NOTIZIARIO Neutroni e Luce di Sincrotrone - Issue 3 n.1, 1998

Rivista del Consiglio Nazionale delle Ricerche

Vol. 3 n. 1

NOTIZIARIO Neutroni e Luce di Sincrotrone

Giugno 1998 - Aut. Trib. Roma n. 124/96 del 22-03-96 - Sped. Abb. Post. 70% Filiale di Roma - C.N.R. p.le A. Moro 7, 00185 Roma

NOTIZIARIO Neutroni e Luce di Sincrotrone Editoriale

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F.P. Ricci RASSEGNA SCIENTIFICA

Electronic Structure and Magnetism in Thin Films and Multilayers .....................................................................................................................................................

C. Carbone and E. Vescovo Structure Determination of Low-Melting Point Molecular Crystals

by Neutron Powder Diffraction ..............................................................................................................................................

R.M. Ibberson Hard X-Ray Waveguides as a New Tool in X-Ray Microscopy ..............................................

S. Lagomarsino et Al. Structural Characterization of Semicrystalline Polymer Blends: SAXS and SANS Complementarity...................................................................................................................................

A. Triolo and R. Triolo

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DOVE NEUTRONI

Status of the TOSCA Project: The Back-Scattering Portion TOSCA-1 ..........................

M. Zoppi et Al.

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SCUOLE E CONVEGNI

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VARIE

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FACILITIES .............................................................................................................................................................

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CALENDARIO SCADENZE

In copertina: Disegno schematico dello strumento “back-scattering” TOSCA-1 installato ad ISIS, vista posteriore dello strumento. Cover photo: Schematic design of the back scattering spectrometer TOSCA-1 installed at ISIS, back view .

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EDITORIALE

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partire da questo numero gli articoli del Notiziario verranno pubblicati in lingua inglese. La redazione ha preso questa decisione ritenendo che faciliterà la diffusione del Notiziario anche all'estero, dove peraltro viene già distribuito in modo significativo.

Alcuni mesi fa, in una fredda notte di febbraio, TOSCA ha attraversato la Manica ed è sbarcata in Inghilterra. Ancora qualche solfeggio, in attesa della prima, e lo spettrometro a geometria inversa, finanziato interamente dal Consiglio Nazionale delle ricerche nell'ambito di un accordo di con il Central Laboratory Research Council inglese - CLRC, progettato da un gruppo di lavoro guidato da F.P. Ricci e costruito presso i laboratori dell'Istituto di Elettronica Quantistica di Firenze, inizierà a fornire dati, al servizio della vasta comunità di biologi, chimici e fisici che, da anni, utilizza i neutroni prodotti dalla sorgente pulsata ISIS del Rutherford Appleton Laboratory. Nonostante i numerosi problemi che è stato necessario superare in fase di realizzazione (pensate a Marco Zoppi, vedi articolo in questo numero, dentro una muta da palombaro, presa in prestito da un amico, che prova a saldare fogli di cadmio su blocchi di berillio), la costruzione dello strumento è stata portata a termine in un tempo da record: dalla prima riunione del gruppo di progetto, tenutasi il 10 dicembre del 1996, alla spedizione dello strumento sono passati meno di 14 mesi. Se le prestazioni effettive corrisponderanno a quelle nominali, TOSCA darà certamente un contributo rilevante agli studi di spettroscopia molecolare e porterà, anche in questo campo, l'applicazione dei neutroni ad un alto livello qualitativo, e a quella piena maturità che è stata finora preclusa dall' inadeguatezza della strumentazione disponibile.

Dal punto di vista della comunità italiana impegnata nella ricerca con i neutroni, la realizzazione di TOSCA rappresenta il conseguimento di un altro importante obiettivo ed il segnale che si è pronti a partecipare, da protagonisti, alle iniziative continentali che si prefigurano, nell'ambito della neutronica, per il prossimo futuro.

