniques such as reflection small angle scattering and high-resolution diffraction and spectroscopy. The instruments on the new target will be designed for the growing fields of softcondensed matter and bio-molecular science. This project represents an important opportunity, in view of the renewal of the international agreement between CNR and CLRC planned for the year 2001, for the definition of scientific collaborations among the Italian and British neutron scattering communities, both in basic research and in development of neutron instrumentation; We recall that the fifth edition of the Italian ‘Scuola di Spettroscopia Neutronica’ was held in Palau, SS, at the Hotel Capo D’Orso, in the period 23rd September - 3rd October. The neutron school, from this year entitled in memory of its first Director Francesco Paolo Ricci, was addressing the theme of the Neutron Inelastic Scattering and was directed by Prof. A. Deriu (Universita' di Parma) and Dr. M. Zoppi (Istituto di Elettronica Quantistica del CNR - Firenze). Teachers were from Italy, from ILL and from Los Alamos and 18 students were attending the school, partecipating to both theoretical seminars and experimental activities with great enthusiasm.

As far as the TOSCA project is concerned, in the last months the installation and commissioning of TOSCA II have been completed at ISIS. The spectrometer, realised at th Istituto di Elettronica Quantistica of the CNR in Florence, is now available for allocation of beam time as part of the instrument suite at ISIS.

S

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Carla Andreani

RASSEGNA SCIENTIFICA Articolo ricevuto in redazione nel mese di Ottobre 2000

THE TOMOGRAPHY EXPERIMENT AT THE SYRMEP BEAMLINE AT ELETTRA S. Pani1,2, F. Arfelli1,2, D. Dreossi1, R. Longo1,2, R. Menk3, A. Olivo1,2, P. Poropat1,2, L. Rigon1,2, G. Tromba3, E. Castelli1,2.

1

Introduction Obtaining the best possible information with the minimum risk for the patient: this is the challenge of modern radiology. Radiologic examinations represent the main cause artificial exposure to the population: the exposure level due to such exams is about one half the total exposure from the natural radioactivity background (100-200 mrem/year).

On the other hand, a late diagnosis of a tumour can dramatically decrease the probability of full recovery; breast tumours, particularly, need to be detected in asymptomatic women as soon as possible.

Early detection of breast cancer is very difficult due to the characteristics of early signs of tumour: these may be nodules, whose X-ray absorption characteristics are very close to those of breast tissue, thus resulting in a very low image contrast, or microcalcifications, whose dimensions are about 100 µm and require therefore a very high spatial resolution of the detection system [1].

Furthermore, breast is one of the most radiosensitive organs, and a high delivered dose can increase the risk of carcinogenesis.

Therefore, it is necessary to optimize radiologic examinations, and in particular mammography, on two sides: first, it is necessary to reduce the dose delivered in a typical exam; second, the image quality must be increased, giving a higher information content, in terms of spatial resolution and contrast resolution, at the same delivered dose. The main limitations of a conventional mammographic apparatus lie in the limited possibility of varying the X-ray beam energy and in the low dynamic range of the radiographic film, which may not allow a simultaneous visualization of very dense and very soft tissues.

The radiation source for a conventional apparatus is an X-ray tube, giving in output a bremsstrahlung spectrum in which the maximum energy corresponds to the tube voltage, and featuring two or more superimposed peaks, due to the characteristic emission of the anode material.

In the case of a mammographic tube, the anode material is usually Molybdenum, whose characteristic energies are 17.5 and 19.6 keV. The effective energy of the output spectrum, about 17 keV, is regarded as an optimal energy for mammography. The low flux from a clinical tube does not allow the use of monochromatization devices.

On the other hand, the high flux from a synchrotron radiation machine allows the use of a monochromator, in order to select the most suitable energy for each examination, depending on organ thickness and composition. In this way, there is no dose delivery due to low energy components, which are almost fully absorbed in the organ and thus do not contribute to the radiographic information, as it happens with a conventional X-ray tube.

The SYRMEP (Synchrotron Radiation for Medical Physics) experiment at the synchrotron radiation facility Elettra in Trieste consists in developing a mammographic apparatus with monochromatic synchrotron X-ray beams and with a linear Silicon pixel detector coupled to a single photon counting electronics. The combination of monochromatic beams and high efficiency detector has already given very promising results in mammographic phantoms and breast tissue imaging [2,3,4].

Planar imaging, however, does not allow the determination of the depth of a tumour inside an organ, and may also not allow the detection of very low variations in attenuation coefficients. These are the typical features of Computed Tomography (CT), a technique capable of providing the attenuation coefficient map by reconstructing a section of an object from attenuation profiles acquired along 180°.

Synchrotron radiation CT is currently investigated by several researchers [5,6,7], because a monochromatic and highly collimated beam obtainable from a SR machine would be an optimum tool for tomography: SR tomograms are not affected by artifacts due to beam hardening, which occur in tomograms acquired with polychromatic beams [8]; furthermore, reconstruction algorithms for divergent beam tomograms need geometrical corrections, which are not required for SR tomograms: this results in the use of the simplest possible reconstruction algorithms. A feasibility study of SR tomomammography, combining the advantages of SR mammography and SR tomography, has been carried out. Moreover, we have applied to tomography an innovative technique, called Diffraction Enhanced Imaging (DEI), in order to verify the velocity vIP possibility of detecting very low contrast details, which would be undetectable by means of conventional tomography.

Dipartimento di Fisica, Università di Trieste.

INFN Sezione di Trieste.

3 Sincrotrone Trieste SCpA. 2

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Materials and methods: the beamline and the detection systems The radiation source of the SYRMEP beamline is the bending magnet n. 6 of the Elettra storage ring. A schematic of the beamline is shown in figure 1. The only optical element of the beamline is the monochromator: it consists of a channel-cut Si (111) crystal, which provides X-ray beams in the energy range 15-35 keV. The beam width at the experimental hutch is 10 cm; the FWHM in the vertical direction is 4 mm at 20 keV.

Fig. 1. Sketch of the SYRMEP beamline.

Upstream the monochromator, a copper slits system allows one to select the dimensions of the beam impinging on the crystal. At the entrance of the experimental area, a tungsten slit system is used to match the beam size to the detector sensitive area, thus avoiding unnecessary dose delivery. Several stepping motors allow the positioning and the movimentation of the sample and of the detection systems. The translation stages allow an accuracy of 1 micron, while the rotation stage provides an accuracy of 10-3 degrees. After the tungsten slits, a parallel plate ionization chamber is used as a flux monitor to evaluate the dose delivered to the sample.

The detector designed by the SYRMEP collaboration is sketched in fig 2. It consists of a silicon microstrip detector used in "edge on" geometry, with the radiation impinging on the side rather than on the surface of the chip. The chip depth is 1 cm, the strip pitch (determining the pixel width) is 200 µm and the chip thickness (determining the pixel height) is 300 µm. The "edge on"

configuration allows a very high absorption efficiency in the mammographic energy range; the detection efficiency is actually limited by the presence of a "dead volume" in front of the strips; in this volume, the charge collection efficiency is reduced. Due to this fact, the detection efficiency ranges from 70% to 90% in the mammographic energy range [9]. The overall detector width is 5.12 cm, corresponding to 256 pixels. The detector is coupled to a single photon counting VLSI readout chain, named CASTOR (Counting and Amplifying sySTem fOr Radiation detection) [10]; each CASTOR chip consists of a preamplifier, a shaper-amplifier, a discriminator and a 16 bit counter. A photon interacting within the detector active volume produces a signal, which is processed by preamplifier and amplifier. Its amplitude is then compared to the discriminator threshold, and, if it is higher, the content of the counter is incremented by one unit.

When acquiring a tomographic image with the SYRMEP detector, the detector is kept stationary with respect to the beam, while the sample is rotated in discrete steps in front of the detector; for each angular position, a profile of the object is acquired and is stored in a row of a matrix (the sinogram); the map of the attenuation coefficients inside the sample is then reconstructed by means of the standard CT reconstruction technique called filtered backprojection [8].

Furthermore, a commercial photostimulable phosphor imaging plate (IP) BAS-MP2025 has been used; the plate area is (20x25)cm2. The IP is read out by a BAS-1800 reader. The reader scanning step, corresponding to the image pixel size, can be 50, 100 or 200 µm.

Each sinogram is acquired by simultaneously rotating the sample and translating the IP; the ratio of the sample rotation speed to the IP translation speed must be small enough to avoid artifacts on the reconstructed image. As

Fig. 3. Artifacts due to the motion of the imaging plate during the angular scan. a) vθ/vIP=1/2 degrees/mm; b) vθ/vIP=1/8 degrees/mm.

shown in figure 3, if the sample cannot be regarded as stationary during an IP translation corresponding to the pixel dimension, the image may appear blurred. Two images, corresponding to different ratios of the sample

Fig. 2. Sketch of the SYRMEP detector.

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angular velocity (vθ) to the IP translation (vIP), are shown.

In the present study, velocities where chosen in order to scan a 7.2 cm long portion of the plate during a 180° angular scan, thus providing an angular step of 0.125 degrees per image line when the IP is read out with a 50 µm step.

Absorption tomography As previously underlined, an ideal mammographic apparatus should provide both a high spatial resolution and a high contrast resolution. Furthermore, a good tomographic apparatus should provide a precise determination of attenuation coefficients, in order to allow one to determine the composition of tissues and to detect neoplastic formations.

Therefore, the first tomography tests at the SYRMEP beamline have been carried out on custom designed test objects, developed in order to check the fulfillment of the above conditions. Due to the small size of the SYRMEP detector, the test objects diameters are 3 or 4 cm.

Furthermore, a procedure was adopted to give a reasonable estimate of the dose which would be delivered to a full breast to obtain a comparable image quality in a real breast tomography.

The best indicator of biological risk in mammography is the mean glandular dose (MGD), which is defined as the dose delivered to the glandular component of the breast tissue. An average breast is in fact composed, according to NCRP [11], by 50% glandular tissue and 50% adipose tissue, the latter being non radiosensitive.

In planar mammography, MGD is evaluated for a compressed breast and thus the thickness along the beam direction is constant. In the present study, MGD for a circular breast section, with a variable thickness, was evaluated as the average of the MGD’s delivered to

Fig. 4. Images of the spatial resolution test object acquired at 25 keV. a) SYRMEP detector; b) Imaging plate.

infinitesimal width slices along the beam direction.

The MGD needed to image small objects is much smaller than the dose delivered to a standard size breast; in order to conduct a meaningful dose estimate, we evaluated the

dose needed to obtain a comparable image quality of a 12 cm diameter sample, assuming 12 cm as the maximum diameter for a breast section in tomographic geometry.

For each image of small objects, the average output photon fluence is calculated; then, the entrance fluence N12, which would give the same average output from a 12 cm diameter breast section, is computed. The MGD delivered by a photon fluence N12 to a 12 cm diameter breast, defined MGD12, represents, to a good approximation, the maximum MGD that would be delivered to a real breast.

The spatial resolution test has been carried out with a 4 cm diameter object, with five series of holes. The diameter of the holes ranges from 500 µm to 3 mm.

Figure 4 shows the image of the test object acquired at 25 keV with both detectors with the minimum dose necessary to obtain a signal-to-noise ratio equal to 5 on the 500 µm details. According to Rose’s criterion [12], 5 is the minimum SNR necessary to visualize a detail.

The MGD12’s corresponding to these images were 0.16 and 1 mGy for the SYRMEP detector and for the IP, respectively. These values are both lower than the dose delivered for a conventional mammography, which ranges between 1.2 and 1.8 mGy. The relevant difference between the MGD12 delivered in the IP and in the STRMEP image is due to two reasons: first, the SYRMEP detector has a much higher efficiency at 25 keV; moreover, the angular sampling step used for IP images is much smaller than the step actually needed to detect 500 µm details. This was done in order to avoid the motion artifacts previously described. This effect can be solved by moving the IP in discrete steps, and using a fast shutter to avoid IP irradiation during the translation.

Figure 5 shows the 500 µm details from images acquired with the SYRMEP detector with an increasing angular step size and a correspondingly increasing entrance fluence on the sample. The details can be detected even with a 4 degrees angular step, but their shape is better resolved with a fine angular scan.

A second test object has been built to evaluate the contrast resolution of the detection systems. It consists of a 2 cm diameter BR12 [13] cylinder with embedded chalk, silicone, wax, polyethylene and water details. All details are 5 mm in diameter. The images of this test object at 34 keV are shown in figure 6.

Table I shows the theoretical and the measured attenuation coefficients for images acquired with both detectors for water, polyethylene (PE) and BR12 at 34 keV, compared to the theoretical values.

The MGD12 required to obtain a SNR equal to 5 for the water detail, simulating a nodule embedded in breast tissue, were 0.1 mGy and 1.0 mGy for the SYRMEP detector and for the IP, respectively.

All attenuation coefficients are correctly reproduced.

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water (cm-1)

PE (cm-1)

BR12 (cm-1)

Theory

0.31

0.23

0.27

SYRMEP

0.28±0.04

0.21±0.04

0.24±0.05

IP

0.33±0.03

0.24±0.03

0.27±0.05

Table I. Theoretical and measured attenuation coefficient from images shown in figure 6.

Diffraction enhanced tomography A widely applied technique in synchrotron radiation imaging is the so called Diffraction Enhanced Imaging (DEI). DEI consists in placing a Silicon crystal, called analyzer, between the sample and the detector. DEI

Fig. 5. 500 µm details from images acquired at a constant delivered dose and with different angular steps: a) 180 steps, 100 ms/step; b) 90 steps, 200 ms/step; c) 45 steps, 400 ms/step.

would be useful in biological imaging, because the typical scattering angle of biological tissues is very close to the angular acceptance of a typical monochromator.

planes are parallel, only undeflected photons will be transmitted by the analyzer, which thus acts as a scatter rejecter.

On the other hand, when a slightly misalignment is introduced between the two crystals, other photons will meet the Bragg condition and different scattered components will transmitted with different weights, as described by the crystal rocking curve. This technique allows one to almost completely remove the primary radiation and to acquire scatter images. Several studies have already given good results in the visualization of soft tissues [14,15].

Fig. 7. Working principle of Diffraction Enhanced Imaging.

In this study, we applied DEI to tomography.

Reconstruction algorithms in conventional tomography are based on the assumption that most of the scattered radiation is removed by means of collimators; in DEI tomography, one is not dealing with line integrals as in

Fig. 9. X-ray deflection in the paraxial approximation: a) beam impinging on the top of the wire; b) beam impinging on the center of the wire.

Fig. 6. Images a BR12 test object containing chalk (center), water (top), polyethylene (left), wax (right), silicone (bottom): a) SYRMEP detector; b) Imaging plate.

Figure 7 shows a schematic explanation of the DEI working principle.

The radiation transmitted through the sample consists of both primary (undeflected) and scattered photons.

When the analyzer is perfectly aligned with respect to the beamline monochromator, i.e. when the lattice

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transmission tomography. Nevertheless, to some approximation the same reconstruction algorithms can be used.

Due to geometrical configuration of the apparatus, DEI tomograms can only be acquired with the imaging plate, because the beam transmitted after the analyzer is not parallel to the entrance beam, and the SYRMEP detector cannot be aligned with respect to the beam.

Two possible approaches are possible: the first one is based on contrast enhancement effects, while the second one is based on edge enhancement effects.

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a) Contrast enhancement effects.

Contrast enhancement effects are based on the difference in the shape of the scattering curves of different

tomogram (on the top of the rocking curve), can be detected when working on the slope of the rocking curve and becomes more visible as the transmitted/primary

Fig. 8. DEI tomograms acquired at 17 keV: a) "top" of the rocking curve; b) 3% transmitted intensity; c) 0.3% transmitted intensity.

materials: low scattering materials have a high scattered intensity in a very small angular interval, while a high intensity can be obtained from highly scattering materials at larger angles.

In figure 8 we show the reconstructed tomograms of the same test object shown in fig. 6. The images were acquired at 17 keV. The ratio of the transmitted intensity by the analyzer over the primary intensity, corresponding to the position on the rocking curve, is indicated below each image. The delivered MGD12 is 1.5 mGy for all images.

The water detail, which is not visible in the scatter-free

intensity ratio is made smaller: since BR12 scatters more photons at a wider angle, if we introduce a large misalignment between the two crystals, a high intensity from BR12 can still be detected, while the intensity from the water detail is very low. b) Edge enhancement effects.

When imaging a sample with a very poor absorption contrast but very sharp edges, this can be detected because of the phase shift introduced on the incident wave by the sharp refractive index variations corresponding to the detail edges: the so called phase

Fig. 10. Detail from images a test object showing a 100 Âľm wire: a) transmission; b) positive misalignment; c) negative misalignment.

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contrast imaging is based on this principle. In the paraxial approximation [16], the phase shift for a ray path through an object is proportional to the integral of the refractive index along the beam direction, and the angular deviation is directly proportional to the gradient of the phase shift in the direction transverse to the direction of propagation.

DEI images can also be described by means of this principle.

As shown in figure 9, a thin beam impinging on a nylon wire can give rise to different effects, since the angular deviation becomes as large as the distance from the center of the wire increases: when the beam is impinging on the top of the wire (a), a high intensity from the wire can be detected when the misalignment of the analyzer is positive, a small intensity is detected when the two crystals are pefectly aligned and the signal is completely removed by a negative misalignment. On the other hand, when the beam strikes the centre of the wire (b), a high intensity is detected when the two crystals are aligned, and the detected intensity is smaller when moving on the slopes of the rocking curve of the crystal.

The effects are shown clearly in figure 10: figure 10.a shows the detail of a transmission image of a custom made test object with a 100 µm nylon wire. The wire is visible, but no information about its orientation can be obtained.

Figure 10.b and 10.c are acquired with a 100 µm beam and two opposite positions of the analyzer with respect to the beamline monochromator: in figure 10.b the misalignment is positive, while in figure 10.c it is negative. The total intensity diffracted by the crystal is 10% of the primary one. A higher intensity comes from the zone called "A" with a positive misalignment of the analyzer (figure 10.b), and the signal is completely removed in the region named "B".

The opposite situation takes place with a negative misalignment (figure 10.c). This demonstrates that the beam is impinging on the upper part of the wire in A, and on the lower part in B. Hence, this gives us an information about the orientation of a structure, not obtainable from the transmission image.

The delivered dose is the same for the transmission and for the DEI images.

further information about the scattering properties of materials and about the orientation of structures. The delivered dose is comparable to the one delivered in clinical mammography, thus opening the way to clinical applications of both techniques.

References [1] Shaw De Paredes E, Radiographic breast anatomy: Radiologic signs of breast cancer, in Syllabus: a categorical course in physics - Technical aspects of breast imaging, M Yaffe ed., Oak Brook, IL, RSNA Publications, 1993, 35-46.

[2] Arfelli F et al. The digital mammography program at the SR light source in Trieste. IEEE Trans Nucl Sci 1997: 44; 2395-2399.

[3] Arfelli F et al. Low dose phase contrast x-ray medical imaging. Phys Med Biol 1998: 43; 2845-2852.

[4] Arfelli F et al. Improvements in the field of radiological imaging at the SYRMEP beamline. SPIE 1999: 3770; 2-12.

[5] Salome M et al. A synchrotron radiation microtomography system for the analysis of trabecular bone samples. Med Phys 1999: 26; 2195-2204.

[6] Dilmanian FA et al. Single-and dual-energy CT with monochromatic synchrotron x-rays. Phys Med Biol 1997: 42; 371-387.