La qualità e la quantità dei contributi offerti dalla nostra comunità alla ricerca scientifica europea è oramai misurabile non solo sulla base degli articoli scientifici pubblicati, ma anche dalla capacità di sviluppare strumentazione avanzata fondamentale che ha un'innegabile importanza strategica per lo sviluppo tecnologico dell'Europa. Alla costruzione di TOSCA, infatti, si affiancano altri progetti ora in fase di sviluppo che sono altamente qualificanti, come lo spettrometro a tempo di volo BRISP e lo spettrometro a retro-diffusione IN13 presso l'ILL di Grenoble, il diffrattometro DIANE del Laboratoire Leon Brillouin di Saclay, il progetto VESUVIO per la spettroscopia con neutroni epitermici e l'attività svolta nell'ambito della rete europea XENNI per la realizzazione di nuovi rivelatori allo stato solido.

La neutronica italiana sembra, dunque, attraversare un periodo caratterizzato da molti aspetti positivi. Il prossimo passo da compiere è di attivarsi affinché l'Italia firmi il memorandum d'intesa che ha dato vita allo Sviluppo della European Spallation Source, il laboratorio che dovrebbe garantire all'Europa la possibilità di conservare, anche durante la prima metà del prossimo secolo, la posizione di preminenza che, in questo importante campo di ricerca, le è riconosciuta oggi nel mondo. A questo proposito alcune importanti iniziative sono già state intraprese (vedi artcolo sullo stato della partecipazione all'ESS nelle Varie).

Il 1997 è stato anche un anno di particolare rilievo per la luce di sincrotrone in quanto è stato celebrato il cinquantesimo anniversario della prima osservazione nei laboratori della General Elec-

tric. È superfluo sottolineare l'enorme sviluppo qualitativo e quantitativo del campo da quella prima osservazione. A proposito, è utile ricordare che la rivista Science ha posto i risultati ottenuti con luce di sincrotrone, in particolare nel campo della bio-cristallografia, tra i dieci più significativi del 1997.

Per celebrare il cinquantesimo anniversario della luce di sincrotrone ESRF ha organizzato in novembre a Grenoble una conferenza dal titolo "Highlights in X-ray Synchrotron Radiation Research". Di tale conferenza, nella quale i maggiori esperti hanno esposto sia un panorama storico che lo stato di avanzamento delle ricerche con luce di sincrotone, riferiamo nell'interno con una relazione di R. Rizzo.

Dal punto di vista della strumentazione a disposizione della comunità italiana di luce di sincrotrone nuove stimolanti possibilità di ricerca sono rese possibili dai recenti sviluppi a Grenoble e Trieste. Ad ESRF (laboratorio al quale l'Italia partecipa al livello del 15%) è in fase di conclusione la installazione delle beamline. Dalla scadenza di febbraio che sono aperte all'utenza internazionale tre nuove beamline: due dedicate alla cristallografia macromolecolare (ID14 e BM30, quest'ultima CRG Francese) ed una allo scattering magnetico (BM28, CRG Britannico). Queste linee vanno ad aggiungersi alle 28 linee pubbliche e 4 CRG già disponibili; per gli utenti italiani è riservato, inoltre, il 66% del tempo macchina sul CRG italiano GILDA. Per quanto riguarda la facility nazionale ELETTRA di Trieste sono ora aperte all'utenza sette beamline, mentre per luglio di questo anno si prevede l'apertura di altre tre linee. Sono stati recentemente firmati accordi di collaborazione fra la Sincrotrone Trieste (ST) e gli enti di ricerca per la gestione delle seguenti beamline: fotoemissione con alta risoluzione VUV (fra ST e CNR), cristallografia diffrattometrica da cristallo singolo (fra ST e CNR), fotoemissione in fase gassosa (fra ST, CNR ed INFM) e la beamline ALOISA (fra ST ed INFM). Questo panorama di offerta verrà ulteriormente arricchito dalle nuove linee già finanziate, alcune delle quali in fase di commissioning: APE (INFM/ETH), BACH (INFM), LILIT (CNR/INFM), X-MOSS (INFM), EXAFS (ST/Università di Trieste), Materials Science (Accademia delle Scienze Ceca) e CIPO (CNR).

Pubblichiamo in questo numero nella sezione "Rassegna scientifica" quattro articoli che riguardano settori in cui la spettroscopia neutronica e la luce di sincrotrone hanno dato, e sicuramente continueranno a dare, contributi significativi. Nell'articolo di Ibberson si descrive l'efficacia della tecnica della diffrazione di neutroni da polveri ad alta risoluzione per ottenere la struttura microscopica di cristalli molecolari con basso punto di fusione. Nell'articolo di Triolo di presentano risultati sulla caratterizzazione strutturale di polimeri semicristallini utilizzando in modo complementare lo scattering a basso angolo X e di neutroni. Nell'articolo di Carbone e Vescovo è illustrata l'applicazione della fotoemissione, risolta in spin, con luce di sincrotrone allo studio di sistemi magnetici quali epistrati e multistrati metallici. Oltre alla descrizione del panorama delle conoscenze attuali viene data una prospettiva sulle future direzioni di ricerca di questo importantissimo campo. Ricordiamo che recenti risultati hanno permesso la comprensione di fenomeni, quali l'accoppiamento magnetico attraverso strati nonmagnetici, e che hanno già trovato applicazione in dispositivi.

Nell'articolo di Lagomarsino et. al. è descritto lo sviluppo di una guida d'onda per raggi X duri che permette di produrre un fascio di ridottissime dimensioni trasverse e con alta coerenza spaziale.

Queste prestazioni sono rese possibili dalla alta brillanza delle è pubblicato a cura del Gruppo Nazionale di Struttura della Materia del C.N.R. in collaborazione con il Dipartimento di Fisica dell’Università degli Studi di Roma “Tor Vergata”.

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macchine di luce di sincrotrone di terza generazione e costituiscono un significativo sviluppo nel campo della microscopia con contrasto di fase, la quale sta rivoluzionando le tecniche di imaging; ricordiamo a proposito che l'imaging con contrasto di fase ha già fornito risultati di notevole rilievo per esempio nelle indagini di materiale biologico nel quale il contrasto di assorbimento è molto limitato.

F.P. Ricci

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tarting from this issue we have decided to publish all articles in English. "Notiziario" is already distributed in significant numbers abroad but we trust that this decision will increase the number of non-Italian readers.

A few months ago, in a cold February night, TOSCA crossed the English channel and reached England. Within a few months the inverse geometry spectrometer, funded entirely by Consiglio Nazionale delle Ricerche within an agreement with the British CLRC, designed by a group under the leadership of F.P. Ricci and built at the Istituto di Elettronica Quantistica in Florence, will start providing data to the vast community of biologists, chemists and physicists who already use the ISIS pulsed source.

Notwithstanding the numerous problems which were found, and solved, during construction (see the article within, and think of Marco Zoppi in a diving suit as he tries to weld Cadmium foils on Berillium blocks), the instrument was completed in record time: from the first meeting of the group in December 1996 to the shipping of the instrument only 14 months have passed. If the design goals will be met, TOSCA will give a significant thrust to molecular spectroscopy studies and will bring the application of neutrons to a high qualitative level which was impossible up to now because of the inadequacy of the instrumentation available.

From the point of view of the Italian neutron community, construction of TOSCA meets an important objective and implies that this community is ready to participate, as leaders, in future European initiatives. The quality and the quantity of contributions offered by our community to European reasearch is measurable not only on the base of published papers, but also on the base of realization of advanced istrumentation which serves a field which is at the overlap of fundamental and applied interests an which is of great strategic interest for European technological development. Along with TOSCA, other highly qualified projects are in the design phase, such as the BRISP time-of-flight spectrometer, the back-diffusion IN13 spectrometer at ILL in Grenoble, the DIANE diffractometer in Saclay, the VESUVIO project for epithermal neutron spectroscopy and the participation in the XENNI European network for the realization of new solid state detectors.