[7] Beckmann F et al. X-ray microtomography (µCT) using phase contrast for the investigation of organic matter. Journal of Computed Assisted Tomography 1997: 21(4); 539-553.

[8] Kak AC, Slanley M. Principles of Computerized Tomographic Imaging. New York. IEEE Press.

[9] Arfelli F et al. At the frontiers of digital mammography: SYRMEP. Nucl Instr Meth in Phys Res A 1998: 409; 529-533.

[10] Colledani C et al. CASTOR: a VLSI CMOS analog-digital circuit for pixel imaging application. Nucl Instr Meth in Phys Res A 1997: 395; 435-442.

[11] Mammography: a users’ guide. NCRP Rep. No. 85. Bethesda, Md. NCRP 1985.

[12] Rose A. Vision : human and electronic. New York. Plenum 1973. 21-23.

[13] White DR et al. Epoxy resin based tissue substitutes. Br J Radiol 1977: 50; 814-821.

[14] D. Chapman et al. Diffraction enhanced X-ray imaging. Phys Med Biol 1997: 42; 2015-2025.

[15] Arfelli F et al, Mammography with Synchrotron Radiation: Phase Detection Techniques. Radiology 2000; 215: 286-293.

[16] S. W. Wilkins et al. Phase-contrast imaging using polychromatic hard X-rays. Nature 1996: 384; 335-338.

Conclusions At the SYRMEP beamline we have built a setup which allows the acquisition of both conventional and diffraction enhanced tomographic images, with a particular attention to possible applications to mammography. Conventional (absorption) tomograms allow a good visibility of small size and low contrast details, while diffraction enhanced tomograms give

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Articolo ricevuto in redazione nel mese di Ottobre 2000

NEUTRON REFLECTIVITY STUDIES OF IMMISCIBLE POLYMER/POLYMER INTERFACES Michele Sferrazza Department of Physics, University of Surrey, Guildford, Surrey GU2 5XH, U.K. Abstract In this paper, a detail study of the interfacial properties between two immiscible polymers is reported. The structure of the interfacial width between immiscible polymer pairs is studied using neutron reflection. Neutron reflectivity, probing difference in the density profile perpendicular to the plane of the sample, is ideal for this type of studies. Deuterated polystyrene (d-PS) on poly(methyl methacrylate) (PMMA) over a range of thickness and molecular weights were used. A logarithmic dependence of the interface width on film thickness is observed, characteristic of an interface broadened by thermal induced capillary waves, whose spectrum is cut-off by dispersion forces across the polymer layer. If the system is inverted, a thin dPMMA layer on a thick PS film, the Hamaker constant of the system becomes negative and dispersion forces will tend to destabilise the interface. In this case, thermally capillary waves are amplified and grow until the top film dewets. This case was studied with both specular and off-specular neutron reflections.

Introduction Polymer surfaces and interfaces play an essential role in many commercial applications of polymers, such as coatings, adhesives, blends and resists. Understanding the microscopic processes taking place at interfaces is, therefore, increasingly important for the wide ranging uses of polymers in many industrial applications. The nature of the interface between immiscible polymers is important to investigate, both because these interfaces provide model systems to elucidate fundamental problems of the statistical mechanics of surfaces and interfaces, and because an understanding of the microscopic structure of the polymer interface will help to address technological questions connected to the adhesion of polymers and the properties of multiphase polymer systems.

The interface between two immiscible polymers is not atomically sharp. This is because the unfavourable enthalpy of mixing that occurs at a diffuse interface is offset by a gain in chain entropy. The self-consistent field theory predicts the width and the interfacial tension for an immiscible polymer pair. The volume fraction profile of one components through the interface is predicted to take the hyperbolic tangent profile form, the interfacial width varies as χ--1/2 for small χ (χ is the Flory-Huggins interaction parameter), and the surface energy is proportional to χ1/2 [1]. The experimental interfacial

width obtained for incompatible polymers is in general broader than the value predicted by the self-consistent field theory developed by Helfand and Tagami [1]. An understanding of the reason for the discrepancy is fundamental for developing a better understanding of the properties of polymer interfaces.

The experiments performed on the PS/PMMA bilayer system described in this paper show the importance of long-range dispersion interactions across a thin polymer film in modifying the structure of the interface between the two polymers. While for a thin PS film on a thick PMMA substrate, the long range van der Waals forces tend to stabilise the interface and an equilibrium interfacial width is obtained, for some cases the dispersion forces will instead tend to destabilise the interface and the dewetting of the top layer on the substrate film will take place. This process is caused by the instability of the film against thermally excited capillary waves at the interface and/or surfaces. For the liquid/liquid system, the instability of the films against capillary wave fluctuations at the interface is important and difficult to study in detail, since the interface of interest is buried. We have studied this case of dewetting using specular and off-specular neutron reflection.

Neutron reflection Reliable experimental measurements to test in detail the theoretical prediction for the interfacial width between two polymers have become available quite recently, with the technique of neutron reflection providing the most accurate information. The wavelength of cold neutrons is of the order of a few tenths of manometers, and this sets the length-scales probed by reflection experiment.

Typically the experiment is sensitive to structural features perpendicular to the plane of the film with length scales between 0.5 and 50 nm [2,3,4]. Two other advantages of neutrons make them particularly suited for studying organic film: their penetration power is larger than for example, X-rays, and so it is straightforward to study buried interfaces, and the great difference in neutron scattering length density between deuterium and hydrogen. When neutrons propagate through a medium in which the scattering centres are small compared with the wavelength of the neutrons, the effect of the medium can be represented by a pseudo-potential. This pseudopotential has a magnitude that is related to the scattering

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length density of the material by V eff =

rn − 2 , n =

2πh2 m bV

where b is the sum of all the scattering lengths in a volume V. The neutrons obey to the Schrödinger equation and if the potential is a function of only one direction, z, the perpendicular component of the motion then obeys a one-dimensional equation  h2  d 2 Φ − + Vz = EΦ  2  2 m  dz

where E = (h–2 k2)/em, and k is the perpendicular component of the wavevector of the incoming beam. This equation can be solved to give the amplitude reflectance r and then the reflectivity R = |r|2. If neutrons propagate from vacuum into a uniform material of scattering length density b/V, the perpendicular component of the wavevector within the material ki is expressed by ki = k 2 − 4π

b V

In the case 4πb/V > k2 , ki is imaginary and then the neutrons propagate into the material only as an evanescent wave, giving rise to total external reflection (reflectivity is unity). In the case of 4πb/V < k2 and a sharp interface (between vacuum and medium), then the reflectivity is given by the Fresnel expression 2

R= r =

k − ki k + ki

that, for high value of k, has the limiting form of R ~π2

2

 b 1  V  k4

If the interface is rough or graded, the reflectance of the interface is modified and a roughness factor is introduced that describes the interface, given by

r = rFresnel e −2 kkiσ

2

where σ is the Gaussian width of the interface. The reflectivity of a multilayer of stack of thin films can be calculated by the following recursive scheme. For each slab i and i-1, the reflectance denoted by ri-1,i , is given by the Fresnel expression: ri −1, i =

ki −1 − ki ki −1 + ki

The combined reflectance of the interfaces between the substrate and layer n-1, and layer n-1 and n-2, denoted by rn-2,n, is given by the combination of these individual interfaces:

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rn − 2 , n −1 + rn −1, ne2ikn−1dn−1 1 + rn − 2 , n −1rn −1, ne2ikn−1dn−1

where d is the thickness of the slab. By combining this reflectance with the reflectance of the interface between layers n-3 and n-2 to yield the reflectance of all interfaces from n-3 to the substrate, the process can be continued recursively until the top interface is reached and therefore the reflectivity is obtained. This algorithmic can be applied to calculate a general profile. By approximating a continuous profile by a stack of such thin layers (normally one chooses layer thickness to give a constant increment in scattering length density between each layer), one can calculate the reflectivity of any profile to the accuracy required within a given k range.

The continuum limit of this expression, in the limit of high k, is r( 0 ) =

π d ( b / V ) 2ikz e dz ∫ dz k2

Thus, the reflectivity is simple related to the Fourier transform of the derivative of the scattering length density profile.

In practice data analysis has a number of difficulties.

Since only the potential V determines the density profile, if the sample contains more than two unknown compositions then the composition profile cannot be uniquely determined.

Also, there is no reliable way of inverting a neutron reflectivity profile to recover a unique potential profile V. Instead a trial function needs to be used as a starting point and its parameters refines until a best fit is achieved between the experimental and predicted reflectivity profiles. This implies that neutron reflectivity is not suitable to analyse films of unknown composition; it should instead be considered as a powerful tool for characterising the structure of thin films at high resolution when their unknown morphology is already know at some coarse level and one has some knowledge of the materials from which they are composed.

Neutron reflection is at its most powerful when samples are specifically made for the technique, using in particular a scheme of labelling selected components with deuterium substituted for hydrogen. For neutron reflectometry, in fact, the contrast can be generated by the deuteration of one component. This consists of exchange, for example in polymers, of the hydrogen in the molecule's chains with deuterium.

Figure 1 shows a schematic diagram of the CRISP neutron reflectivity instrument at the ISIS spallation facility, Rutherford Appleton Laboratory (UK). A white neutron beam, with a wavelength range of order 0.5-6.5 Å is incident on a sample at grazing angle of incidence, which is typically between 0.25 and 1.5 degree. These

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give a q range between 0.008 Å-1 and 0.3 Å-1. The neutron beam is inclined at 1.5o to the horizontal, which allows easier study of liquid surfaces. Fine collimation is obtained by two slits before the sample, which define the resolution and illuminated area of the sample position.

There are another two slits before the detector position to suppress the background noise. Neutrons specularly

methacrylate) (PMMA) blend, the discrepancy between the theory and the experimental result is still present. For this case, the theoretical interfacial width w between PS/PMMA can be estimated using the expression

.

With a reference volume equal to the volume of a PS

Fig. 1. Schematic diagram of a neutron reflectometer - CRISP, at the Rutherford Appleton Laboratory.

reflected are detected and sorted by wavelength using their time-of-flight. If one is interested only in the specular reflection a single 3He detector can be used, but often a liner or two dimensional position sensitive detectors is used which has the advantage that the intensity of neutron in the off-specular direction can be studied.

This allows the investigation of in-plane correlation.

After data processing, the intensity of the reflected neutrons are displayed as a function of the wavelength, or more normally as a function of the change in wavevector on reflection – the scattering vector q, which is related to the wavelength of the neutron λ and the incident angle ϑ by the expression q=

4π sinϑ λ

A study of the PMMA/PS interface The main result of the self-consistent theory is that the interfacial width between two immiscible polymers varies as χ--1/2 . Measured values of the polymer-polymer interfacial width, obtained with the neutron reflection technique for various types of polymer interfaces, ranging from block copolymers to polymer brushes in polymer matrices, are typically higher than the values extracted from the self-consistent field theory. For the system of a polystyrene (PS) and poly(methyl

repeat unit, with χ=0.04, and with the statistical segment lengths for PS and PMMA as a(PS) = 6.7 Å and a(PMMA)=7.5 Å, the calculated interfacial width is 29 Å [1,5]. Experiments by different groups gave a value for the interface width between PS and PMMA of around 50 Å, independently of the molecular weight (Mw) and film thicknesses used [6,7]. This result for the interfacial width between immiscible polymers is not reproduced by the self-consistent field theory. A possible explanation has been suggested recently: the experimental result agrees with the theoretical prediction if a correction to the interfacial width due to capillary wave fluctuations is considered [5].

To study in detail the contribution to the interfacial width given by the capillary wave term, neutron reflection experiments using the reflectometers CRISP and SURF at the Rutherford Appleton Laboratory have been performed [8]. Bilayers of d-PS on h-PMMA were prepared by spin-casting a PMMA layer onto a silicon substrate, producing a thick film between 4000 Å to 9000 Å. The silicon substrates used were disks polished on one side, of diameter 5 cm and thickness 0.5 cm with orientation (111). The d-PS film was first spun-cast on a glass slide and then floated onto the PMMA. The thickness of the top d-PS layer was varied between ~50 Å and ~20000 Å. All the film thicknesses were also measured using ellipsometry. The samples prepared in

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this way were then dried at 60 oC in a vacuum oven for some time and then annealed at 120 oC for 6 hours: this time was found to be sufficient to reach equilibrium, as neutron reflectivity experiments at different annealing times have shown. All the polymers were obtained from Polymer Laboratories (UK) and were prepared by anionic polymerisation with Mw/Mn of 1.1 or less. The Mw of the polymers used was ranging from 200K to 400K. Neutron reflectivities were measured for both the unannealed and the annealed samples.

Figure 2 shows the reflectivity curves for a system as an example. For all pairs used, the film thickness of the bottom PMMA layer was quite thick and the film thickness of the d-PS layer was varied: the reflectivity curves in fact showed interference fringes characteristic of the d-PS layer thickness and not of the PMMA layer.

The solid lines in figure 2 are fits to the reflectivity curves

The interfacial width parameter ∆ plotted in figure 3 is connected to the hyperbolic tangent profile 2w by ∆ = 2 w / 2π , therefore a ∆ value of 20 Å corresponds to a hyperbolic tangent value of 50.1 Å, in very satisfactory agreement with the values obtained by other workers.

From figure 3 it is clear that for thinner d-PS films there is a thickness dependence of the value of the interfacial width with the film thickness, and it is at least approximately logarithmic. This logarithmic dependence persists to up d-PS layer thickness of ~1000 Å, after which the data clearly levels off to a value of ~20 Å. To analyse the result expressed by figure 3 more quantitatively, the assumption that the contributions to the interfacial width due to the intrinsic interface and to the capillary wave broadening add in Gaussian quadrature is taken [9,10].

The theoretical total interfacial width is therefore expressed by [8]: k BT (2π / ∆ o ) ln 4πγ (2π / λcoeh )2 + (2π / adis )2 2

∆2 = ∆2o + ∆ζ 2

= ∆2o +

where ∆0 is the intrinsic width, λcoeh is the neutron coherence length and adis is a dispersive capillary length.

The second term is the capillary wave contribution imaging the interface as if it were a membrane in a state of tension characterised by a bare interfacial tension γ that sustains a spectrum of waves, each of whose average energy is determined by equipartition of energy. Since one of the polymer is rather thin, the dispersive or Van der Waals forces across the film are important and are responsible of the dispersive capillary length in the above expression given by [8] Fig. 2. Neutron reflectivity curves as a function of the transfer momentum Qz for d-PMMA/d-PS pairs. The bottom layer was thick ~ 5000 Å and the d-PS layer was varied from 60 Å to 20000 Å. The solid lines are fits to the data as described in the text. The curves are shifted down by factors of 100 from each other for clarity.

obtained by a least-squares fit to a multilayer model with Gaussian roughness introduced at the surface and film interfaces. The parameters involved in the fit are measured independently, except for the roughness between the d-PS and PMMA layer, which is the parameter fitted. The silicon dioxide layer thickness, PMMA layer and d-PS layer thickness were also measured by spectroscopic ellipsometry. The surface roughness was kept constant during the fitting at a value of 7 Å: this value has been extracted by independent measurements and is in line with literature data [8]. The results of the interfacial roughness extracted by the fitting of the reflectivity curves for all pairs measured, are plotted in figure 3 as a function of the thickness of the d-PS layer.

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2 = adis

4πγl 4 A

where l is the top film thickness and A is the Hamaker constant for the interactions between the substrate polymer and the air across the thin film. The Hamaker constant can be estimated from refractive index and dielectric constant data using an approximation based on Lifshitz theory [11].

For our system a value of 2⋅10-20 J is obtained. For film in the thickness range of 50 Å to 500 Å, the dispersive capillary length may be estimated as falling between 300 Å and 3 microns. Thus for films in this range of thickness, this dispersive capillary length rather the neutron coherence length dominates the capillary waves expression. This explains the logarithmic behaviour of the width up to ~1000 Å and the level off of the width forth thicker d-PS thickness, the region where the neutron coherence length is dominant. Using the expression, the data of figure 3 have been fitted varying the intrinsic interface width ∆0 and the interfacial tension γ. The solid

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line in figure 3 is the fit obtained. The value of the neutron coherence length has been fixed at 20 µm. The fitted parameter values are ∆0= 9.3 ± 1.4 Å and γ=2.7 ± 0.3 mJm-2.

It should be recognised that the estimate of the Hamaker constant A is quite uncertain: in fact over the range of the thickness used in our experiments, retardation effects are likely to become important, and so the effective Hamaker constant will also depend on the thickness. Luckily since A appears in the logarithm, the results are rather insensitive to the value chosen for A: this is demonstrated by the fact that a change of 50 % on the value for the Hamaker constant results in a change of 10 % on the best fit of ∆o, and has even less effect in the best fit value of γ.

The values of the interfacial width ∆0 and interfacial tension γ extracted from the best fit can be compared with the ones predicted by the self-consistent field theory: γ = aρkBT

χ 6

where a is the statistical segment, χ the interaction parameter, and 1/ρ is the volume of the monomer repeat unit.

For the temperature used in the experiments described in this section, the value of the Flory-Huggins interaction parameter is χ = 0.037 ± 0.002 [12]. Using this value for χ,

Fig. 3. The interfacial width parameter ∆ for the d-PS and PMMA interface as a function of the d-PS film thickness, as measured by neutron reflection. Different symbols are for different systems.

a statistical segment length a=7 Å and a volume of the monomer repeat unit which takes a value of 174 Å3, ∆0 and γ as predicted by the self-consistent field theory are ∆0= 11.8 ± 0.6 Å and γ=1.7 ± 0.05 mJm-2. Therefore the comparison between the deduced bare interface width ∆0 extracted from the experimental data and the value determined from the self-consistent field theory is

excellent, while our deduced interfacial energy is somewhat larger than the one predicted. However the approximation implicit in using the capillary wave expression to fit the data has to be considered. For example in using the equation, the assumption that the free surface of the d-PS film behaves like a rigid wall is taken, whereas it will have its own spectrum of capillary waves that will be coupled to the waves at the interface.

But, since the surface energy of d-PS is about an order of magnitude larger than the d-PS/PMMA interfacial energy, the rigid wall assumption is a reasonable starting point.

Early stage of spinodal dewetting for the PS/PMMA system As we have seen in the previous section, if the bottom layer is a thick PMMA film on top of a silicon substrate and the top layer is a thin PS film, the Hamaker constant for this system is A=2⋅10-20 J. The inverted system, the PS is a thick layer now at the bottom (on a silicon substrate) and the top layer is a thin PMMA layer, has a negative Hamaker constant of A=–1.7⋅10-20 J. In this case, dispersion forces amplify thermally excited capillary waves at a fluid interface until dewetting takes place: this process is known as spinodal dewetting. The study of the process of dewetting in thin polymer films, because of the importance of thin films and coatings in technology, has received much attention recently [13-19]. The excess free energy per unit area due to a sinusoidal perturbation of wave vector q and amplitude ζq is Aζ q2 / 8πh 4 [20]. Thus if the Hamaker constant is positive capillary waves lead to an increase in the system's free energy - dispersive forces in this case stabilise the interface against capillary wave fluctuations.

On the other hand, if the Hamaker constant is negative, the growth of long wavelength capillary waves leads to a lowering of the system's free energy and the onset of instability, with the wave vector for marginal stability qc given by qc=(A/2πh4σ)1/2 where σ is the interfacial tension.