Italian neutron research is therefore going through a very positive phase. The next important step will be to assure that Italy signs the memorandum of understanding for the European Spallation Source, the laboratory which will assure for Europe the leading role which it now has well into the next century. For more details see the report on the ESS we publish in this issue.

1997 has been also a very significant year for synchrotron radiation research because it marked the 50th anniversary of its first (visual!) observation at the General Electric laboratories. It is not necessary here to stress the enormous quantitative and qualitative development of the field since then. We note that the journal Science has placed results obtained with synchrotron radiation, in particular in the field of bio-crystallography, among the 10 top

DIRETTORE RESPONSABILE: COMITATO DI REDAZIONE:

SEGRETERIA DI REDAZIONE:

F.P. Ricci C. Andreani, F. Boscherini, R. Caciuffo, R. Camilloni

GRAFICA E STAMPA:

om grafica, via Fabrizio Luscino 73, Roma

Finito di stampare nel mese di Giugno 1998

D. Catena

HANNO COLLABORATO A QUESTO NUMERO:

breakthroughs of 1997.

To celebrate the 50th anniversary of synchrotron radiation ESRF organized in November the conference "Highlights in X-ray Synchrotron Radiation Research". The major experts in the various areas provided historical revies and up-to-date results; we publish a report from this conference by R. Rizzo.

From the point of view of the instrumentation available to Italian users, at ESRF (to which Italy contributes 15%) the beamline installation phase is approaching conclusion. From the February deadline three more beamlines are open to the users: two are devoted to macromolecular crystallography (ID14 and BM30, the latter being a French CRG) and one to magnetic scattering (BM28, a British CRG). Already available are 28 public beamlines and 4 CRGs; Italian users also have access to 66% of beamtime on GILDA, the Italian CRG. As for ELETTRA, the national facility in Trieste, seven beamlines are now open to users and three more will be available in July. Recently, collaboration agreements for beamline operation have been signed between Sincrotrone Trieste (ST) and research institutes for the following beamlines: VUV photoemission with high energy resolution (between ST and CNR), single crystal diffraction (between ST and CNR), gas phase photoemission (between ST, CNR and INFM) and ALOISA (between ST and INFM). ALOISA (fra ST ed INFM). In the future new research opportunities will be offered by already funded beamlines, some of which are already in the commissioning phase: APE (INFM/ETH), BACH (INFM), LILIT (CNR/INFM), X-MOSS (INFM), EXAFS (ST/Università di Trieste), Materials Science (Czech Accademy of Sciences) and CIPO (CNR).

In this isssue we publish in the "Rassegna Scientifica" section four papers on subjects in which synchrotron radiation and neutron spectroscopy have given and will continue to give, significant results. In the paper by Ibbersen the effectiveness of high-resolution neutron powder diffraction in obtaining the structure of molecular crystals with low fusion temperature is described. Triolo describes the structural characterization of semicrystalline polymers using in a complementary fashion X-ray and neutron small angle scattering. In the paper by Carbone and Vescovo the application of spin-resolved photoemission with synchrotron radiation to the study of magnetic systems such as metallic epilayers and multilayers is described. The present understanding of the field is illustrated, together with perspectives for future reaserch in this important field; we recall that recent results have allowed the understanding of phenomena such as magnetic coupling through non-magnetic layers and that such phenomena already find application in devices. Lagomarsino et. al. describe the development of an X-ray waveguide which allows the production of a beam with very small transverse dimensions with high spatial coherence. This kind of performance is made possible by the high brightness of third generation synchrotron radiation sources and is a significant contribution to phase contrast microscopy, one of the most promising fields of imaging; we recall that phase contrast imaging has already provided important results, for example in the investigation of biological materials in which absorption contrast is very limited.

F.P. Ricci

C. Carbone, E. Vescovo, R.M. Ibberson, S. Lagomarsino, A. Triolo, R. Triolo, M. Zoppi

Per numeri arretrati: Grazia Ianni, GNSM-C.N.R., viale dell'Università 11, 00185 Roma. 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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Articolo ricevuto in redazione nel mese di Aprile 1998

ELECTRONIC STRUCTURE AND MAGNETISM IN THIN FILMS AND MULTILAYERS C. Carbone Institüt für Festkörperforschung des Forschungszentrums Jülich, Germany

E. Vescovo NSLS Brookhaven National Laboratory - Upton, New York, USA

Ultrathin films and multilayers have distinct magnetic properties without counterpart in bulk systems. They represent a new class of magnetic materials which begins to find technological application. The investigation of their electronic structure provides a ground to understand the magnetic behaviour.

This article illustrates the application of spin-polarized photoemission spectroscopy with synchrotron radiation to magnetic films and multilayers. Examples are presented on the electronic and magnetic properties of new 3d metal phases and on the role of magnetic quantum size effects in determining the interlayer coupling.

ved photoemission. Moreover, the possibility of controlling the polarization of the photons - both circular and linear - has enormously enhanced the possibilities of magnetic measurements. In fact, by introducing polarization control in traditional techniques such as absorption, reflection and photoemission, magnetic dichroism effects can be easily observed. Synchrotron radiation spectroscopies can provide a detailed description of magnetic and electronic properties of thin films a few atomic layers thick. For example, the band dispersion, the magnetic moments, the magnetic anisotropy and susceptibility, the Curie temperature and the critical behaviour at the magnetic phase transitions can be determined.

Moreover, extension of spectroscopic methods - which are sensitive to the orientation of magnetic moments - in the field of microscopy allows the observation of the structure of magnetic domains in surfaces and thin films.

In this paper we present a brief review of a few synchrotron radiation photoemission studies of magnetic systems. The examples we have selected show how it is possible to detect the effects of atomic geometry - and in particular of the reduced coordination and dimensionality - on electronic and magnetic structure.

In the next section we illustrate the application of photoemission to simple cases of elementary monocrystalline films whose structural and magnetic properties are different from those of the bulk. In the following section we consider more complex systems composed of multilayers obtained by combining in various ways magnetic and non-magnetic layers. In particular, we will show how band structure analysis explains the origin of the exchange interaction between magnetic layers separated by non-magnetic ones. These long range interactions give rise to indirect magnetic coupling which are observed in many multilayers. Of particular technological relevance are the effects of magnetic coupling which affect macroscopic properties.

The foremost example is magneto-resistance, which is already employed for novel magnetic field sensors. Lastly, in the final section, we will describe the even more complex case of thin films of alloys and compounds. Here the situation is still not well defined, being this area of research quite new. We will take then this opportunity to indicate the expectations and perspectives for new ap-

Introduction The study of thin films and multilayers has recently become one of the principal fields of research in magnetism. The interest in these two-dimensional structures stems from their peculiar magnetic properties, which often have no counterpart in the bulk. Furthermore these new magnetic properties, such as indirect exchange coupling between non-adjacent magnetic layers, spin-dependent transport phenomena, and interface and surface magnetic anisotropies, have both fundamental and technological relevance. For applications it is important to note that the properties of magnetic thin films can in part be controlled by appropriately choosing materials, substrate, growth conditions, thickness and sequence of the various layers. Growth methods of nanostructured 2d systems can now produce in practice new magnetic materials which are starting to find applications in information technology. In the realm of fundamental research in solid state magnetism, multilayers and thin films allow one to study the effects of structure, chemical composition and dimensionality on the magnetic properties.

Recent progress in the study of magnetic multilayers and thin films is mostly due to the development of advanced growth techniques, experimental methods and computational tools. 1 In recent years, the increasing availability of synchrotron radiation has contributed greatly to the development of spectroscopic methods for the study of magnetism in thin films. The high intensity available - especially on insertion-device beamlines - has made possible complex techniques such as spin - resol-

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plication of spectroscopic methods in the study of magnetic systems.