At early times the dynamics of the instability can be solved in the linear approximation [21]; for a thin liquid film A on a liquid substrate B [22] the mode that leads to dewetting is called 'peristaltic mode': in this case the displacements of the free surface and the fluid/fluid interface are in antiphase.

Moreover the polymer/polymer interfacial tensions are typically an order of magnitude smaller than the polymer surface tensions, so for a polymer film on a polymer substrate it is the interfacial tension that dominates.

Similarly, in a pure peristaltic mode the ratio of the amplitudes of the displacement of the surface and the displacement of the interface is equal to the ratio of the surface tension to the interfacial tension so in a polymer/polymer system we expect the displacement of

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the surface to be much smaller than the displacement of the polymer/polymer interface. In the liquid/liquid case, the expression for the rise time of the unstable mode is different to that for a film on a solid substrate (dissipation in the substrate becomes important or dominant); it takes the form

τm ~

h6ηBσ LA

2

measured first. After measuring the reflectivity, for each sample long scans were taken for study of the offspecular scattering at the same qz value. The resolution was relax in the vertical direction and very well defined in the horizontal direction. The intensity of the offspecular scattering as a function of the detection angle from the specular position was extracted. Reflectivity and off-specular scattering measurements were measured for different annealing times. On D17 the diffuse scattering

where L the thickness of the substrate film [22].

Spinodal dewetting has been predicted to be the main dewetting process for ultra-thin films with a negative Hamaker constant. The main predictions of the theory are that the wavelength of the fastest growing wave at the interface or surface is proportional to the square of the thickness of the film; and that the rising time of the amplitude of the wave increases with the inverse of the nth power of the thickness, where n depends upon the type of substrate on which the thin film is deposited.

For the Si/PS/PMMA system, we then expect that the fluctuations at the interface between the two polymers should now be amplified until rupture of the film takes place. The characteristic wavelength of the fast mode is: qm ~

a l2

If the expression for a=(A/6πσ)1/2 is recalled and a thickness of 100 Å is considered, the fast mode has a wavelength of the order λ~1µm, where a value of 2mJm-2 for the interfacial tension between PS/PMMA is taken.

The interface under interest is buried, and the only way of monitoring the development of the unstable mode at the interface between the two polymers is the study of off-specular neutron scattering. Off-specular scattering is in fact sensitive to the in-plane structure of the surface or interface, whilst specular scattering is sensitive to the change of refractive index in the direction perpendicular to the interface.

The characteristic size of the fluctuations in the plane of the interface can be extracted from off-specular neutron analysis. The change of its intensity with time can be connected with the characteristic rising time of the unstable mode. From the theory introduced previously, the wavelength of the unstable fastest mode and its characteristic rising time depend strongly on the thickness of the top layer. To study this effect in detail, samples with different top film thickness were used. The neutron reflection experiments were performed using the reflectometer D17 at ILL (Grenoble, France) and SURF at the RAL (Didcot, UK) [23]. Bilayers of PS/PMMA were prepared: the top d-PMMA layer had a thickness in the range of 95 to 150 Å, while the bottom h-PS layer had a thickness around 1300 Å. The annealing temperature was fixed at 155 oC. For each sample, the reflectivity was

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Fig. 4. Reflectivity curves for the 95 Å samples for different annealing times, from unannealed (top curve) to 24 hours (bottom curve). Fits to the data for some curves are also shown and described in the text. The data have been shifted by factors of 100 with respect to each other for clarity.

In the inset the surface and interface roughness are reported as well the surface roughness of a contrast match experiment (see text for details).

was acquired for each sample at a fixed qz value, while on SURF the off-specular neutron scattering was measured at a fixed angle of 1.2o. The qz value at which the long measurements on D17 were performed was qz=0.0127Å-1.

For each annealing time, the reflectivity was measured first. Figure 4 shows the reflectivity curves for a sample with top d-PMMA thickness of 95 Å for different annealing times. The solid lines for the first 4 curves are fits to a 3-layer model, with roughness parameters both at the surface and interface. Clear features were visible from the reflectivity measurements: for the unannealed sample and up to 6 hours of annealing time, interference fringes due to the bottom layer were visible, corresponding to a thickness of around 1300 Å; fringes corresponding to the top d-PMMA layer were also clearly visible. The intensity of these fringes decreased with increasing annealing time and they were still present up to 6 hours, although quite reduced in intensity.

After 6 hours, a clear difference in the reflectivity was visible, with the top d-PMMA fringes disappearing. The fits with a multilayer model were good up to 6 hours of annealing time. The solid line for the 6 hours reflectivity

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was the best fit obtained with the model used. From the fitting of the reflectivity data, the values of the surface and interface roughnesses were extracted and are reported in the inset to the figure. It is possible to observe that the top surface roughness was around 8 Å up to 2 hours of annealing.

The reflectivities for the 3 and 6 hours annealing time samples were fitted if the top roughness was increased to 13 Å and 18 Å respectively. The surface and interfacial widths, as compared with a hyperbolic tangent profile width, assumed values of 45 Å and 48 Å for the 6 hours annealing time sample, comparable to the total thickness of the top d-PMMA film. In the fitting procedure the surface roughness and interface roughness are correlated, so to be confident that it was the interface roughness that was undergoing the largest increase we performed an analogous experiment in contrast matched conditions.

We made a PS/PMMA bilayer where the top 100 ± 6 Å film was a mixture of h-PMMA and d-PMMA whose composition was such that the top layer had the same scattering length density as PS. For this sample no reflection at all is expected at the polymer/polymer interface and the data fitting is sensitive only to the surface roughness, not to the roughness of the polymer/polymer interface. The fitted surface roughness values as a function of time (shown in

Fig. 5. Reflectivity curves for the 110 Å samples for different annealing times, from unannealed (top curve) to 72 hours (bottom curve). Fits to the data for some curves are also shown and described in the text. The data have been shifted by factors of 100 with respect to each other for clarity.

In the inset the surface and interface roughness are reported as well the surface roughness of a contrast match experiment (see text for details).

the inset to figure 4) confirmed that the loss of visibility in the fringes seen for the non-contrast match sample is dominated by the roughening of the polymer/polymer interface. This was also confirmed by an Atomic Force Microscopy (AFM) study [23]. Thus even at early times,

long before gross dewetting is visible using optical microscopy and while the surface remains essentially smooth, the growing capillary waves that ultimately lead to dewetting manifest themselves in a continually increasing roughness of the polymer/polymer interface.

Figure 5 shows as another example the reflectivity curves for samples with top d-PMMA film of thickness of 110 Å corresponding to the unannealed, 6, 12, 24 and 72 hours of annealing time. The fringes due to the top 110 dPMMA layer were clearly present up to 12 hours of annealing, and still visible after 24 hours of annealing time. However, they instead disappeared for the 72 hours case. This was different from the previous case, where the fringes due to the 15 Å smaller d-PMMA film disappeared after just 6 hours of annealing. In the inset of figure 5, the surface and interface roughnesses are plotted as a function of the annealing time. In the inset also the surface roughness for the contrast match experiment is also reported.

The interface roughness that is probed by specular neutron reflectivity represents a combination of the intrinsic diffuseness of the interface with a contribution from capillary waves integrated over all possible wavelengths up to an upper limit imposed by the lateral coherence length of the neutron beam, which in this geometry is of the order of 20 µm.

Clearly in order to provide a more searching test of the theory it would be desirable to study the growth of capillary waves in a way that discriminates between modes of different wavelength. To do this we turned to measurements of the intensity of neutrons scattered out from the specular beam. A quantitative analysis of this can be achieved if a long scan at a fixed qz is performed for all the samples, which have been annealed for different lengths of time. The intensity of the diffuse scattering as a function of the angle from the specular position, or the in-plane transfer momentum, can be plotted for the long scan measurements.

Figure 6 reports the diffuse scattering for different annealing times as a function of the angle from the specular position for the samples with top thickness of 97 Å. The measurements reported in figure 6 were taken at a constant qz=0.0127 Å-1, and the intensities were normalised to the specular intensity. The data for the unannealed sample shows a rapid and monotonic fall of intensity from the specular peak; after annealing, however, a prominent shoulder appears which grows in size with increased annealing time. The top inset plots in figure 6 shows the excess scattering relative to the unannealed sample plotted against the scattering angle.

These show that there is a peak in intensity at a specific scattering angle, which for this case correspond to a scattering object with a wavelength of ~1 µm.

The intensity of the scattering peak grows with annealing

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time, and there are indications that at later times the peak moves to smaller wavevectors. We interpret the data as arising from diffraction from capillary waves at the polymer/polymer interface, with the peak arising from scattering from the fastest growing capillary wave. The analogy with bulk spinodal decomposition is obvious.

Once again, we used contrast matching to confirm our

for samples with various PMMA thickness is reported in the bottom inset to figure 6. The dashed line corresponds to a quadratic dependence of the characteristic wavelength λm on the film thickness h:λm =bh2 , where the best fit value of b is 1.1(± 0.1) 1010 m. We expect such a quadratic dependence from theory; this also predicts for the value of the pre-factor b=a(π3σ/A)1/2 [22].

The value of the PS/PMMA interfacial tension calculated from self-consistent field theory is 2.0mJm-2 and the surface tension of PMMA at 150 oC is 31mJm-2, so we can deduce a value of the Hamaker constant A= -0.8⋅10-20 J. This is of a plausible order of magnitude, though it is somewhat smaller that our estimate of A= -1.7⋅10-20 J derived from an approximation of Lifshitz theory.

Nonetheless, given the uncertain inherent in making this kind of estimate this degree of agreement is satisfactory.

One point that may be important is that for these film thicknesses retardation effects should begin to be significant, decreasing the effective Hamaker constant with increasing thickness, and producing a slightly weaker than quadratic dependence of characteristic wavelength on film thickness.

Finally we consider the time dependence of the

Fig. 6. Diffuse scattering for different annealing times as a function of the angle from the specular position for the sample with top d-PMMA thickness of 95 Å. The legends in the figures distinguish different annealing times. The Qz value for these measurements was 0.0127 Å-1.

The top inset shows the excess scattering for the different measurements showing a characteristic wavelength that grows fastest. In the bottom inset the characteristic wavelengths as a function of the d-PMMA thickness extracted form the diffuse scattering are also shown (see text for details).

supposition that the scattering is dominated by the polymer/polymer interface; when the PMMA film is contrast matched to the PS film, so no scattering occurs at the polymer/polymer interface, we saw no particular change in the off-specular intensity as a function of the annealing time.

The off-specular neutron scattering peaks observed at earlier times, when the surface was still flat, were therefore connected to the instability of the PS/PMMA interface. In the case studied here, considering that an estimation of the wavelength characteristic of the fastest unstable mode for the PS/PMMA case is 1 µm (as has been shown previously), the case where a dependence of the top PMMA thickness of l6 is expected for the rising time of the unstable wave. It is worthwhile to mention that the theoretical approach is valid for the early stage which leads to the rupture of the film, and off-specular scattering is a powerful technique to investigate this for a buried interface.

The characteristic wavelength for the instability at early times, extracted from the off-specular neutron scattering

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Fig. 7. Diffuse scattering intensity as a function of annealing time for the 3 different thickness systems: 95 Å, 110 Å, and 147 Å. In the inset the characteristic rise times of the interfacial instability are shown as a function of the d-PMMA film thickness. The line is the dependence of the rise time to the sixth power on the thickness.

interfacial instability. An important features of spinodal dewetting is the very strong dependence of the rise time of the interfacial instability on film thickness; it is this strong thickness dependence of the kinetics of dewetting that clearly distinguishes spinodal dewetting from, for example, nucleation and growth of holes. Figure 7 shows the relative intensity at the maximum of the off-specular peak as a function of film thickness for three films measured on D17. At early times the growth of intensity

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is exponential, saturating at later times. The rise times are shown in the inset. They show the very strong dependence of rise time on film thickness characteristic of spinodal dewetting; a 50 % increase in film thickness leads to a 770 % increase in rise time. Of course, given the sparsity of the data it is not possible to test predictions for this dependence quantitatively, but the inset shows the curve for dependence on the sixth powers of thickness, as predicted for liquid/liquid dewetting. In the future more experiments and a more detailed analysis of the offspecular scattering should be possible, allowing one to test the predictions of a linear theory for the growth rates of capillary wave modes of arbitrary wavevector and to probe the breakdown of the linear approximation.

Conclusions In conclusion we found that the interfacial width between two immiscible polymers has a logarithmic dependence on film thickness, indicating that capillary waves contribute substantially to the interfacial width as measured by neutron reflectivity.

When corrected for the effect of capillary waves, there is reasonable quantitative agreement between the predictions of the self-consistent field theory and experimental measurements. We have also identified and characterised the earlier stages of the interfacial instability that leads to the dewetting of one polymer film by another. Dispersive forces amplify thermally excited capillary waves at the polymer/polymer interface; we have monitored the growth of these waves both by measuring a continual increase in the overall diffuseness of the polymer/polymer interface and by detecting neutrons diffracted in grazing incidence from the fastest growing capillary wave modes.

Our results are consistent with the predictions of a linear theory of spinodal dewetting at a liquid/liquid interface both in respect of the length-scale and the characteristic time of growth of the unstable capillary wave.

References [1] E. Helfand and Y. Tagami, J. Chem. Phys. 56, 3592 (1972) [2] T.P. Russell et al., Mater. Sci. Rep. 5, 171 (1990) [3] R.W. Richards and J. Penfold, Trend Poly. Sci. 2, 5 (1994) [4] M. Stamm and D.W. Schubert, Ann. Rev. Mater. Sci. 25, 325 (1995) [5] K.R. Shull et al., Macromol. 26, 3929 (1993) [6] M.L. Fernandez et al., Polymer 29, 1923 (1988) [7] S.P. Anastasiadis et al., Macromol. 92, 5677 (1990) [8] M. Sferrazza et al., Phys. Rev. Lett. 78, 3693 (1997) [9] F.P. Buff et al., Phys. Rev. Lett. 15, 621 (1965) [10] J.S. Rowlinson and B. Widom, Molecular Theory of Capillarity, Claredon Press, Oxford, (1992) [11] J. Israelachvili, Intermolecular and Surface Forces, Academic Press, (1992) [12] T.P. Russell et al., Macromol. 23, 890 (1990) [13] G. Reiter, Phys. Rev. Lett. 66, 715 (1991) [14] G. Reiter, Macromol. 27, 3046 (1994) [15] G. Krausch, J. of Phys.: Condens. Matt. 37, 7741 (1997) [16] Lambooy et al., Phys. Rev. Lett. 76, 1110 (1996) [17] S. Qu et al., Macromol. 30, 3640 (1997) [18] Sharma and G. Reiter, J. Colloid. and Interf. Scie. 178, 383 (1996) [19] R. Xie et al., Phys. Rev. Lett. 81, 1251 (1998) [20] T.E. Faber, Fluid Dynamics for Physicist, Cambridge University Press, Cambridge 1995 [21] F. Brochard-Wyart and J. Daillant, Can. J. Phys. 68, 1084 (1990) [22] F. Brochard-Wyart et al., Langmuir 9, 3682 (1993) [23] M. Sferrazza et al., Phys. Rev. Lett. 81, 5173 (1998

Acknowledgement The results reviewed in this paper were the outcome of enjoyable collaborations with R.A.L. Jones (University of Sheffield) and C. Xiao (University of Surrey).

The author also would like to thank R. Cubitt (ILL), D. Bucknall, J. Webster and J. Penfold (RAL) for their help during the experiments.

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Articolo ricevuto in redazione nel mese di Settembre 2000

NEUTRON-ELECTRON SPECTROSCOPY E. Balcar Atominstitut, TU Vienna, Stadionallee 2, A-1020 Vienna, Austria.

S. W. Lovesey ISIS Facility, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, England, UK.

Inelastic scattering of neutrons by electrons in a solid is an established technique by which to investigate processes that occur on a scale of energy up to about 0.2 eV, and the processes include spin-wave excitations and crystal-field states. The article looks at some examples of what can be gained from investigations conducted above this energy and up to several electron volts.

4πÅ-1 might achieve a magnitude that is a few percent of its maximum value. With regard to this second problem one can realistically be optimistic about future activities because of what has been achieved with existing instrumentation. In what follows, we do not dwell on the challenge to the experimentalist of detecting neutrons deflected through small angles.

The emphasis in this article is on processes that occur at an energy of about 0.2eV and beyond. In particular, we have little to say about crystal-field states even though they are an active field of research (Mesot 1995, Lovesey and Staub 2000, and Staub and Soderholm 2000).

Orientation The aim of this article is to stimulate, or possibly help renew, interest in the use of energetic neutrons to study properties of electrons in solids. To this end, we bring together basic features of the scattering of neutrons by electrons and look at what is expected if the electrons are localized in space (e.g. electrons in the f shell of a lanthanide ion) or mobile and band-like.

Of course, the use of thermal neutrons to study electrons in solids is very well established. Physical properties in this energy range include crystal-field states and collective spin excitations, often called magnons or spin waves.

There are at least two practical problems to be faced when it comes to using neutrons to investigate properties at higher energies, above about 0.2 eV; one problem, a sufficient supply of neutrons, in principle is largely resolved and the second, satisfying kinematics in an experiment, is handed down by Nature and we must learn to live with it. First, one needs a copious supply of energetic neutrons, and for these we turn to spallation sources. In the not too distant future, the supply will be very much improved by the operation of the Spallation Neutron Source under construction in Oak Ridge, USA (Mason et al. 2000) and perhaps also the AUSTRON source if the current project in Austria is brought to fruition (Rauch et al. 2000). Existing sources of energetic neutrons are more than adequate to make some measurements but new sources will surely open new vistas of research.

The second problem stems from the huge difference in the masses of the electron and the neutron. The challenge which then arises is to satisfy the twin objectives of a large transfer of energy, exceeding 0.2 eV, say, and only a modest change in the wavevector so as not to unduly suffer a shortfall in scattered intensity that will come about because of a monotonically decreasing form factor in the scattering length, which at a wavevector around

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Neutron-electron interaction The contribution to the scattering length of the neutron due to its interaction with an electron has a size set by the classical radius of an electron, re = α2ao = 0.282 x 10-12cm, which is similar to the magnitude of many nuclear scattering lengths. Not surprisingly, the huge difference between the spatial sizes of a nucleus and an orbital for an electron in a solid leads to significant differences in the scattering profiles for nuclei and electrons.

There are two physically different components in the neutron-electron interaction. One is due to the spin of an electron and the second, quite often called the orbital interaction, is due to the magnetic field created by a moving charge.

Hence, the interaction involves two degrees of freedom belonging to the electron, namely, its spin and its momentum (or velocity). The momentum can be reexpressed in terms the orbital motion of the electron, much as one does in a multipole expansion of a photon wave function to expose electric and magnetic absorption events. For elastic neutron scattering there is a one to one correspondence between the momentumdependent interaction operator and orbital angular momentum; the correspondence was demonstrated by Trammell (1953) in a landmark paper. For inelastic scattering processes, of interest to us here, the correspondence is not one to one and, in fact, there is no benefit in using orbital angular momentum operators to describe truly mobile electrons. Instead, calculations of the cross-section, also called a profile, for itinerant electron systems are best done in terms of the linear momentum operator.

In discussing localized electron systems, we will not

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consider the algebra involved in relating the linear momentum operator to the orbital angular momentum operator for the algebra is quite involved (Balcar and Lovesey 1989) and a discussion of it adds little to the main aim of this article. Instead, we settle for quoting a result for the neutron-electron interaction operator for one ion expressed in terms of the total spin, S, and total orbital angular momentum, L, of the valence electrons (a matrix element of the interaction with electrons in core states is zero because electrons in these states are paired).

Restricting our attention, for the moment, to scattering events in which the primary beam is deflected through a modest angle the neutron-electron interaction operator is, Q( k ) = 12 [{ < jo ( k ) > + < j2 ( k ) > }L + 2 < jo ( k ) > S]

(1)

Here, 〈jn(k)〉 is a radial integral for electrons in the valence shell created by weighting the radial density of electron charge in the valence shell by the spherical Bessel function jn(kr) where k is the magnitude of the wavevector. It follows from the properties of jn(kr) that 〈jo(k)〉 tends to a non-zero value as k → 0, and it is customary to arrange normalization of the radial charge density such that 〈jo(o)〉 = 1. For n > 0 radial integrals evaluated with k → 0 tend to zero. Hence, in the extreme limit of forward scattering (i.e. no deflection of the beam) the interaction operator (1) is proportional to the magnetic moment carried by the valence shell.

Let us consider a matrix element of (1) taken between states of different total angular momentum with quantum numbers J and J′. The situation J ≠ J′ which we consider is realized by a scattering event that induces a transition between states with different J values in a multiplet, and between J-multiplets belonging to different term energies.

The key result is that, non-diagonal matrix elements of the operator J = L + S are zero, and < J |Q | J ′ > = − 12 < J |L | J ′ > {< jo ( k ) > − < j2 ( k ) > }

(2)

The characteristic dependence of this matrix element on k is a signature of an inelastic event, and the dependence is quite different from that encountered with elastic scattering, e.g. Bragg diffraction.

To complete the present discussion let us consider the cross-section which arises from the result (2). Non-zero off-diagonal matrix elements of L must have J′ = J ± 1.

Hence, for modest deflections of the beam inelastic events are subject to the dipole selection-rule. Consider J = L + S and the event J′ = J - 1 where the states labelled by J and J′ are separated in energy by an amount ∆. The

cross-section corresponding to this situation derived from (2) is found to be proportional to, 1 r 2 ( LS ){ < j ( k ) > − < j ( k ) > } 2 o 2 J 6 e

δ ( hω − ∆ ),

(3)

where hω is the change in energy of the neutrons; k = q q′ and hω = h2(q2 - q′2)/2m with h2/2m = 2.072 meVÅ2.

The result for J = L - S  and J′ = J + 1 is very similar to this, of course, and the dependence on the atomic quantum numbers is a bit more complicated.

In general, the dipole selection rule does not limit the events observed in neutron-electron spectroscopy. The foregoing expressions actually refer to a very special case and experimental evidence for this is discussed later. For an arbitrary value of k there are no simple and general theoretical expressions for the cross-section, and each ion needs to be examined on an individual basis. Results for all the rare-earth (tripositive) ions are tabulated by Osborn et al. (1991).

As the last topic in the present discussion of inelastic scattering by an ion we wish to emphasize that, the intensity distribution as a function of k can be very different in inelastic and elastic scattering events. In the latter case the k-dependence of the scattering length is embodied in a so-called atomic form factor.

To illustrate the difference in the k-dependence of intensity for inelastic and elastic events we show in Fig. 1 calculated results for Sm3+. The inelastic event is dipole-allowed so the intensity for k = 0 is different from zero. The k-dependence of elastic scattering is unusual in so far that it is not monotonically decreasing, and this feature can be traced to a near cancellation of the spin and orbital contributions of the magnetic moment which also leads to a small value of the gyromagnetic factor, namely, g = 2/7.

The calculated elastic and inelastic structure factors displayed in Fig. 1 are in accord with experimental findings (Moon and Koehler 1979, Williams et al. 1987).

We next consider mobile electrons and find that the crosssection for inelastic scattering is likely to be profoundly different from results we have just discussed for localized electrons. One model we can treat completely is a gas of electrons which do not interact, the so-called ideal electron fluid or jellium model of electrons in a solid.

Even though the Coulomb interactions between electrons, and between electrons and ion cores in the crystal are not included in the ideal electron fluid the electrons are correlated because of the quantum mechanical exchange force originating from the Pauli exclusion principle.

Since the electrons in the ideal fluid are identical and correlated it is not correct to consider scattering by a single electron. Instead, to calculate the profile one must

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employ a formalism that describes all electrons in the fluid on an equal footing and accounts for the Pauli exclusion principle. There are various ways of handling the calculation; perhaps the simplest is to use second quantization of the spin density and momentum density of the electrons. One finds that the cross-section is proportional to, re2

no ( 2π )3

∫ dq{1 +

2

k4

( k x q)2 } fq (1 − f k + q )δ [ hω + ε ( q) − ε ( k + q)].

(4)

negligible and f k+q → 0. The corresponding value of (4) is often referred to as the Compton limit of the crosssection, namely, re2 ∫ dq fqδ [ hω + ε ( q) − ε ( k + q)] no ( 2π )3

(5)

Although this result has been derived for ideal electrons, in fact, it is correct in the general case when fq is the appropriate momentum distribution and the bare mass of the electron, m e, is replaced by an effective mass.

Let us return to (4) and consider its behaviour in the limit of zero temperature. In this case, fq = 1,|q|< p f : fq = 0,|q|> p f

where the Fermi wavelength pf satisfies, p f = ( 3π 2 no )1 / 3

In the description of the profile it is quite convenient to use reduced variables for the energy transfer x = hω/εf and wavevector transfer y = k/pf where the Fermi energy εf = (hpf)2/2me. Scattering is restricted to a domain in x-y space as a direct consequence of the Fermi functions.

One finds the intensity profile for a degenerate ideal electron fluid is different from zero in the domain specified by the following conditions (the domain is often called the particle-hole continuum); 0 ≤ x ≤ (2y + y2) : 0 ≤ y ≤ 2

(y2 – 2y) ≤ x ≤ (y2 + 2y) : y > 2.

Fig. 1. Calculated structure factors for Sm3+, showing the significant differences to be found for elastic and inelastic (dipole-allowed) events.

The spin and orbital profiles, calculated from (4) with T = 0 K, pf = 0.96 Å-1, εf = 3.5 eV, and y = 0.57, are displayed in Fig. 2 as a function of hω = xεf. Two features merit attention. First, the profiles extends over a wide range of energies and, secondly, the orbital contribution exceeds the spin contributions. The latter feature is due to the factor 1/k2 in the orbital contribution to (4). In a real material and k → 0 this contribution will saturate due to diamagnetic screening, for example.

In this expression, fq is the Fermi occupation function for an electron with energy ε(q) = (hq)2/2me and no is the density of electrons. The expression (4) is the sum of contributions due to the spin and the orbital interactions, with the latter distinguished by the vector product k x q.

The Fermi functions in (4) are a signature of quantum mechanics, and their influence is profound at low temperatures (the so-called degenerate Fermi fluid). One general feature that merits comment is the form of the cross-section for large values of k. On taking the limit k → ∞, the contribution from the orbital interaction is

Intermultiplet transitions Fig. 3 shows experimental data collected on thulium metal at 20K. The incident neutron energy = 2.14 eV and the deflection of the beam = 5°. Calculations (Osborn et al. 1991) for tripositive thulium ion predict the lowest Coulomb transition 3H6 → 3F4 at 693.5 meV, with transitions to the 3F3 and 3F2 levels at just under 2eV. Coulomb transitions, which are designated by ∆L ≠ 0, have energies that depend on the Coulomb repulsion between electrons in the valence shell. Since the Coulomb integrals are likely to be more sensitive to changes in the local environment than spin-orbit interactions, Coulomb transitions are a useful probe of the way intra-atomic

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correlations are influenced by forming the metallic state.

There are strong deviations from the Landé interval rule because of intermediate coupling. Looking at Fig. 3, the Coulomb transition 3H6 → 3F4 is lower in energy than the dipole-allowed transition 3H6 → 3F5 and it is more intense.

Measured and calculated intensities are gathered in Fig. 4. At a sufficiently small value of k the intensity of the dipole-allowed transition exceeds the intensities of all other transitions because these intensities vanish in the limit k → 0.

EuBa2Cu3Ox ( x = 6.1 and 7) by Staub et al. (1997). There appears to be a more efficient screening of the Coulomb interaction by conduction electrons in Pr metal. Looking to the future, it will be valuable to measure the dependence on wavevector of structural factors for (intermultiplet) transitions split by the influence of the crystal field. Calculations (Staub et al. Private communication) using all the current information on this material predict different dependencies for crystal-field split terms belonging to a multiplet.

Significant departures from isolated-ion behaviour can be expected in metallic systems where the 4f moment is inherently unstable. These include intermediate valence compounds, where two nearly degenerate electronic configurations are hybridized, and heavy-fermion compounds where the valence is nearly integral but strong band-4f hybridization suppresses the 4f moment.

Similar phenomena are found in actinide compounds. In all cases, intermultiplet spectroscopy can provide valuable information, including the mixing of configurations and the degree of hybridization of the valence shell. A review of early work can be found in Osborn et al. (1991).

The density of an electron gas and rs are related through, 4π 3 1 rs = 3 no ao3

where ao is the Bohr radius. In units of eV, the Fermi energy and plasmon energy are, ε f = 50.13 / rs2 and hω p = 47.15 / rs3 / 2 , and pf = (εf /3.80)1/2 Å-1.

Fig. 2. Spin and orbital profiles for scattering from mobile electrons.

Smooth dotted curves are derived from the spin and orbital contributions to (4). The reduced wavevector y = 0.57, and for this relatively small value the orbital contribution exceeds the spin contribution. The full and broken curves are the spin and orbital contributions obtained from a bandstructure model of sodium and k = (1/4, 1/4, 0) in units of (2π/a) = 1.55Å-1.

Results are taken from Blackman et al. (1987).

Comparison of the transition energies for thulium metal, obtained by neutron-electron spectroscopy, with corresponding energies for thulium in LaF3, obtained by optical studies, shows no appreciable differences. This finding suggests that for thulium metal there is no additional screening of the Coulomb interaction to that occurring in the isolated ion. A similar finding has been reported for

εf(eV)

hωp(eV)

1.12

4.78

8.08

5.23

0.69

1.83

3.94

Zn

2.30

1.58

9.48

13.52

Al

2.07

1.75

11.70

15.83

Sn

2.22

1.63

10.17

14.25

pf (Å-1)

Metal

rs

Li

3.24

Rb

Table I.

Neutron scattering from mobile electrons As already mentioned, the physical processes we consider occur on an energy scale beyond about 0.2 eV. No experimental studies of mobile electrons in this domain of energy have been reported. In contrast, there is a multitude of studies of processes occurring at lower energies and, in particular, we have in mind the many studies of spin wave excitations in metallic systems.

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Energetic neutrons might be used to conduct Compton scattering experiments. Because the scattering wavevector in these experiments is by design made very large the scattering is (spatially) incoherent. Results are essentially to do with individual particles and reveal nothing about spatial properties. The value of Compton scattering for studying the motion of nuclei in condensed matter is well established (Watson 1996 and references therein). We have seen that the Compton cross-sections for nuclei and electrons are the same; the information in

Fig. 3. Neutron scattering cross-section for thulium metal showing intermultiplet transitions from a ground state 3H6. The peaks are labelled by the final state of the transition. After Osborn et al. (1991).

them is to do with distributions in momentum space, and in the case of electrons the distribution in question is that of the spin density. The same physical information is available from the Compton scattering of x-rays, and many successful experiments using synchrotron sources have been reported (Sakai 1996).

Let us turn to the inelastic and coherent scattering of neutrons by mobile electrons. Our starting point is the discussion in section 3 of scattering by an ideal Fermi fluid. First, we consider the modification to this result brought about by switching on the Coulomb interaction between electrons. The plasmon is a collective oscillation in the density of electrons. The plasmon frequency at k = 0 is ωp = (4π n o e2/m*)1/2 where m* is the effective mass and some values calculated with m* = me are given in Table I. The dispersion of the plasmon is quite weak. For k beyond about pf the plasmon is subject to strong damping from particle-hole states, which is also known as Landau damping.

The plasmon is invisible in neutron scattering, in zero applied field. The origin of this effect, essentially a selection rule, is orthogonality of the particle density that carries the oscillation and the spin density to which neutrons couple; the first density is n↑ + n↓ and the second is n↑ - n↓. Application of a magnetic field breaks the selection rule, and thereby a magnetic field is an

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excellent switch to use in picking out in the profile the contribution due to the plasmon.

Secondly, we look at the effect due to electron scattering by ion cores in a crystal on the spin and orbital profiles.

Figs. 2 and 5 display profiles obtained from bandstructure models of sodium and paramagnetic iron. In all the cases illustrated, spin underperforms orbital scattering.

The unit of intensity used for the profiles is such that on multiplying by 0.29 the intensity is barns sr-1 eV-1.

(Noise arises in the profiles calculated from bandstructure models from band degeneracies at the zone boundaries and the handling of the delta function that expresses conservation of energy, cf. (4).)

Fig. 2 includes profiles calculated for an ideal degenerate Fermi fluid. These are found to be a good guide for hω less than about 2 eV. Dramatic differences between results for the ideal fluid and band-structure models appear just above 3 eV. The origin of the distinctive features from the band-structure model is the splitting of the free electron degeneracy at the zone boundary, which is perpendicular to the (1, 1, 0) direction. This has been discussed extensively in the literature under the title of zone-boundary collective state.

An estimate of the profiles in terms of the density of electronic states G (ε) can be obtained by averaging the cross-section over the directions of k. For the spin profile one arrives at, ∞

re2 ∫ dε f ( ε ){1 − f ( hω + ε )} G( ε ) G( hω + ε ) , −∞

(6)

where f(ε) is the Fermi occupation function for an electron with energy ε. The expression (6) has the form of a joint density of states. For simple metals εf is typically a few eV, as can be seen by reference to Table I. Hence, at room temperature it is appropriate to replace f(ε) by a step function at εf, and in this case (6) reduces to, hω

re2 ∫ dε G( ε + ε f − hω ) G( ε + ε f ) : hω < ε f , o

or

(7) εf

re2 ∫ dε G( ε) G(ε + hω ) : hω > ε f . o

In the limit hω << εf one obtains re2 hω G 2 (ε f ) as an estimate of the profile. The density of electronic states depends on the spatial dimension of the system. This observation leads one to anticipate that, cross-sections for highly anisotropic systems, which are quasi-one or –two dimensional, will be quite different from those featuring in Figs. 2 and 5.

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For a ‘top hat’ electron density of states of total width εo the profile derived from (7) is zero except within an interval of energy between 0 and ε0, where it is a triangle

Fig. 4. Inelastic structure factors for intermultiplet transitions in thulium.

Experimental data is obtained from spectra such as the one displayed in Fig. 3, which is taken at an angle = 5°; 3F4 filled circles, 3H5 filled triangles, 3 H4 open circles and 3F3 filled squares. The continuous curves are results obtained from theory based on an isolated ion. After Osborn et al. (1991).

of height ( re2 / 2ε o ) . This finding indicates that scattering from mobile electrons is particularly intense for materials with a narrow band width.

Calculated spin and orbital profiles displayed in Fig. 5 are for a d-band metal, and the potential used in the band-structure calculation models paramagnetic iron. At the high energies of interest here enhancement of the spin response, required at low energies to reproduce the spin wave, is not significant and calculations reported in Fig. 5 contain no such enhancement. As noted already, the orbital outweighs the spin contribution to scattering.

Another feature to note is the effect on a profile of increasing the magnitude of the scattering wavevector.

In Fig. 5 the two panels correspond to wavevectors that differ by a factor = 4.3, while the scale for the intensity differs by an order of magnitude. Thus, increasing k reduces the signal from inelastic scattering by mobile electrons, and in this respect the scattering is not unlike intermultiplet transitions discussed in section 3. The cross-section to be observed in an experiment is the sum of the spin and orbital profiles separately displayed in Figs. 2 and 5.

Spin wave excitations in metallic systems continue to

Fig. 5. Calculated spin and orbital profiles obtained from a band-structure model of paramagnetic iron (Blackman et al. 1987). In the upper panel k = (1/2, 1/2, 0) and the bottom panel k = (3, 0, 0) and the unit is (2π/a) = 2.3Å-1. For k = 1.6Å-1 (upper panel) and k = 6.9Å-1 the spin profile (solid curve) is weaker than the orbital profile.

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pose a major theoretical challenge, even elemental magnets, and there is a need for experimental investigations. For example, there remain basic questions about the spin waves in iron and nickel (Karlsson and Aryasetiawan 2000). Some calculations for nickel predict a spin wave branch that extends up to an energy of 0.5eV at the zone boundary, in addition to a softer spin wave that achieves half this energy. The dispersion, and temperature dependence, of the spin waves in nickel and other so-called simple metallic magnets are not understood and interplay between theoretical and experimental studies are much needed.

Magnetic materials very much at the centre of current research also support high-energy spin excitations.

Measurements on La2CuO4 (Coldea et al. 2000) possibly include the most energetic excitations to have been observed. By using a spectrometer with many counters supplied with neutrons from a pulsed spallation source (the ISIS Facility), the investigators were able to convincingly demonstrate dispersion at the level of 10% in an excitation centred around 300 meV. This observation of dispersion has a very direct bearing on properties of the appropriate spin Hamiltonian.

the delicate nature of the neutron as a probe of condensed matter. The many-body effects in x-ray spectroscopy just mentioned are a measure of the disruptive nature of xrays in this mode of investigation. If the goal of an experiment is to measure properties of electrons in the sample without a violent disturbance the method of choice is neutron-electron spectroscopy.

Acknowledgement We are grateful to Dr. R. Coldea and Dr. U. Staub for useful comments that improved a first version of the article.

References Balcar E and Lovesey SW (1989) Theory of Magnetic Neutron and Photon Scattering (Clarendon Press: Oxford) Balcar E and Lovesey SW (1993) J. Phys.: Condens. Matter 5, 7269 Blackman JA et al. (1987) J. Phys. C: Solid State Phys. 20, 3887 ibid 20, 3897 Coldea R et al. (2000) Physica B 276 â&#x20AC;&#x201C; 278, 592 Karlsson K and Aryasetiawan F (2000) J. Phys.: Condens. Matter 12, 7617 Lovesey SW and Staub U (2000) Phys. Rev. B61, 9130

Discussion In bringing the article to a close it is fitting to add a few words about spectroscopic techniques applied to metals that utilize beams of x-rays. Synchrotron sources of xrays developed over the past decade or so have made these techniques much more valuable than before (Margaritondo 1988).

X-ray absorption lineshapes and photoemitted electron energy distribution curves contain a wealth of useful information. The distribution curves measured in photoemission spectroscopy, for example, contain information on the initial electronic states and it appears in the curves convoluted and mixed with other effects that could be the prime interest. Effects in question include bulk and surface plasmons, secondary electrons from inelastic scattering processes, and the orthogonality catastrophe which is a many-body effect that arises with photoelectrons excited from core levels of an ion. The absorption of x-rays contains another many-body effect, namely, excitons which arise from the addition of electrons to the conduction band. Excitons and the orthogonality catastrophe cause power law behaviour at the absorption edge. Because of these and other effects measurements using x-ray spectroscopy are indeed rich in information content. On the other hand, the interpretation of data is both subtle and demanding, and very much more so than one anticipates with data gathered using neutron-electron spectroscopy.