Simple magnetic films Progress in preparation and characterization techniques of epitaxial metallic films has opened the possibility to study the correlation between structural and magnetic properties. One of the most attractive characteristics of thin films is indeed the possibility to modify the atomic structure of the magnetic material. Consider for example the planar geometry of a single atomic plane or the epitaxial stabilization of films under compressive or tensile strain, leading to strong deformation of the unit cell. Sometimes, as a result of the interaction with the substrate, it is even possible to obtain magnetic films with structures different from those stable in the bulk. For example, metastable fcc and hcp Fe or fcc and bcc Co films can be obtained, the bulk structure of which is bcc and hcp, respectively. By epitaxial growth techniques ferromagnetic films with ordered crystallographic structure can be obtained on many substrates. For example the three classical ferromagnetic metals - Fe, Co and Ni - can be stabilized as fcc thin films with similar lattice parameters, if deposited on a Cu(100) substrate.

The influence of atomic geometry on electronic structure is the microscopic cause of the particular magnetic properties of thin films. The band structure of thin films can be studied by the conventional methods of angular-resolved photoemission. A typical problem which is encountered in the analysis of the data is, however, the identification and separation of spectral components.

There are particular difficulties in the case of magnetic materials as the exchange interaction removes spin degeneracy. In ferromagnetic materials one can obtain further information from the polarization analysis of the photoemitted electrons. This decomposes the total spectrum in two components: that due to electrons with magnetic moment parallel and anti-parallel to the average macroscopic magnetization direction. The results of spin-resolved measurements thus reflect the energetic distribution and consequently the different occupancy of states with opposite spin in a magnetic material; these are the microscopic bases of ferromagnetism. Following the methods used normally in angular-resolved photoemission, spin-resolved measurements determine the dispersion relation E=E(k,Ďƒ) which completely describes the electronic structure of a ferromagnet in the limits of a non-relativistic and single-particle description.

In Fig. 1 we compare spectra of thin films of three fcc 3d ferromagnetic metals grown by epitaxy measured with and without spin resolution.2 Measurements of the conventional type exhibit only one asymmetric structure close to the Fermi level due to the 3d states. It is clear how the spin-polarized measurement provides additional information: more spectral structures are resolved and the spin character of each is directly determined ex-

Fig. 1: Photoemission spectra of epitaxially grown fcc magnetic thin films.

Spin-integrated spectra (left) are decomposed in the two spin components (right) corresponding to photoelectrons with spin magnetic moment parallel (darker area) or anti-parallel (lighter area) to the macroscopic magnetization direction. The dependence of the energy distribution with the spin character decreases with the magnetic moment, from Fe to Co and Ni, and disappears in Cu.Figure 1. Structure of ice Ih showing the oxygen atoms (empty circles) and possible positions of the hydrogen atoms (filled circles), which are disordered within the Bernal-Fowler rules.

perimentally. The comparison between the electronic structure of the different metals shows the typical modifications deriving from the gradual filling of the 3d valence band; the exchange splitting between bands of opposite spin gradually diminishes in passing from Fe through Co to Ni, and completely disappears for Cu. This corresponds to the diminishing magnetic moment. For each spin component the band dispersion as a function of the wave vector can be examined by applying methods normally used in photoemission experiments.

For example, by appropriately varying the experimental geometry and energy of the photon one can select transitions from initial states with different wave-vector along high symmetry directions in the Brillouin zone. This type of analysis is reported in Fig. 2 for a relatively thick film composed of 20 atomic layers of Co. The experimental results2 are in good agreement with calculations for

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thickness ranges with different electronic and magnetic properties can be distinguished (Fig. 3). Up to 4 monolayers films are ferromagnetically ordered with a high and practically uniform magnetic moment of more than 2 µB per atom. When the thickness reaches 5 mono-

Fig. 2: Band structure of fcc Co. The symbols indicate the binding energy of majority and minority spin states (filled and empty symbols, respectively), determined by photoemission experiments. As a comparison the results of spin-polarized electronic structure calculations are also shown.