One attraction of neutron-electron spectroscopy has to be

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Margaritondo G (1988) Introduction to Synchrotron Radiation (Oxford University Press : New York) Mason TE et al. (2000) Proceedings of the LINAC 2000 Conference, paper FR203 Mesot J (1995) in Magnetic Neutron Scattering edited by A Furrer (World Scientific:Singapore) Moon RM and Koehler WC (1979) J. Mag. & Mag. Mat. 14, 265 Osborn R et al. (1991) in Handbook on the Physics and Chemistry of Rare Earths, Vol. 14, Chapter 93 (Elsevier Science Publishers: Amsterdam) Rauch H et al. (2000) Physica B 276-278, 33 Sakai N (1996) J. Appl. Cryst. 29, 81 Staub U et al. (1997) Phys. Rev. B55, 11629 Staub U and Soderholm L (2000) in Handbook on the Physics and Chemistry of Rare Earths, Vol. 30, Chapter 194 (Elsevier Science Publishers : Amsterdam) Trammell GT (1953) Phys. Rev. 92, 1387 Watson GI (1996) J. Phys.: Condens. Matter 8, 5955 Williams WG et al. (1987) J. Phys. F17, L151

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Articolo ricevuto in redazione nel mese di Novembre 2000

CHEMICAL-SHIFT NORMAL INCIDENCE X-RAY STANDING WAVE DETERMINATION OF ADSORBATE STRUCTURES Vincenzo Formoso I.N.F.M., Unità di Cosenza, Dipartimento di Fisica Università degli Studi della Calabria Via P. Bucci I-87036 Arcavacata di Rende (CS) The technique of X-ray standing wavefield (XSW) absorption is a very sensitive tool to probe the position of implanted or adsorbed atoms on the sample surface. Its application to the determination of adsorbate structures at the solid-vacuum interface is now very well established. For many years, the use of this technique was limited because of the need of highly perfect samples. The angular range of total reflectivity is, in fact, typically of a few of arc-seconds. As the XSW range around a Bragg reflection is very narrow, not only the incident x-ray beam must be highly collimated and monochromatic, but, also the crystal sample must be perfect at a similar degree. This latter requirement had the effect to restrict studies on semiconductor materials, which typically do have the necessary high degree of crystalline perfection. Therefore, relatively few XSW experiments were done on metal surfaces in UHV conditions. For 10 years Woodruff [1] and co-workers pursued such aim by using the Synchrotron Radiation Source (SRS) at Daresbury, and developed the technique of Normal Incidence X-ray standing waves (NIXSW), a new variant of the well established XSW technique. They demonstrated that at incidence normal to the Bragg scattering planes, NIXSW becomes insensitive to the crystal mosaicity, to the incidentbeam collimation, and to the exact incidence angle, making metal single crystals perfectly adequate.

At the European synchrotron radiation facility (ESRF), where I worked as scientist for five year on the ID32 beamline (the Surface-EXAFS and X-Ray Standing Waves Beamline), an UHV system well adapted for several surface science techniques (SEXAFS, Resonant Photoemission, XPS, etc.), and also for NIXSW studies, was installed. ID32 offered an excellent compromise between a good flux gain and a good resolution. For this reason, Woodruff and co-workers started a program to exploit the special performances of the ESRF synchrotron as far as the capabilities of the NIXSW technique.

I had the big chance to be involved and collaborate to that project. The general aim of the project was to develop the NIXSW method and to apply this to a range of adsorbate structures on metal single crystal surfaces of general relevance to the area of heterogeneous catalysis. The high flux and high spectral resolution available on ID32 opened new possibilities.

First of all, it was possible to exploit ‘chemical shifts’ in photoelectron binding energies and perform NIXSW structure determinations of coadsorbed molecular fragments resulting from a surface reaction. Second, we could study the local

structure of adsorbates at low coverages. Third, we could investigate low Z (C, N and O for example) adsorbate atoms which have low photoionization cross-section at the X-ray energies used in the NIXSW experiment.

The potentiality of the chemical-shift NIXSW technique (CSNIXSW) will be discussed and recent results will be presented.

We were able to demonstrate that this technique offers the possibility to shed new light on the determinations of several surface structures.

Introduction The study of the nature of the chemical bond, of the absorption site and, in general, of the geometry of adsorbed atoms is the starting point for a quantitative knowledge of a well characterised metal single crystal surface. Many peculiar techniques have been developed and applied to investigate the proprieties of solidvacuum surfaces and interfaces. A large part of the well established surface structures was obtained by methods using coherent interference of elastic scattered electrons (Low Energy Electron Diffraction (LEED), photoelectron diffraction (PhD), and surface extended X-ray absorption fine structures (SEXAFS).

SXRD (Surface X-ray Diffraction) has been also extensively employed to study surface and interface structures. It is a very powerful technique for structural studies on crystalline surfaces, but it presents, nevertheless, some restrictions: a1) It is not trivial to achieve the necessary surface sensitivity because atoms are weakly scattered from Xrays. a2) It has no elemental sensitivity. a3) It probes the long-range order part of a surface. Only when the surface has an excellent long-range order of the adsorbate layer, this technique is able to supply detailed local adsorbate-substrate structural information. Over the last few years scanning probe microscopes have highlighted the poor average quality of many surfaces showing that surface patches with a lack of long-range order, coexist with long-range ordered domains.

We will show that surface structure information may be obtained from X-ray Standing Wavefield technique (XSW), even when a surface contains regions of good long-range order as well as regions of local order or

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complete disorder. We would like to stress this real crucial point because structural data obtained from diffraction methods, may give us the misleading view of a surface structural perfection. Commonly, there is no long-range order in the adsorbate layer.

Basic physics of the XSW technique The existence of a X-ray Standing Wavefield within a crystal was demonstrated by Borrman2 (Bragg reflection in the Laue case geometry) and by Batterman3 (Bragg reflection in the Bragg case geometry). In this latter case, the incident, ε 0 = E0 e 2 π i (ν 0 t − K 0 • r ) , and the reflected, e2π i

ε H = EH

(ν H t − K H • r )

, coherent X-ray plane waves interfere to set up a XSW field. This XSW field is parallel to the diffraction planes of the crystal and has the same spatial periodicity. ν0 and K0 are the frequency and the propagation vector of the radiation, respectively. Because the two waves are coherent, we can define, a phase factor between the two amplitudes E0 and EH: EH = E0 R e iΦ . It is independent on time and space. The intensity of the X-ray standing wavefield in the crystal is the squared modulus of the sum of the incident and reflected X-ray amplitudes and can be written as:

I=

ε ε* E0

i Φ − 2π H • r ) = 1+ R e (

2

2

(1)

.

We note that the time dependence vanishes out in the expression for the intensity.

H=KH - K0 is the reciprocal lattice vector associated with the Bragg reflection, r is a real-space vector defining the position of the atomic absorber at which the intensity is measured, R is the amplitude factor given by 2

R=

E IH = H2 I0 E0

ϕ Φ= ϕ + π

where

and Φ is the phase factor written as : for Re( EH E0 ) > 0

for Re( EH E0 ) < 0

[

]

ϕ = arc tan Im( EH E0 ) Re( EH E0 ) 



and EH E0 = − FH FH η ± η − 1  are given in terms of the geometrical structure factors FH and FH for the H and H reflections.

2

In the standard form of XSW experiments, the η parameter is related to the incident angle θ, and it is given by η=

(θ − θ B ) sin (2θ B ) + ΓF0 P Γ FH FH

.

Here F0 is the structure factor for the (000) reflection, P is

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Fig. 1. Calculated reflectivity and phase for a non-absorbing crystal versus the η parameter. The shape of the XSW profile is strongly dependent on the adsorber position, z, measured with respect to the scattering planes.

a

polarization

e2 Γ= 4πε 0 mc 2

λ2 πV

factor

and

Γ

is

expressed

by

,

where V is the volume of the unit cell, λ is the x-ray wavelength, m and e the mass and charge of the electron, ε0 the permittivity of the free space, and c the speed of light.

It fig.1 we plot the calculated reflectivity R, the phase Φ and the XSW profile (eq. 1) in terms of the dimensionless η parameter.

In case of a non absorbing crystal, when the incident angle θ is scanned through the nominal Bragg angle, θB , we have that in the Bragg diffraction condition (-1< η < 1) the reflectivity is equal to 1 and the phase changes of π .

The superposition of the two coherent plane waves creates a planar standing wave. The intensity (see equation 1) at a particular location r in space is determined by H and by Φ and is spatially modulated: minima (nodes) or maxima (antinodes) of the wavefield intensity lie on scattering planes. The XSW is set up inside the crystal and it extends well outside. The

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Fig. 2. Relationship of (111) and (-111) layer spacing for atop, hcp, fcc adsorption sites.

Fig.3. The angular distribution of electrons ejected from an atom in the XSW field. In our experiment, the angle θ between the detector and the x-ray A vector was 45°, β =2, δ =0, γ =1.

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scattered intensity by a certain atom depends on its position with respect to the maxima or minima of the Xray interference field.

The intensity of the planar wavefield, I = 1 + R + 2 R cos(Φ − 2π H • r)

is a periodic function whose period is given by H • r = H d H = 1 where λ

d H = H −1 = K H − K 0 −1 = 2 sinθ

is the spacing between the nodes of the standing wave. Adjusting H and/or θ, it is possible to choose a well-adapted wavefield spacing for any structural problem.

The XSW method provides information on the spatial distribution of absorbers perpendicular to specific scattering planes by measuring the X-ray absorption profile for an adsorbate atom. When we scan through a Bragg reflection, the X-ray standing wave shifts its phase in a systematic and predictable fashion. One way to change the phase, Φ, is just rocking the crystal around the Bragg condition. The maximum scattered intensity is observed when the maxima of the intensity of the XSW coincide with the atomic position. In the case of a single absorbing atom at a single high symmetry adsorption site, the XSW measured intensity is:  z  I = 1 + R + 2 R cos  Φ − 2π  dH  

(2)

By fitting the intensity versus photon energies, we determine the structural parameter z, i.e. the distance between the adsorbed atom and the scattering planes. In order to take into account some distribution of z positions, due to vibrational or static disorder, or to different adsorption sites, with a probability f(z)dz, the XSW profile can be written  D I = 1 + R + 2 R fC 0 cos  Φ − 2π  dH  

(3) D is the coherent position, the adsorbate position with respect to the diffraction planes. The coherent fraction, fC0, provides a measure of the local order of adsorbate atoms on the surface.

The NIXSW The Normal Incidence X-ray Standing Wave (NIXSW) technique retains not only the basic capabilities of standard XSW experiments to determine adsorbate structures, but also allows us to use commercial metal single crystals due to the relaxed constraint on the crystalline perfection of the sample.

For a symmetric Bragg reflection of σ polarized X-rays the Darwin width, Wθ ,is expressed as[4]: Wθ = 3.6 * 10 −5 Å −1

d H2 FH tan(θ B ) a

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A different expression for angles close to 90° has to be used [5]. Let us suppose E=15KeV is the energy of the incident X-rays striking on Si(111), we obtain Wθ =3.6 arc sec=17.4 µrad, being the lattice constant a=5.431 Å-1, the diffraction plane spacing dH=3.14 Å the Bragg angle θB=7.6° , and the structure factor FH=6.53.

Commercial single metal crystals have a mosaic structure in the substrate of order 0.5°. On a Cu(111) metal crystal, for example, the standard XSW technique cannot be applied because the total angular range is only 0.016°, if the incidence angle is 45° and the photon energy is 4.2 KeV. But, if we set the incidence angle at 90°, the angular range increases considerably. It has a value close to 1°, providing the desired insensitivity to crystalline quality.

The use of an impinging beam normal to scattering planes, however, gives wide rocking curves and, thus, a low sensitivity to the crystal mosaicity, making metal single crystals routinely usable. In fact, at θB =90° the energy width of the Cu(111) reflection is 0.87eV, comparable with the resolution of double crystal X-ray monochromators used on synchrotron radiation sources.

Since in NIXSW technique the scattering angle is kept fixed while the X-ray energy is scanning, it is important to give a different expression for the η parameter: −2( E − EB ) 2 sin θ B + ΓF0 E η= P Γ FH FH

and, thus, we note that it remains linear in E even at θB=90°.

Moreover, the photoemission or the Auger emission electrons associated with the photoabsorption ensures surface specificity in the absorption profile. At incidence normal to (111) planes of fcc metals, the photon energy is typically in the 2.5-3.5 KeV range. This photon energy range ideally matches to the first harmonic of a standard ESRF undulator, such as that on ID32.

It may be worthwhile to illustrate the procedure we used (see fig.2) to extract quantitative structural information by means of this technique: a1) We selected two independent Bragg reflections and for each, we measured the XSW absorption profile. For fcc crystal surfaces, we used the (111) and the (-111) reflection planes. a2) The measured lineshape was determined by two structural parameters, the coherent position, D, and the coherent fraction, fC0 . These parameters were obtained by fitting the NIXSW profiles. a3) Once we had the coherent position for the (111) reflection, we could deduce the coherent position value for the (-111) reflection, D(-111). See fig.2 for the high symmetric adsorption sites. If a bridge site is occupied, the coherent position is D(-111)=( d(111)+1.5 D(111) )/3.

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a4) Then, we compared the D(-111) measure with the predicted one. If the two values are quite close, the adsorption site is determined. a5) The coherent fraction values were very important because an high value of fC0 (-111) is consistent with a fully symmetric adsorption site, while a low value is consistent with lower or mixed symmetry sites.

CS-NIXSW In a surface chemical reaction several co-adsorbed molecular moieties are involved. The coexistence of several different adsorbates on a surface at the same time, a mixture of reactants and products, make intermediate reactions also possible. Structural studies of such multiple chemical state species can supply quantitative information on chemical and physical phenomena at surface. Little progress has been made so far in determining their local adsorption geometries.

As well known, X-ray fluorescence, Auger electron emission and electron photoemission can monitor X-ray absorption in atoms near the surface. By photoemission, then, it is possible to measure the absorption in a specific chemical state. Small changes in the photoelectron yield as well as in binding energies, as measured at a specific core level, are due to changes in the local electronic environment of the atom.

Several surface structural techniques are based on photoabsorption. One successful approach is the Chemical-Shift Photoelectron Diffraction (CSPhD)[6].

Photoelectron diffraction is intrinsically element selective, and, thus, by measuring the photoelectron diffraction signal related to a specific chemically shifted component of the core-level photoemission yield, one may obtain chemically specific structural information.

Another successful experimental procedure to determine local adsorption geometries of adsorbates is the use of the X-ray standing wavefield (XSW) technique [7]. This, in fact, selects chemical species at a crystal surface or interface by measuring the photoabsorption yield. A first attempt to resolve a chemical state by X-ray standing wave analysis using chemical shifts was performed [8] recently, but their spectral resolution was not enough high to distinguish among several chemically shifted states. It was found, in fact, that for adsorbates of low atomic number, or when chemical state resolution is required, photoemission from the adsorbed atom is the best tool to monitor the x-ray absorption.

The new third generation of synchrotron radiation sources offer several impressive opportunities because of their very high spectral brilliance, their increased spatial and spectral resolution. Such combination of powerful means at ID32 of ESRF allows the researcher to monitor local adsorption sites on surfaces using normal incidence X-ray standing waves, not only in an element-specific

fashion, but also with chemical state specificity, by measuring the intensity of 'chemically shifted' core level photoemission signals from the different states. Then, the enhanced monochromator resolution and output flux, combined with a suitable electron energy analyser give the possibility to deal with the problem of chemicallyresolving coadsorbed species containing the same elements, and allow to study low-Z elements which have only shallow and weakly-absorbing core levels.

Chemical-Shift NIXSW (CS-NIXSW) studies can, thus, be performed routinely at ESRF by using the wide

Table 1. Chemical shifts associated with the fragmentation of PF3 adsorbed on Ni(111) and Ru(0001) surfaces at different temperatures.

Table 2. Coherent fraction and coherent position values obtained from fitting the XSW profiles associated with the PF3 fragmentation on Ni(111).

potentiality of that light source both for the creation of the X-ray standing wavefield and for the detection of the photoabsorption signal. In other words, if a particular elemental species is present on the surface in two or more different states, either in a different local geometry with respect to the substrate or to other atoms to which it may be bonded in a molecular species, the core level photoemission from these atoms will show different photoelectron binding energies associated with these different states.

The detected signal is rather surface specific and can be used to provide information on the structure of the outermost layer especially if photoelectron energies are reasonably low, below 1 KeV. The photoemission process, then, involves only primary photoionisation events. The elemental and surface specificity is, in fact, not clouded

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by secondary electron ionisation events because they are not detected.

Non-dipole photoemission angular effects in NIXSW The differential photoelectric cross section for an atom in an external X-ray interference field (XSW) is given by [9] E dσ H 0 = Mif + H exp(i( Φ − 2π H 〈 r) ) M if dω E 0

2

where Mif0 = f A 0 • p exp(iK 0 • r) i

and Mif = f A H • p exp(iK H • r) i are the matrix elements, k is the photon wave vector, A is H

Table 3. Coherent fraction and coherent position values obtained from fitting the XSW profiles associated with the SO2 fragmentation on Ni(111).

the photon polarization vector, r the electron position , p the electron momentum operator, f is the final state (outgoing electron), i is the initial state (core level), O the incident wave, and H the reflected wave.

Let consider the expansion: exp(iK • r) ≈ 1 + iK • r −

1 (K • r)2 + ⋅⋅⋅ 2

For sufficiently low photon energies, Kr << 1, the socalled dipole approximation exp(iK • r) ≈ 1 is generally adopted in photoemission. Note also that in the NIXSW case the incident and reflected X-rays are collinear, therefore, we will have A 0 • p = A H • p . With these considerations we can write the differential cross section as dσ ≈ 1 + R + 2 R cos(Φ − 2πH • r) dω

because the matrix element

f A•p i

2

factorises out.

The conclusion is that, in the dipole approximation, the photoemission intensity measured in NIXSW does correctly monitor the XSW absorption profile.

The experimental geometry of NIXSW measurements is: the photon beam hits the sample at incidence normal to

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the scattering planes and the photon polarization vector A of the incident and reflected waves are collinear. The photoemission signal is detected by an electron energy analyser placed at an angle θ with respect to the photon polarisation vector A, such that the incident x-rays provide backward photoemission and, the reflected x-rays forward photoemission.

Because of the angle resolving nature of typical electron spectrometers, the measured photoemission signal is the differential cross section given by [10, 11] dσ = dω σ  β  1 + 3 cos2 θ − 1 + δ + γ cos2 θ sinθ cosϕ  4π  2  (4)

(

) (

)

where β is a dipolar asymmetry parameter. The parameter γ represents the correction term corresponding to the dipole-electric quadrupole interference and the parameter γ is the magnetic-electric-dipole term, which is present only if core relaxation occurs. Finally, θ is the angle between the photoemission direction and the photon polarization vector, ϕ is the angle between the photon propagation direction and the projection of the electron wave vector in the plane perpendicular to A. For many years, the dipole approximation has been widely used to interpret photoemission from atoms, molecules, and solids if the photon beam energy was lower than 10–20 keV. Quadrupole and magnetic dipole effects can, however, substantially influence the angular dependence of the photoemission even at much lower photon energies as recent theoretical [ 4] and experimental data [12,13] from inert gas atoms showed. In addition, it was proved that quadrupole and magnetic dipole effects are important also for the photoemission detection in x-ray standing wave determinations of surface structure [14].

If a forward/backward asymmetry exists in the photoemission, and this depends only on the geometry of the experiment, the detected photoemission signal does not monitor the true absorption profile [7] because the measured signal will not monitor the two components of the XSW in an equivalent fashion. In fact, in this case the coherent fraction f obtained from the standard fitting procedure of the data would be too high, physically meaningless because greater than one.

Thus, in order to take into account of quadrupole and magnetic dipole effects we introduced a forward/backward asymmetry parameter Q, defined (see equation 4) as the ratio of the photoemission signal detected in the forward direction to that detected in the backward direction:  dσ forward     dω cos ϕ = 1

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 dσ backward  1 + Q  =  dω cos ϕ = −1 1 − Q

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Beyond the dipole approximation and including magnetic dipole and electric quadrupole contributions, ( exp(i K • r) ≈ 1 + i K • r ), the differential cross section can be written in term of the Q parameter [15] dσ dω

∝ 1+ R

+2 R

1+ Q 1− Q

1+ Q + 1− Q

f c0

 D  cos ϕ − 2π  d H 

Let us write this equation in the standard form defining an apparent reflectivity, Ra = R dσ dω

∝ 1 + Ra + 2 Ra

f C0

1+ Q , 1− Q

 D  cos ϕ − 2π  d H 

For positive Q values the apparent reflectivity and the apparent amplitude of the interference term are enhanced with respect to the dipole case where Q=0.

Fig. 5. CS- NIXSW technique.

The (111) XSW absorption profiles obtained from the four distinct peaks of the photoemission energy distribution curve. Clearly, they differ significantly, implying different local adsorption sites for the four species.

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energy is far above threshold for the ionisation of the 1s core level and quadrupolar contribution to photoemission becomes increasingly important as the photon energy rises above the threshold.

Using these photoemission XSW profiles without the Q correction, significant errors in both coherent position and coherent fraction can occur, in particular, an anomalously large coherent fraction is obtained.

Recently, a new simple method for measuring the quadrupole-dipole interference asymmetry factor for photoemission from core levels, was presented in a very interesting paper [16].

Fig. 4. Photoelectron energy spectra in the P 1s peak region recorded: - at140 K immediately after PF3 dosing; - after exposure to a monochromatic incident X-ray beam for 10,50,90 min; - at 300 K, soon after the exposition for an extended period to radiation damage at 140 K; - a freshly prepared PF3 layer at 300 K.

This means that when we use the measured photoemission signal as a monitor of the intensity of the X-ray standing wave, we obtain a profile with anomalously large modulation amplitude as compared with a true XSW absorption profile. These effects strongly influence the NIXSW measurements because their typical utilisation range is 2.5-3.5 KeV and are still important, even at these photon energies, if we study low atomic number absorbers. In this case, in fact, the beam photon

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Recent results We applied the chemical shift XSW technique to the study of three different model systems. The local X-ray absorption of the adsorbed atoms on a surface in a specific chemical-state was investigated by resolving the different chemically shifted components of the core level photoemission.

First of all, we set up a demonstrative experiment of the new capabilities of the CS-NIXSW technique when used in conjunction with a third generation X-ray synchrotron source. The chosen system was a very well known model system, PF3/Ni(111). The investigation was extensively performed with several techniques. [17] Then, the same technique was applied to two different systems, SO2/Cu(111) and CH3SH/Cu(111), to learn on the structure of coadsorbed reaction products, yielding quantitative information on their local adsorption geometry.

We demonstrated the utility and the capabilities of the CS-NIXSW technique, particularly for the characterisation of coadsorbates produced by a chemical reaction and, moreover, that it is a quite useful and powerful tool for the investigation of adsorbates with low atomic numbers.

The experiments were performed at the European Synchrotron Radiation facilities (ESRF) on beamline ID32. The samples, Ni(111) and Cu(111), were prepared by the usual combination of in situ argon-ion bombardment and of annealing cycles until a clean and well ordered surface was obtained. Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and the width of the substrate XSW profile, were used to check the cleanness and the quality of the crystal.

Coadsorption site determination of PFx fragments on Ni(111) by CS-NIXSW [18] The coadsorbed PFx speciess on Ni(111) were produced

by X-ray induced fragmentation of an initially adsorbed PF3 overlayer. PF3 was introduced into the chamber to the

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Fig. 7. NIXSW profiles of (111) and (-111) obtained by fitting the individual components of the P 1s, the F 1s and Ni 2p3/2 photoemission peaks. The Ni(111)- PF3 surface was exposed to the X-ray beam at a temperature of 300 K. Fig. 6. NIXSW profiles of (111) and (-111) obtained by fitting the individual components of the P 1s, the F 1s and Ni 2p3/2 photoemission peaks. The Ni(111)- PF3 surface was exposed to the X-ray beam at a temperature of 140 K.

typical pressure of 10-8 mbar. The total exposure of the surface was 10-5 mbar s which was enough to produce a saturation coverage at room temperature and at 140K. Our results revealed some significant and unknown details in the surface chemistry of the PF3 photodissociation on Ni(111). The incident X-ray beam caused a reduction of the intensity of the P 1s peak associated with the initial intere PF3 adsorbate, and induced new chemically shifted components.

An important distinction between the behaviour at room and low temperatures was observed for the photoninduced decomposition of PF3 (fig.1). At low temperature exposure, four resolved components were seen (PF3, LT1, LT2 and LT3 labels). In the case of the room temperature exposure or warming up of the sample from lowtemperature, instead, only one new peak (RT2 label) appeared. It was clear that, in addition to the peak

associated with the intere PF3 species, the spectra obtained after low temperature radiation exposure showed three additional components.

In a previous photoemission study of P on Ru(0001) [19] the four components of the P 1s, corresponding to successively larger chemical shifts , were assigned to PF3, PF2, PF, and P. We showed that these suggestions were not consistent with our measurements.

On the basis of the measured chemical shifts (see table 1) we could remark the following points: b1) The values were almost similar but not identical.

Table 4. Values for the NIXSW fitting parameters of the coherent position (D) and coherent fraction (fco) for the four distinct chemically shifted S 1s states associated with the CH SH interaction with Cu(111).

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b2) The close similarity of the LT1 chemical shift (2.13 eV) with that attributed to PF2 on Ni(111) (2.1eV ), suggested that LT1 fragment was very likely PF2. b3) Even the 1s and 2p core levels of P on the Ni(111) surface, do have a significantly different degree of localisation.

b4) Since none of the chemical shifts associated with the other P 1s states reproduces the amount of the P2p shift attributed to PF ( 3.3 eV), any doubt on the assignment of PF is ruled off. b5) It is worthwhile to notice, moreover, that the chemical shift (4.02 eV ) of the single additional state

Fig. 8. Schematic diagram of the adsorption geometry of PF3 on Ni(111).

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produced at room temperature (RT2) is significantly different ( by more than 0.3 eV ) from its closest-energy peak (LT2) seen after low temperature exposure. It opens the very real possibility that these do not correspond to the same surface species.

The main conclusions for the different coexistent P sites as implied by our chemical-shift P 1s NIXSW data, are summarised in Table 2 and figs. 6 and 7. The different chemically shifted peaks, that is PF3, LT1, LT2, LT3, show differences in their XSW profiles for low and roomtemperature surface preparations.

This is consistent with the occurrence that each P species occupies a different site on the surface. The NIXSW profiles for the P 1s peaks having the closest chemical shifts (LT2 and RT2) correspond to different local adsorption sites for their associated P atoms, supporting the suggestion that these two peaks do not correspond to the same species. It turns out that after low temperature irradiation the NIXSW profile of the peak with the largest chemical shift, LT3, appeared to be very similar to the RT2 peak. Of course, while the consequent implication is that the P atoms of the LT3 and RT2 surface species have

Fig. 9. S 1s photoelectron energy spectra recorded from the Cu(111) surface at a sample temperature of 140 K, and after heating to 300 K.

the same local geometry, the large difference in their photoelectron binding energies made clear that they did not correspond to the same adsorbed species.

A powerful aid for the assignments of these states was the additional information regarding the local geometry of the associated P atoms provided by the NIXSW data: c1) In the case of PF3 species, the NIXSW coherent position values were consistent with atop adsorption at both temperatures (see Table 2). The coherent fraction values in both (111) and (-111) NIXSW for these species were high, implying a single high-symmetry adsorption site. c2) The coherent position values found for LT1 species were in agreement with the occupation of bridge sites.

The low value of the (-111) coherent fraction was also consistent with single lower symmetry sites. We therefore assigned LT1 to a bridge-bonded PF2 species (see Fig. 8b). c3) The second species observed at room temperature (RT2) corresponds to P atoms that occupy fcc hollow sites (with a possible minority co-occupation of hcp hollow sites) (see Table 2). On the other hand, the trend of the results was consistent with the possibility that RT2 states actually correspond to P atoms (see Fig. 8a). c4) The structural parameters for the LT3 species were clearly correspondent to the fcc hollow site occupation. c5) The structural parameter values associated with the LT2 species were not consistent with any single high symmetry site. The (-111) coherent fraction value was very low and could even be zero, implying very poor lateral coordination of the adsorption site on the surface.

On the other hand, the (111) coherent fraction was high, implying good order perpendicularly to the surface.

Thus, the (111) coherent position was easily identified as a true layer spacing. Since that the two LT2 and LT3 species were seen together on the surface prepared at low temperature, we suggested that these two chemically shifted states would correspond to the two distinct P atoms in some P2Fx surface species. The LT3 state would be associated with the lower atom bonded to the Ni(111) surface in the hollow site, whereas the LT2 state would be the upper atom. The P-P distance in this species was expected to be approximately 2.20 Å, so that implied that the P-P bond was tilted at an angle of 50° relatively to the surface normal. Notice that if this tilted bond had no preferred azimuthal orientation, as seems likely, the upper P atom would occupy a multitude of low symmetry sites.

Interaction of SO2 with Cu(111) studied by CS_NIXSW[20].

Sulphur dioxide, SO2, is an important air pollutant created by the burning of sulphur-contaminated fossil fuels. It does not dissociate on noble and transition metal surfaces at low temperatures. Several coadsorbed molecular moieties may coexist on a surface at the same

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energy of the O 1s peak. As the temperature was raised, chemical interaction occurred, and coadsorbed SOx species appeared on the surface via molecular dissociation. In fact, the spectrum taken at 100K was dominated by a single peak identified as adsorbed SO2.

Fig. 9 shows that increasing the temperature , three distinct S1s chemical states appeared. Heating the surface had the result to convert much of the SO2 species to a second state with a binding energy about 1 eV higher, while further heating gave rise to a third state. It appeared at a binding energy about 4 eV smaller. The SOx species, characterised by the largest S 1s binding energy state seen in Fig. 9, were associated with adsorbed SO3 produced by the reaction (3SO2 --> 2SO3 +S). The highest temperature (lowest binding energy) state was associated with atomic sulphur.

Atomic sulphur appeared to occupy a mixture of facecentred cubic and hexagonal close-packed hollow sites on the surface. The measured value of D[111]=1.75Å allowed us to find the values of D[-111] for atop, hcp hollow and fcc hollow: they were respectively 0.58Å, 1.28Å and 1.97Å (with near-unity value expected for D[111] ). The measured value of D[-111] is actually 1.68Å, essentially at midway between the values for the two hollow sites.

So, sulphur atoms occupy these two hollow sites with equal probability: in this case the predicted value of d [111] is 1.63Å, in agreement with the measured values.

SO2 was found to adsorb with its molecular plane essentially perpendicular to the surface, and the data were most readily interpreted in terms of a bridging geometry bonding through the oxygen atoms, although the CS-NIXSW data indicated that oxygen atoms could not occupy only static near-atop sites.

SO3 species adsorbed with its axis perpendicular to the surface, atop a surface copper atom with S-O bonds out of plane such that oxygen atoms were closer to the surface. The polarisation angle dependence of the NEXAFS data indicates that the SO2 molecular plane is essentially perpendicular to the surface, as is the axis of adsorbed SO3.

Fig. 10. Local adsorption geometries at low temperature and experimental energy spectra in the region of the S 1s binding energy.

time. A structural change of the adsorbed SO2 occurs at low temperature as a result of the temperature increase and/or of the molecular cracking under monochromatic undulator radiation.

In the case of the X ray irradiation, we could see the radiation induced state conversion of the S 1s spectra; O 1s spectra, instead, did not show significant changes in total oxygen coverage on the surface and in the binding

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A surface chemical reaction, CH3SH on Cu(111), investigated by CS-NIXSW[21] The interaction of alkane thiols with surfaces is of interest for desulfurization catalysts and catalyst poisoning.

These species form self-assembled thiolate monolayers (through deprotonation at the interface) which are of potential interest for molecular electronics.

Methanethiol CH3SH, is the simplest of such species.

The chemical shifts of the S 2p photoelectron binding energies of the intact molecular thiol, the LT thiolate, and the HT thiolate, relatively to the atomic sulphur, were

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identified as 2.3, 0.6, and 1.4 eV, respectively. These photoemission experiments showed that four distinct Scontaining surface species resulted from the interaction with the Cu(111) surface. They were assigned to: - an adsorbed methyl thiol molecule (present only at the lowest temperatures ≤150 K); - two distinct states, both attributed to methyl thiolate (CH3S ) adsorbed in different geometries. The two thiolate species were conveniently labelled low and high temperature (LT and HT) thiolates; both thiolate species were present at the lowest temperature, but the LT thiolate transformed to the HT thiolate as the temperature was raised; - all species present on the surface transformed on atomic sulphur when the temperature was highest than 400 K. For the S atom in the intact thiol, the experimental d(111) S-Cu layer spacing of 2.38 Å given by the coherent position, provides a (-111) coherent position of 0.79, 1.48, or 2.18 Å, depending on whether the S site occupation, that is, to atop, hcp hollow or fcc hollow sites.

Thus, the NIXSW data for the adsorbed intact thiol, suggested a single high-symmetry adsorption site because of the relatively high value of D(-111)=0.8 Å for the sulfur absorption signal.

The experimental value D(-111) =0.79 Å was in perfect agreement with the prediction for the atop site, so we deduced that the intact thiol was bonded to the Cu(111) surface through its S atom, which is directly atop an outermost Cu atom layer.

If the LT thiolate would occupy a high symmetry site, the measured (111) layer spacing of 1.88 Å would give (-111) coherent positions of 0.63 Å (atop), 1.32 Å (hcp hollow) and 2.01 Å (fcc hollow). The measured value of 1.55 Å would be intermediate between the last of these two, and the most likely interpretation would be a mixed occupation of the two hollow sites.

The RT thiolate showed a much smaller (111) coherent position (D(111)=1.07 Å), and its (-111) coherent fraction was close to zero (D(111)=0.15 ).

The data could be only reasonably interpreted with a local reconstruction of the outermost Cu atom layer to a more open packing structure, allowing the S atoms of the RT thiolate to penetrate deeper into the top layer. The very low (-111) coherent fraction was consistent with this reconstruction provided be incommensurate or have a large unit mesh with many nonequivalent local site registries.

The measured coherent positions for the atomic S, triangulated quite well to the occupation of fcc hollow sites. In fact, the measured (111) coherent position of 1.56 Å would lead to an expected (-111) coherent position for this site of 1.91 Å, close to the measured value of 1.97 Å (see table 4). However, the coherent fraction for these species were too low for a single high symmetry

adsorption site. For a single hollow site the Cu-S bond length, is 2.15Å, significantly shorter than that measured, 2.30Å, in a pure atomic S on Cu(111) [22].

Sulphur is known to cause complex reconstruction of the Cu(111) surface. A mixture of an unreconstructed overlayer and a partially penetrated reconstructed surface could account for the apparent bond length and for the low coherent fractions.

Conclusions A brief review of the theory of the XSW absorption, and of the NIXSW technique was presented. Previously, the XSW method for the determination of adsorbate structures on surfaces was applied only on high perfect semiconductor sample using dedicated and special designed beamlines. These restrictions were removed using the normal incidence Bragg reflection geometry: conventional SEXAFS beam line became appropriated and single metal crystals having a high degree of mosaicity could be investigated.

The aim of this paper is to highlight some development of the study of adsorbates on metal surfaces conseguent the use of third-generation synchrotron radiation facilities. Recent experimental results showed that chemical-shift XSW technique provides complementary and quantitative information on several chemically important problems involving the surface chemistry and the structure of adsorbates. These results showed that: - Surface sample temperature and X-ray incident beam induce fragmentation of the adsorbate.

- Reactions between coadsorbed fragments are suppressed at low temperature but occur at enough high temperature.

- CS-NIXSW is able to determine the local adsorption geometries of all coadsorption reaction products on the surface. These local adsorption sites determination can be done at different temperatures allowing us to follow the local adsorption sites through a surface chemical reaction.

Acknowledgement I wish to acknowledge especially the collaboration with D. P. Woodruff, R. G. Jones, and B. C. C. Cowie since together with them I started to explore the possibilities of the chemical-shift normal-incidence X-ray standing waves technique. I would like also to acknowledge G. J. Jackson, N. K. Singh, J. McCombie, J. Ludecke, A. S. Y. Chan, and C. Fisher. Finally, I wish to acknowledge E. Colavita for useful conversation and critical reading of the manuscript.

References 1. B. Krassig, M. Jung, D. S. Gemmell, E. P. Kanter, T.LeBrun, S. H. Southworth, and L. Young Phys. Rev. Lett. 75, 4736 (1995).

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2. M. Jung, B. Krassig, D. S. Gemmell, E. P. Kanter, T.LeBrun, S. H. Southworth, and L. Young Phys. Rev. A 54, 2127 (1996).

3. C. J. Fisher, R. Ithin, R. G. Jones, G. J. Jackson, D. P. Woodruff, and B. C. C. Cowie J. Phys. Condens. Matter 10, L623 (1998).

4. D. P. Woodruff, Prog. Surf. Sci. 57, 1 (1998).

5. G. J. Jackson, B. C. C. Cowie, D. P. Woodruff, R. G. Jones, M. S. Kariapper, C. Fisher, A. S. Y. Chan, and M. Butterfield, Phys. Rev. Lett.

84 (2000) 2346 6. K.U. Weiss, R. Dippler, K. M. Schindler, P. Gardner, V. Fritzsche, A.M. bradshaw,D. P. Woodruff, M..C. Acensio, and A.R. Gonzales-Elipe Phys. Rev. Lett. 71 (1993) 581 7. G. J. Jackson, J. Ludecke, D. P. Woodruff, A. S. Y. Chan, N.K. Singh, J. McCombie, R. G. Jones,B. C. C. Cowie , and V. Formoso Surface Science 441 (1999) 515 8. S.A.Joyce, J.A. Yarmoff, T.E. Madey Surface Science 245 (1991) 1782 9. G. J. Jackson, S.M. Driver, D. P. Woodruff, N. Abrams, R. G. Jones, M.T. Butterfield, M.D. Crapper, B. C. C. Cowie , and V. Formoso Surface Science 459 (2000) 231 10. G. J. Jackson, D. P. Woodruff, R. G. Jones, N.K. Singh, A. S. Y. Chan, B. C. C. Cowie , and V. Formoso Phys. Rev. Lett. 84 (2000) 2346 11. N.P. Prince, D.L. Seymour, M.J. Ashwin, C.F. McConville, P.Woodruff, and R. G. Jones Surface Science 230 12. B. Krassig, M. Jung, D. S. Gemmell, E. P. Kanter, T.LeBrun, S. H. Southworth, and L. Young Phys. Rev. Lett. 75, 4736 (1995).