fcc Co, both with respect to dispersion and to the exchange splitting, while there is a slight discrepancy for the binding energy. The magnetic moment calculated for Co films in this structure is about 1.5 µB, in agreement with dichroism measurements; this value is similar to the value for bulk hcp Co. Within the first few atomic layers the reduction of coordination and finite size effects strongly affect the magnetic properties, which therefore often have a complex behaviour with film thickness. In monolayer films the magnetic moment tends to increase due to the reduced coordination. As the film thickness increases the magnetic moment generally decreases. FCC Fe films are a particular case, in which an opposite trend is in fact observed: this points out the subtle dependence of the exchange interaction on the atomic structure.3 For Fe with fcc structure, theory predicts that the ferromagnetic configuration is favoured, with respect to the anti-ferromagnetic one, by an expansion of lattice parameters and by low coordination. Experimental observations on thin films of fcc Fe confirm these expectations.

For the growth of fcc Fe on Cu(100) and Co(100) three

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Fig. 3: The magnetic moment of fcc Fe films has a complex dependence on thickness, which is reflected in the evolution of the polarization of photoemitted electrons and in dichroic effects at the absorption edges [2].

Three thickness regions with different magnetic properties, electronic and atomic structure can be distinguished.

layers, together with a structural transition characterized by a small decrease of the unit cell, a drastic change in the electronic and magnetic properties is observed. The spin polarization of the valence band, magneto-optical measurements and absorption dichroism show that between 5 and 10 monolayers the average magnetic moment is drastically reduced (0.6 - 1.0 per atom), compared both to thicker and to thinner films. There is strong experimental evidence that only the surface region of the film remains ferromagnetically ordered, while the inner layers are either paramagnetic or anti-ferromagnetic and thus do not contribute to the magnetization. Lastly, above 10 monolayers the structure gradually changes to bcc - the stable phase for bulk Fe - with a consequent increase of the average magnetic moment. Usually this anoma-

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lous behaviour of Fe is associated with the magnetostructural instabilities theoretically predicted for metals in the center of the 3d series. It is well known, in fact, that for bulk Fe various magnetic configurations - ferromagnetic, paramagnetic and antiferromagnetic - are energetically close4. Small changes in the atomic geometry of Fe fcc films clearly affect directly the exchange interaction, which can in fact change sign. Moreover, the electronic structure on the surface is strongly modified by the reduced atomic coordination and thus determines a different magnetic ordering of the surface with respect to the bulk.

Another example which illustrates the strong dependence of the magnetic properties on local coordination is the surface of Gd(0001) films deposited on W(110). Gd has a simple ferromagnetic ordering with a Curie temperature which is very near to room temperature (Tc = 293 K). As a consequence of the strong localization of magnetic moments and of the particular symmetry (L = 0) of the 4f shell, the magnetic behaviour of Gd is well described by the Heisenberg Hamiltonian. Recently this system has received considerable attention. It was in fact discovered that the Curie temperature on the surface of epitaxial films was higher than in the bulk. The microscopic origin of this phenomenon is still not clear, but there is experimental evidence that the enhancement of the Curie temperature at the surface is a consequence of the existence of very strongly localized electronic states at the surface. As illustrated in Fig. 4, the photoemission measurements show a considerable difference in the behaviour as a function of temperature of electronic states localized at the surface with respect to those which extend into the bulk.4 Approaching the Curie temperature the surface state - close to the Fermi level - stays fixed at the same binding energy. On the other hand bulk states - approximately 1 eV below the Fermi level - progressively change their binding energy with increasing temperature, finally converging to a single peak at Tc. These results indicate that in the case of Gd the degree of wave function localization determines a different qualitative behaviour at the surface and in the bulk: bulk bands loose their magnetic moment above the Curie temperature while surface bands - which are more localized - maintain it due to the greater superposition with the localized (magnetic) 4f states. These electronic properties probably control the different magnetic behaviour of the bulk and of the surface (different Curie temperatures). These microscopic differences suggest in fact that the magnetic coupling J has considerably different values in the bulk and on the surface, from which a different phase transition temperature in the two situations results.6 Multilayers: Indirect exchange interaction between magnetic layers Metallic multilayers are artificial materials which may

Fig. 4. Normal emission photoemission spectra as a function of temperature for monocrystalline Gd(0001) epitaxially grown on W(110). Close to the Curie temperature (TC = 293 K) bulk states of the film (approximately 1 eV below EF) converge to one peak while the surface state remains unaltered (from D. Li, J. Pearson, S.D. Bader, D.N. McIlroy, C. Waldfried and P.A. Dowben, Phys. Rev. B51, 13895 (1995)).

exhibit new properties. Among these, electronic transport properties which depend on the magnetic state of the multilayer are of particular applicative interest.