13. M. Jung, B. Krassig, D. S. Gemmell, E. P. Kanter, T.LeBrun, S. H. Southworth, and L. Young Phys. Rev. A 54, 2127 (1996).

14. C. J. Fisher, R. Ithin, R. G. Jones, G. J. Jackson, D. P.Woodruff, and B. C. C. Cowie J. Phys. Condens. Matter 10, L623 (1998).

15. D. P. Woodruff, Prog. Surf. Sci. 57, 1 (1998).

16. G. J. Jackson, B. C. C. Cowie, D. P. Woodruff, R. G. Jones, M. S. Kariapper, C. Fisher, A. S. Y. Chan, and M. Butterfield, Phys. Rev. Lett.

84 (2000) 2346 17. K.U. Weiss, R. Dippler, K. M. Schindler, P. Gardner, V. Fritzsche, A.M. bradshaw,D. P. Woodruff, M..C. Acensio, and A.R. Gonzales-Elipe Phys. Rev. Lett. 71 (1993) 581 18. G. J. Jackson, J. Ludecke, D. P. Woodruff, A. S. Y. Chan, N.K. Singh, J. McCombie, R. G. Jones,B. C. C. Cowie , and V. Formoso Surface Science 441 (1999) 515 19. S.A.Joyce, J.A. Yarmoff, T.E. Madey Surface Science 245 (1991) 1782 20. G. J. Jackson, S.M. Driver, D. P. Woodruff, N. Abrams, R. G. Jones, M.T. Butterfield, M.D. Crapper, B. C. C. Cowie , and V. Formoso Surface Science 459 (2000) 231 21. G. J. Jackson, D. P. Woodruff, R. G. Jones, N.K. Singh, A. S. Y. Chan, B. C. C. Cowie , and V. Formoso Phys. Rev. Lett. 84 (2000) 2346 22. N.P. Prince, D.L. Seymour, M.J. Ashwin, C.F. McConville, P.Woodruff, and R. G. Jones Surface Science 230 (1990) 13

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A SECOND TARGET STATION AT ISIS J. Penfold ISIS Facility, Rutherford Appleton Laboratory, CCLRC, Chilton, Didcot, Oxon, UK

Neutron scattering has made a unique and fundamental contribution to the understanding of the structure and dynamics of condensed matter. Although much of the early work was based on reactor sources; in recent years, through the contributions of ISIS, IPNS. LANSCE and KEK, the potential of accelerator based pulsed neutron sources has been firmally established. The major new pulsed source developments, the SNS project in the USA, the Joint Hadron Facility in Japan, and ultimately the European Spallation Source, demonstrate the importance of pulsed neutron sources to the future of neutron scattering.

ISIS is currently one of the most intense and extensively instrumented pulsed neutron sources, and the effective

and growing exploitation of ISIS over a broad range of condensed matter research has powerfully demonstrated the specific benefits of the time of flight technique in neutron scattering on pulsed sources. The Second Target Station will build on the success of ISIS, and extend its capability into new areas.

The proposed Second Target Station at ISIS is a low frequency, low power target station, which would operate at 10 hz, taking 1 â&#x20AC;&#x201C; 5 pulses from the ISIS Synchrotron (see figure 1).

The low power dissipation and low frequency will enable it to be optimised for the production of cold neutrons, in a way not possible on the existing high power target station. Substantial gains in performance of greater than

Fig. 1. Schematic representation of the Second Target Station at ISIS

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an order of magnitude over the existing target station at ISIS will be achieved for cold neutrons and high resolution instrumentation with a broad spectral range.

These potentially impressive gains in the capability of cold neutron scattering and high resolution studies will provide exciting new opportunities in the technologically significant areas of Soft Condensed Matter, Advanced Materials and Bio-Molecular Sciences. Just as the advent of the high flux reactor at the Institute Laue Langevin, Grenoble in the early 1970’s, with its dedicated cold neutron source, broadened considerably the appeal of neutron scattering, so the development of the Second Target Station at ISIS, optimised for long wavelength neutrons, will have a major impact on the study of complex condensed matter systems. The combination of the cold neutron flux, the simultaneously available broad spectral range and the potential for high resolution, will provide facilities that are not available elsewhere.

Within the UK a number of major technological themes and priorities have been identified (a number of similar exercises have been carried out in many other European countries), many of which are closely associated with the new research opportunities that will be afforded by the development of the Second Target Station at ISIS, and are closely associated with the broad areas of Soft Condensed Matter, Bio-Molecular Science and Advanced Materials.

(See Table 1).

Chemicals

Bio-chemical technology, advanced materials, polymers, processing, sensors.

Energy

Enhanced oil recovery, waste management.

Materials

Sensors, advanced materials, adhesion, surface Engineering.

Manufacturing

Processing, product formulation.

Defence and Aerospace

Sensors, advanced materials, process technology.

Retail and Distribution

Intelligent or smart packaging.

Health and Life Sciences Drug delivery, drug creation, pharmaceuticals, immune response manipulation, metabolic pathways.

Food and Drink

Material changes during processing.

Agriculture

Pesticides, environmental controls.

Table 1.

In the areas of Advanced Materials and Soft Matter the new scientific opportunities are in the study of complex, multi-component or multi-phase systems, the use of complex sample environments, and the investigation of non-equilibrium systems. In such systems the dimension scales of importance often range from molecular to mesoscale. This dictates the need for a broad wavelength range with a particular emphasis on cold neutrons.

Kinetic studies (ranging from chemical reactions to

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probing dynamic surface tension) require the same broad spectral range with higher fluxes of cold neutrons. Multicomponent or multi-phase systems are only tractable with the parametric studies possible using enhanced flux and high resolution. The Life Sciences are currently making an immense impact, with many health-related issues underlying this importance. The success of X-ray crystallography in solving complex proteins and virus structures to high resolution has been critical for understanding structure-function relationships, and this has been further boosted by the Human Genome Project and the prospects of post genome research on a large number of newly identified protein sequences. The role of neutron crystallography is secondary to these high resolution x-ray studies. However, there are important contributions that neutrons will make in, for example, determining water or hydrogen location at lower resolution. In the broader context the post Genome era will provide exciting and important new opportunities in the broader bio-molecular sciences remit, in fields of pharmacy, food science, sensors, bio-compatibility and bio-functionality. In these particular areas there is much overlap with the areas of interest identified in Soft Matter and Advanced Materials, and their specific requirements.

The main scientific areas in which significant developments are envisaged are summarised as follows, Soft Condensed Matter Surface, interfacial and bulk properties of complex fluids (polymers, surfactants, colloids).

• Interfacial studies: self-assembly and ordering of complex mixtures of surfactants, polymers and proteins at interfaces; with emphasis on kinetic processes and multi-component systems at technologically relevant interfaces (liquid-liquid and liquid-solid), thin film devices.

• Processing of soft solids: relationship between microscopic structures and bulk properties (rheology) in industrially relevant fluid fields.

• Self-Assembly: structure of lyotropic meso-phases, micro-emulsions and vesicles, with emphasis on dynamics of structural phase changes, association and disassociation, and self-assembly in super-critical fluids.

• In-situ electrochemistry.

Bio-molecular Sciences Pharmaceuticals, drug delivery formulations, membrane-protein, interactions, bio-compatibility and functionality, food technology.

• Interfaces and membranes: structural organisation of membranes and membrane-protein systems.

• Macromolecular assemblies: low resolution studies on macromolecular assemblies, in systems not tractable by

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high resolution crystallography, viruses, glycoproteins, protein folding, protein-nucleic acid interactions. Solvent structure. Meso-scale structure.

• Pharmaceuticals: determination of new drug structures, where the role of hydrogen atoms is essential in understanding drug-receptor interactions. Molecular engineering.

• Food Technology: study of solvent distribution and structural changes in complex assemblies (for example, starches) during processing. Protein fouling.

Advanced Materials Crystalline, magnetic, disordered and engineering materials; including complex inorganic and organic assemblies, clathrates, intercalates, zeolites, nanostructured materials, high temperature superconductors, giant magneto-resistance materials, magnetic films and multi-layers, spin valves, glasses, complex fluids, porous media.

• Structural details of giant/collossal magnetoresistive materials, non-stoichiometric oxides, piezoelectric, ferroelectric and negative thermal expansion materials.

• Composite multi-crystalline materials, such as mixed geological phases, magnetoresistive manganates, toughened engineering materials and high Tc materials.

• Studies under extreme conditions: catalysis, in-situ chemical reactions (including electrochemistry and battery function), ultra-high pressures, novel processing routes.

• Structure of nano-scale materials: sol-gel processing, ceramics.

• Magnetic thin films, multilayers, spin electronics.

• Nucleation and growth processes, molecular clustering, glasses.

• Complex fluids in porous media; oil-recovery; waste disposal; supercritical fluids.

• Stress/strain mapping in engineering materials.

To realise this broad area of inter-disciplinary research a preliminary instrument suite has been identified, in the areas of Large Scale Structures, Diffraction and Spectroscopy (see figure).

The specific benefits of the time of flight method on pulsed neutron sources include improved instrumental resolution, wide simultaneous coverage of momentum and energy transfer, intrinsically low backgrounds, the ease of use of fixed geometry sample environment equipment, the high fluxes of epithermal neutrons, and the broad spectral range. The second target station will extend and reinforce those benefits for the instrumentation described above to which include: •A wider dynamical range in momentum transfer for crystalline and non-crystalline diffraction, SANS and reflectometry, and in energy and momentum transfer for spectroscopy, by virtue of the increase of the time frame from 20 to 100 ms.

•Enhanced flux for long wavelength/low energy neutrons, arising from a fully optimised target/moderator assembly.

•A coupled cold moderator will provide a further enhancement of flux for those instruments that do not require a sharp pulse structure.

•The availability of new technologies provides the opportunity to enhance further the performance of new instruments on the Second Target Station. The larger time frame will allow resolution improvements through the use of supermirror guides for the efficient transport of neutrons along longer flight paths.

The key areas of instrumentation that will benefit most are neutron reflectivity, small angle neutron scattering (SANS), very high resolution spectroscopy, noncrystalline diffraction, high resolution crystalline diffraction and large scale crystallography.

Further details of the preliminary instrument suite can be found on www.isis.rl.ac/target station 2/instruments/.

The second target station will be situated on the southern side of the existing high power target (see figure 6), and one pulse in five from the ISIS synchrotron will be

Fig. 3. The study of protein-membrane interactions.

• Structure of complex materials: Role of zeolites in ion exchange materials and catalysis, novel electrodes and electrolytes, clathrate formation, nano-structured materials.

• Determination of new magnetic structures: materials with novel ground states.

directed along a new proton beam line. The relatively low power transfer tantallum will have two cryogenic moderators in “wing” geometry (a 25K decoupled solid methane moderator, and a 25K coupled liquid hydrogen moderator), and surrounded by a beryllium reflector. 9

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Fig. 2.

Fig. 4.

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Fig. 5. Possible Instrument suite for the Second Target Station

Fig. 6. Schematic representation of the Second Target Station

Fig. 7. The target, moderator and reflector assemblies.

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instrument beam points will view each moderator (see figure 7).

The relatively low proton power will allow target and moderator designs which are optimised for the production of long wavelength neutrons, and hence more

provide the detailed design criteria. With an estimated construction period of 3 to 4 years, it is anticipated that this project could be completed by 2005/6. With the funding of the new synchrotron radiation source, Diamond, secured, it is CCLRC’s highest priority. These two new facilities will provide a powerful complementary combination for inter-disciplinary research in Soft Condensed Matter, Bio-Molecular Sciences and Advanced Materials over the next 5 to 10 years. Further details can be obtained from Andrew or Jeff Penfold Taylor (a.d.taylor@rl.ac.uk) (j.penfold@rl.ac.uk).

Fig. 8. Some recent Monte Carlo calculations of the target / moderator assembly. The triangles are for a broad pulse coupled moderator, and the circles for a sharp pulse moderator suitable for high resolution applications.

efficient than those on the existing target station. In particular, the use of a pre-moderation and a coupled hydrogen moderator will provide a significant enhancement in cold neutron flux (with a relaxed pulse structure, ∆t ≤ 300 msec) (see figure 8).

It is now recognised that neutron flux does not simply scale with proton power/current, and that at modest target power levels there are gain factors and efficiency factors which are not available at higher power levels.

This is particularly true for for cold neutron production; and this coupled with the broad wavelength band more easily available at low source frequencies, is an attractive proposition.

The scientific case for the Second Target Station at ISIS has been developed, and was enthusiastically endorsed by the current (and potential) UK user community at a meeting at RAL in May (see “Second Target Station at ISIS: New opportunities for interdisciplinary research using neutrons”, RAL report, RAL-TR-2000-032, and http://www:isis.rl.ac.uk/ targetstation2/). Formal approval and funding is now being sought. In the meantime detailed programme of calculations is underway; in order to optimise and refine the neutronic performance of the target/moderator assembly and to

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Assessment Committee supports INFM at ILL

The INFM Nucleo di valutazione1 (Assessment Committee) voiced their complete satisfaction with the relationship between ILL and INFM during an informal visit to the facility in July. They were accompanied by Roberto Felici and Matilde Bolla, respectively the responsible and the administrator of the on-site INFM Outstation which acts as a gathering point for Italian Users.

The Committee was personally welcomed by the ILL British Director, Colin Carlile, who described the role of the Institute focussing particularly on its partnership with Italy. He gave an overview of the type of neutron beam instrumentation available at the ILL using an interactive computerised model emphasising the investment programme recently launched to upgrade the instrument suite at the ILL - the Millennium Programme. Italian use of the ILL beam time over the past three years was detailed by Giovanna Cicognani, former INFM employee and now Head of Scientific Coordination at the ILL. The statistics show that Italy makes full use of its 3% budget allocation and, in fact, demand exceeds current availability demonstrating vividly the health of the Italian neutron scattering community.

The Assessment Committee were treated to a tour of the swimming pool of the reactor Level D, where they looked at the reactor pool and

1 Members of the Committee are: Prof. S. Carra (Milan, Chairman), Dr. L. Scarpitti (ENEA, Rome), Prof. M. Calderini (Turin), Prof F Toigo (Padua), Prof. C. Zannoni (Bologna) The Committee was accompanied by Dr. F. Calvi (INFM, Genoa), Dr E. Narducci (INFM, Genova).

saw the Cherenkov effect, which is always an impressive sight. They said that they considered it to be a real privilege to be guided by the Head of the Reactor Division, Ekkehardt Bauer, who took the time to answer in detail their many questions.

They finished the tour by a visit to the experimental hall, where they met Giovanna Fragneto (instrument responsible of the ILL diffractometer D16), Francesca Natali (post-doc on the Italian CRG IN13 from Parma), Ferndinando Formisano (local responsible of the BRISP Brillouin Spectrometer), Claudia Mondelli (PhD at the ILL) and Italian users on site.

The visit complemented the scientific presentations which had been given on the IN13 backscattering spectrometer and BRISP Brillouin Spectrometer by Francesco Sacchetti to the Panel at its meeting held in Genova in June this year.

Many thanks to everyone at the ILL who helped us in the organisation of the day. Both the ILL team and the INFM Assessment Committee considered the visit as very positive and one which underlined the excellent relations between the two parties.

Giovanna Cicognani ILL-SCO

A glance at the ILL future: Colin Carlile explaining the ILL interactive mock-up during the INFM Assessment Committee visit to the facility (from the left: C.M. Bertoni, N. Narducci, F. Calvi, S. Carra, C. Carlile and C. Zannoni.

Ekkehardt Bauer (second from the left) explaining the Cherenkov effect at the reactor pool.

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Commissione Luce di Sincrotrone del CNR Il CNR ha approvato nel 2000 un

chetto stesso di elettroni.

aminoacidi con luce polarizzata

nuovo Progetto di Ricerca e Svilup-

E’ stato recentemente dimostrato,

circolarmente presso il FEL di

po nel campo delle applicazioni del-

presso i laboratori di Stanford, che

Amburgo.

le sorgenti di luce di sincrotrone di

l’effetto SASE è possibile nell’infra-

Alle attività sperimentali suddette

quarta generazione basate su sorgen-

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verrà aggiunta una attività di forma-

ti free electron laser (fel).

progetti verso la realizzazione di

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sorgenti FEL ad energie più alte

delle macromolecole. Tale attività

di esperimenti su sorgenti di luce di

Uno di tali progetti è in corso di rea-

verrà per ora realizzata presso le sor-

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lizzazione ad Amburgo presso i la-

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basate su sorgenti Free Electron La-

boratori DESY.

ELETTRA di Trieste ed ESRF di Gre-

ser (FEL). Tali sorgenti permettono

Il programma del Progetto si svilup-

noble, con l’intenzione di continuar-

di raggiungere un flusso di fotoni ed

perà su due linee parallele: la prima

la, appena possibile, su una sorgente

una brillanza di diversi ordini di

prevede l’utilizzo di una sorgente

X-FEL.

grandezza superiori a quelli otteni-

FEL nell’infrarosso già operativa a

Il Progetto, che ha una durata di tre

bili nelle migliori sorgenti attuali.

Nashville (USA); la seconda l’utiliz-

anni, é coordinato dal Dr. P. Perfetti

Se a tali eccezionali caratteristiche si

zo della sorgente DESY.

ed ha come Unità operative:

aggiunge la coerenza tipica delle sor-

In particolare il programma di ricer-

- Istituto di Struttura della Materia,

genti laser, si capisce come sia possi-

ca si articolerà su tre diverse classi di

CNR, Tor Vergata (Dr. P. Perfetti)

bile pensare esperimenti completa-

esperimenti:

- Istituto di Chimica dei Materiali,

mente nuovi ed esplorare nuovi

1. accoppiamento di un microscopio

CNR, Montelibretti (Dr. T. Prosperi)

campi di ricerca della fisica, della

ottico a campo vicino (SNOM)

- Istituto di Biologia Cellulare, CNR,

chimica, della scienza dei materiali

con il FEL di Vanderbilt per utiliz-

Campus Buzzati Traverso, Monte-ro-

in generale, della biologia e della

zarlo in modo spettroscopico. So-

tondo (Prof. G. Tocchini Valentini)

medicina.

no previsti esperimenti su films

Attualmente sorgenti di tipo FEL so-

sottili; esperimenti su fluttuazioni

P. Bosi

no state realizzate nell’infrarosso e

laterali di barriere di potenziale

Segretaria Scientifica

sono iniziati progetti per la realizza-

presenti in particolari interfacce

zione di FEL nel vicino ultravioletto

di interesse per la microelettroni-

e nella regione dei raggi X molli. Le

ca; esperimenti su campioni bio-

sorgenti nell’ultravioletto e nei raggi

logici.

X si basano sull’effetto SASE (Self

2. spettroscopia di assorbimento a

Amplified Spontaneous Emission),

due fotoni presso la sorgente di

in cui la radiazione coerente origina

DESY su campioni di alogenuri

dall’interazione tra la radiazione

alcalini, bromuri e floruri, quarzo

spontanea, emessa dagli elettroni

e ad alcuni tipi di ossidi.

iniettati in un ondulatore, ed il pac-

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

46

3. fotorisoluzione di mescolanze di

•

Vol. 5 n. 2 Dicembre 2000

VARIE

SISN: Documento programmatico della SISN sulle future sorgenti neutroniche su scala europea Premessa

stituzione dell'associazione.

Prospettive per le nuove

Nel contesto delle diverse ipotesi che

Un precedente forum di discussione,

sorgenti europee

circolano attualmente sul futuro del-

tenutosi in occasione del convegno

Fermo restando che ESS rappresenta

la neutronica in Italia, anche in rela-

nazionale dell'INFM a Genova, ha

l'obiettivo principale per la comunità

zione alle diverse iniziative che si

evidenziato alcuni punti che posso-

neutronica italiana, in quanto realiz-

stanno sviluppando su scala Euro-

no rappresentare una chiave di lettu-

zerebbe la creazione di una large-sca-

pea, si ritiene opportuno che la So-

ra per quanto concerne le future sor-

le facility europea, occorre considera-

cietà Italiana di Spettroscopia Neu-

genti neutroniche in ambiente euro-

re che i tempi di realizzazione di tale

tronica (SISN) assuma il ruolo che le

peo. In tale riunione sono state di-

obiettivo sono diluiti nel tempo e so-

spetta, a livello di consulente nazio-

scusse sia le prospettive di nuova

no attualmente stimabili in circa un

nale, quale portavoce della comunità

scienza, accessibile con le nuove sor-

decennio di ulteriore studio e pro-

neutronica italiana.

genti pulsate, sia le informazioni tec-

gettazione. Inoltre, occorre tener

In generale, basandosi sulle attuali

niche sulle varie ipotesi attualmente

conto che nel breve periodo (5-10 an-

prospettive, si può affermare che le

aperte (ESS, AUSTRON, ISIS-2, LE-

ni) sono prevedibili sensibili diminu-

sorgenti termiche (reattori) non sem-

GNARO). Dalla discussione di Ge-

zioni del flusso medio di neutroni

brano destinate ad espandersi, con

nova sono stati individuati due pun-

accessibile alla ricerca, anche in con-

l'unica eccezione, forse, del reattore

ti importanti:

seguenza dell'invecchiamento, ed al-

di Monaco. Viceversa, si prevede che,

1.prospettive per le nuove sorgenti

la conseguente obsolescenza, di sor-

se ci sarà un'espansione, questa sarà

genti storiche che presumibilmente si

europee

orientata verso le sorgenti pulsate. In

2.ottimizzazione dell'accesso alle

stanno avvicinando alla fine del loro

particolare, tenuto conto delle diver-

sorgenti esistenti per la comunità

ciclo produttivo. In questo contesto,

se ipotesi attualmente in discussione

italiana

deve essere considerato sintomatico

su scala europea (ESS, AUSTRON,

Su questi punti, l'Assemblea della

lo shut-down definitivo del reattore

ISIS-2) la SISN ritiene doveroso dare

SISN, riunitasi a Roma il 19 Ottobre

di Risø e la crisi momentaneamente

una valutazione obiettiva degli inte-

2000, ha ulteriormente discusso rag-

intervenuta negli USA a seguito del-

ressi della comunità nazionale.

giungendo un'opinione comune, che

la chiusura di BNL e del lungo shut-

A questo scopo, è importante che la

viene descritta nel presente docu-

down di Oak Ridge.

comunità neutronica italiana, infor-

mento programmatico, e che costi-

Nel contempo, la costruzione del se-

mata di quanto sta accadendo sulla

tuisce il punto di vista ufficiale della

cond target station ad ISIS, denominata

scala europea, e delle possibili azioni

SISN. L'Assemblea della SISN ritiene

ISIS-2, risulta un'ipotesi più che reali-

che possono essere intraprese nell'in-

che questo documento debba essere

stica che, secondo la programmazio-

teresse della comunità nazionale,

opportunamente pubblicizzato e so-

ne prevista, potrebbe essere realizzata

faccia sentire la sua voce tramite la

stenuto sia presso il MURST che

sulla scala di qualche anno. Il proget-

SISN che viene quindi ad assumere

presso gli Enti che attualmente fi-

to prevede, per questa realizzazione,

attivamente quel ruolo di consulen-

nanziano le attività di spettroscopia

un aumento della corrente nell'accele-

za che è previsto dallo statuto di co-

neutronica italiana (INFM e CNR).

ratore da 200 a 300 A e lo splitting del

Vol. 5 n. 2 Dicembre 2000

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

47

VARIE

fascio di protoni su due distinti bersa-

liana ha attualmente accesso, tramite

nium programme prevede, in una pri-

gli di spallazione. Il primo dovrebbe

accordi o convenzioni internazionali,

ma fase, il potenziamento di 5 stru-

mantenere le stesse caratteristiche che

alle seguenti facilities europee: ILL

menti:

ISIS presenta attualmente (a 50 Hz di

(Grenoble), ISIS (Oxford), LLB (Sa-

- Stress residui

frequenza), mentre per il secondo è

clay). Inoltre, pur non essendo ope-

- Thermal LADI

prevista una frequenza più bassa (10

ranti specifici accordi o convenzioni

- Polarizzazione su IN20

Hz) che permetterebbe, con l'allunga-

esistono collaborazioni scientifiche a

- Potenziamento del D3

mento del frame, l'utilizzo più effica-

vari livelli che permettono l'utilizzo

- Time resolved SANS

ce dei neutroni freddi ed il conse-

da parte dei ricercatori italiani di al-

L'Italia ha proposto che IN13 (CRG

guente trasferimento su ISIS-2 della

tre sorgenti neutroniche europee

con la Francia) venga incluso nel MP

strumentazione che li utilizza (basso

(Svezia e Germania) o negli USA

e, probabilmente, questo rientrerà

angolo, riflettometria, diffrazione ad

(Brookhaven, Oak Ridge, Los Ala-

nel quadro più generale del miglio-

alta risoluzione, spettroscopia quasi-

mos, etc.). La partecipazione attuale

ramento delle guide per neutroni

elastica, etc.). Ovviamente, questo

dell'Italia presso ILL prevede un ac-

con un conseguente incremento di

comporterà la contemporanea messa

cesso a livello del 3% del tempo

flusso al campione. Un contributo

a disposizione di beam-lines attual-

macchina totale disponibile. Tale ac-

italiano per il miglioramento delle

mente occupate su ISIS-1 e permet-

cordo è gestito da una convenzione

guide di IN13, che venisse incluso

terà lo sviluppo di nuova strumenta-

con l'INFM. Per quanto concerne

nel millennium programme, sarebbe

zione che viceversa utilizza più effi-

ISIS, all'Italia è garantita una parteci-

altamente auspicabile.

cacemente i neutroni termici e/o epi-

pazione del 5% che viene gestita da

Inoltre, per venire incontro alle cre-

termici.

un accordo internazionale con il

scenti esigenze scientifiche della co-

Dal punto di vista realizzativo, non

CNR. La convenzione con LLB è ge-

munità italiana sarebbe anche auspi-

si prevedono problemi sostanziali in

stita anch'essa da INFM.

cabile un incremento della quota di

quanto la tecnologia necessaria è già

La costruzione di un second target ad

partecipazione italiana ad ILL. A

acquisita ed il problema si riduce al-

ISIS, con la conseguente liberazione

questo proposito, si consideri che, a

la reperibilità di mezzi finanziari. I

di un discreto numero beam-lines, e

fronte di una richiesta ben più eleva-

tempi caratteristici dell'operazione

la realizzazione del millennium pro-

ta, la quota di utilizzo italiana non

sono stimati in 2-3 anni. Infine, oc-

gramme dell'ILL, che prevede l'up-

può mai superare, secondo i termini

corre tener presente che il RAL è sta-

grading di diversi strumenti e la co-

dell'accordo, la percentuale negozia-

to scelto per la costruzione di un sin-

struzione di nuovi, rappresentano

ta del 3% del tempo macchina totale

crotrone (DIAMOND Project, in coo-

un'occasione che vale la pena coglie-

disponibile.

perazione con la Francia) che ren-

re per accrescere le competenze ita-

Per quanto concerne ISIS, si deve os-

derà il polo di Oxford equivalente a

liane nel campo della strumentazio-

servare che la scelta dello strumento

quello di Grenoble per quanto con-

ne neutronica. Inoltre, si sottolinea

inerente il progetto TOSCA, che fa

cerne la disponibilità, nello stesso si-

che la comunità italiana vanta nume-

parte integrante dell'accordo di coo-

to, di neutroni e luce di sincrotrone.

rosi e proficui rapporti di collabora-

perazione tra il CNR ed CRLC, è sta-

zione scientifica con LLB che potreb-

ta condizionata dalla mancanza di

Ottimizzazione dell'accesso alle sorgenti

bero essere ulteriormente potenziati

una beam-line disponibile ed è stata

esistenti per la comunità italiana

da valide proposte nel campo del

quindi, in qualche maniera, imposta

Stante la mancanza di sorgenti neu-

rinnovo del parco strumenti.

da ragioni oggettive ma contingenti.

troniche nazionali, la comunità ita-

Per quanto concerne ILL, il millen-

L'accordo di cooperazione, che giun-

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

48

•

Vol. 5 n. 2 Dicembre 2000

VARIE

ge a scadenza con il 2001, dovrà es-

to che la convenzione venga rinno-

bito del millennium programme, per-

sere rinnovato e si reputa necessario

vata alla sua scadenza naturale di

metterebbe alla ricerca italiana di

proseguire sulla strada oramai trac-

fine anno 2000.

crescere ulteriormente avvicinandosi

ciata di includere la costruzione di

Infine, per quanto concerne l'assenza

ai livelli europei.

uno strumento nei termini dell'ac-

cronica di sorgenti neutroniche na-

La continuazione della partecipazio-

cordo. Ovviamente, la costruzione di

zionali, che indubbiamente potrebbe-

ne italiana ad LLB consoliderebbe il

ISIS-2 apre molte prospettive e tutto

ro facilitare il processo di formazione

proficuo rapporto instaurato da an-

lascia presagire che il prossimo stru-

dei giovani ricercatori, appare oggi

ni, estendendo a nuovi giovani ricer-

mento potrà essere scelto avendo a

chiaro che il reattore della Casaccia

catori possibilità di formazione già

disposizione un maggior numero di

risulta inaccessibile e che, forse, l'uni-

sperimentate nel passato.

gradi di libertà.

ca alternativa nazionale è rappresen-

Un eventuale accordo INFM-INFN

Il CNR ha erogato, nel corrente anno

tata da un'ipotesi di accordo tra

per la realizzazione di una piccola

2000, un primo finanziamento a

l'INFM e l'INFN per realizzare, pres-

sorgente pulsata a Legnaro permet-

fronte della costruzione di una sta-

so i Laboratori Nazionali di Legnaro

terebbe, infine, di disporre di una

zione sperimentale italiana a valle

(PD), una piccola sorgente pulsata di

sorgente nazionale presso la quale,

dello strumento TOSCA. La Com-

neutroni. Questa sorgente, per la

parallelamente alla stazione italiana

missione Neutroni del CNR si è già

quale sono state effettuate positiva-

ad ISIS, all'OGG presso ILL, ed al-

pronunciata affinché tale stazione

mente alcune prove di fattibilità, po-

l’auspicata SANS facility presso LLB,

sperimentale sia dotata di un diffrat-

trebbe fornire un flusso di neutroni

potrebbe essere esteso il progetto di

tometro. Non si esclude, comunque,

sufficiente per costituire un ottimo

formazione di personale giovane,

che questa possa essere dotata di al-

laboratorio finalizzato alla formazio-

che riveste un'importanza strategica

tra strumentazione (p. es. anelastica)

ne del personale ed alla realizzazione

per la nostra comunità, in attesa del-

per scopi di test e/o formazione. In-

di tests su rivelatori e strumentazio-

lo sviluppo delle sorgenti di nuova

fine, è doveroso ricordare che la di-

ne neutronica in generale.

generazione.

sponibilità (se pur parziale) di una sorgente di neutroni epitermici per-

Conclusioni e raccomandazioni

metterebbe alla comunità neutronica

In conclusione, si può affermare che

italiana di presentarsi con altre cre-

la via ottimale per uno sviluppo ar-

denziali, che non quelle attuali, al-

monico della neutronica in Italia

l'appuntamento con ESS.

passa per un coinvolgimento italiano

La convenzione con LLB ha reso

in ISIS-2, che permetterebbe l’acces-

possibile la realizzazione di proget-

so in tempi brevi ad una sorgente di

ti sperimentali che, per le loro ca-

nuova generazione, lo sviluppo delle

ratteristiche, trovano difficile accet-

esistenti linee di ricerca e l’acquisi-

tazione presso facilities di maggiori

zione del know-how (sia per il lato

dimensioni, ha facilitato l’instaurar-

tecnico che per quello delle risorse

si di proficue collaborazioni scienti-

umane) necessario per una efficace

fiche con centri di ricerca industria-

partecipazione ad ESS.

li italiani ed ha permesso l’adde-

La rafforzata partecipazione italiana

stramento di numerosi giovani ri-

ad ILL, con le iniziative di completa-

cercatori italiani. Si auspica pertan-

mento dei CRG già avviati e nell'am-

Vol. 5 n. 2 Dicembre 2000

Marco Zoppi

•

Segretario SISN

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

49

VARIE

TMR TMR

Training and Mobility of Researchers

Support for activities in the field of neutron scattering is available from the neutron round-table.

The neutron round-table is funded by the EC (DGXII) with approximately 100.000 Euro per year. The mission of the round-table is:

1.

To actively encourage co-

3.

To support training of young scientists

5.

The round-table consist of

ordination and collaboration

and other scientists, new to the

representatives from all major

between user facilities - such

field of neutron scattering about

European neutron user facilities,

that the European users will

the potential of the method.

from EC supported networks

benefit through a better quality

developing novel

access to the European neutron

4.

scattering facilities.

national access to summer

user representatives appointed

schools, workshops, training

by ENSA (European Neutron

To spread the

courses, co-ordination activities

Scattering Association). The

knowledge about the

etc. Detailed information on

name of all contact persons can

potential of neutron scattering,

how and when to apply for

be found on the web page

and support studies on future

support can be found on the

mentioned above. The present

prospects with neutron

round-table web page:

chairman/co-ordinator of the

scattering.

http://www.risoe.dk/fys/TMR.

round-table is Kurt Nørgaard

htm

Clausen, and can be contacted

and an increased quantity of

2.

The round-table

instrumentation and techniques

supports non-

for neutron scattering plus 5

as kurt.clausen@risoe.dk

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

50

â&#x20AC;˘

Vol. 5 n. 2 Dicembre 2000

CALENDARIO

18-19 gennaio 2001

FOLGARIA (TN), ITALY

1 Convegno Utenti GILDA Paolo Fornasini, Dipartimento di Fisica, Università di Trento, 38050 Povo (TN), Italia e-mail: fornasin@science.unitn.it http://www.science.unitn.it/~rx/gilda01/bando.html O

23 maggio -3 giugno 2001

ROME, ITALY

VUV-XIII Satellite Meeting "Decay Processes in Core-Excited Species" M.N.Piancastelli, Dipartimento di Chimica, University "Tor Vergata", Rome, Italy coredec@stc.uniroma2.it http://www.uniroma2.it/eventi/coredec/

ERICE, ITALY

International School of Crystallography Paola Spadon, Dipartimento di Chimica Organica, Università di Padova, Via Marzolo 1, 35131 Padova, Italia.

Tel: +39 049 8275275; Fax: +39 049 8275239 e-mail: paola@chor.unipd.it

23-27 luglio 2001

30 luglio - 2 agosto 2001

9-13 settembre 2001

MUNCHEN, GERMANY

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

TRIESTE, ITALY

13O International Conference on Vacuum Ultraviolet Radiation Physics (VUV-XIII) e-mail: vuv13@elettra.trieste.it http://vuv13.elettra.trieste.it/vuv13/

maggio 2002

NIST, USA

American Conference on Neutron Scattering

Vol. 5 n. 2 Dicembre 2000

•

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

51

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 2001 e il 16 ottobre 2001

Le prossime scadenze sono il 15 marzo 2001 (cristallografia macromolecolare) e il 1 giugno 2001 (fisica)

ILL BESSY

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

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

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

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

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

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

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

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

GILDA

RISØ E NFL La scadenza per il prossimo call for proposals è il 1 aprile 2001

(quota italiana) Le prossime scadenze sono il 1 maggio 2001 e il 1 novembre 2001

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

LURE La prossima scadenza è il 30 ottobre 2001

MAX-LAB La scadenza è approssimativamente febbraio 2001

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

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

52

•

Vol. 5 n. 2 Dicembre 2000

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.

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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/ ESS European Spallation Sources Andrea Fournier Tel: + 49 2461 61 2184 E-mail: a.fournier@fz-juelich.de http://www.kfa-juelich.de/ess/ GKSS Forschungszentrum Geesthacht, P.O.1160, W-2054 Geesthacht, Germany Reinhard Kampmann; tel: +49 4152 87 1316 fax: +49 4152 87 1338 E-mail: PWKAMPM@DGHGKSS4 Heinrich B. Stuhrmann; tel: +49 4152 87 1290 fax: +49 4152 87 2534 E-mail: WSSTUHR@DGHGKSS4 IFE Institut for Energiteknikk, P.O. Box40, N-2007 Kjeller, Norway Jon Samseth; tel: +47 6 806080 fax: +47 6 810920 telex: 74 573 energ n E-mail: Internet JON@BARNEY.IFE.NO ILL Institute Laue Langevin, BP 156, F-38042, Grenoble Cedex 9,France 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 fax: +1 708 972 4163 or +1 708 9724470 (Chemistry Div.) http://pnsjph.pns.anl.gov/ipns.html ISIS The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxfordshire OX11 0QX, UK Richard Heenan; tel +44 235 446744 E-mail: RKH@UK.AC.RUTHERFORD.DEC-E Steve King; tel: +44 235 446437 fax: +44 235 445720; Telex: 83 159 ruthlb g E-mail: SMK@UK.AC.RUTHERFORD.DEC-E http://www.isis.rl.ac.uk JAERI Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken 319-11, Japan.

Jun-ichi Suzuki (JAERI); Yuji Ito (ISSP, Univ. of Tokyo); fax: +81 292 82 59227 Telex: JAERIJ24596 http:// neutron-www.kekjpl JINR Joint Institute for Nuclear Research, Laboratory for Neutron Physics, Head P.O.Box 79 Moscow, 141 980 Dubna, USSR A.M. Balagurov; E-mail: BALA@LNP04.JINR.DUBNA.SU Yurii M. Ostaneivich; E-mail: SANS@LNP07.JINR.DUBNA.SU fax: +7 095 200 22 83 Telex: 911 621 DUBNA SU http://www.jinr.dubna.su KFA Forschungszentrum Jülich, Institut für Festkörperforschung, Postfach 1913, W-517 Jülich, Germany Dietmar Schwahn; tel: +49 2461 61 6661; E-mail: schwahn@djukfa54.bitnet Gerd Maier; tel: +49 2461 61 3567; E-mail: meier@djukfa54.bitnet fax: +49 2461 61 2610 Telex: 833556-0 kf d

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FACILITIES

LLB Laboratoire Léon Brillouin, Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur-Yvette Cédex France J.P Cotton (LLB); tel: +33 1 69086460 fax: +33 1 69088261 telex: energ 690641 F LBS+ E-mail: COTTON@BALI.CEA.FR http://bali.saclay.cea.fr/bali.html 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 56 310 2087 fax: +41 56 310 2939 E-mail: SINQ@psi.ch http://www.psi.ch/sinq 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 SNS Spallation Neutron Source SNS Project Office, 701 Scarboro Road, Oak Ridge, TN 37830 tel: +1 865 574 0558 E-mail: snswebmaster@sns.gov http://www.sns.gov/

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