In a ferromagnetically ordered material charge carriers are spin polarized, with a preferential orientation with respect to the macroscopic magnetization.

In a multilayer electric currents are affected by the orientation of the magnetization of the various layers.

It is not surprising, therefore, that research on multilayers gained considerable momentum from the discovery that the relative orientation of the magnetization of magnetic layers can be determined by an indirect exchange interaction which is mediated by thin layers of non-magnetic materials. Transport properties of multilayers are of interest not only for sensors, but also for their future use as magnetic elements in solid state circuits.7 Indirect magnetic coupling was observed for the first time in Fe/Cr multilayers in 1986 at the Institüt für Festkorperforshung der Forshungszentrum Jülich. Light scattering measurements showed that two thin (10 - 50 Å) layers of Fe are magnetically coupled when separated by a non magnetic layer of Cr.8 The magnetic coupling -

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Fig. 5. Relative variation of the electrical resistance through a multilayer system with exchange interaction. The thickness of the non-magnetic film determines an anti-parallel alignment of the film magnetization in the absence of external magnetic field. When the magnetic field increases the electrical resistance decreases and it reaches a minimum value for fields sufficiently strong to align the magnetization of the magnetic layers, overcoming the indirect magnetic coupling.

which determines the parallel or anti-parallel magnetization direction of the two Fe layers in the absence of external magnetic fields depends on the thickness of the Cr film. It is notable that this coupling survives up to considerable thicknesses of the Cr layer (in the order of tens of monolayers). In other words there is an indirect and long range magnetic interaction, mediated by a non-magnetic film. Subsequent experiments in various laboratories with different experimental methods have determined not only that indirect magnetic interaction is a general property of multilayer systems - observable for numerous combinations of materials - but also that their is an oscillatory dependence of this interaction with the thickness of the non-magnetic layer. The interaction changes sign in a periodic manner as the thickness of the non-magnetic layer varies. The oscillation period is characteristic of the materials and their crystallographic orientation.9 Lastly, we recall that in structures with anti-parallel alignment of the magnetization an interesting dependence of the electrical resistivity on the external magnetic field is observed. The resistance decreases with increasing magnetic field and asymptotically reaches the minimum value when the magnetic field is sufficiently strong to overcome the indirect interaction and thus align the magnetization of the layers (Fig. 5). This variation of resistivity is quite strong in some cases (e.g. for Fe/Cr we have ∆R/R = 220%, giant magneto resistance, GMR), especial-

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ly if it is compared to magnetoresistance in alloys used nowadays like permalloy (Ni0.8Fe0.2, ∆R/R = 2%). The first sensors based on this effect are now commercially available and are constructed by films a few nanometers thick.

The origin of the indirect exchange interaction is strictly connected to the electronic structure near the Fermi level. This indirect interaction has been initially analyzed with the aid of models based on the RKKY method, which can explain the oscillatory dependence of the interaction with film thickness. The period of the interaction is associated with the topology of the Fermi surface of the non-magnetic material. For example, the two oscillation periods through Cu(100) films - approximately 2.6 and 5.9 monolayers - correspond to the inverse of the reciprocal space vectors connecting stationary points on the Fermi surface of this material. The limitations of this analysis lies in the intrinsically asymptotic character of the RKKY interaction which does not allow to study correctly the dependence of the magnetic coupling and its intensity as a function of the film thickness.

Further detailed theoretical10 and experimental11 analyses have pointed out the strong connection between finite size effects and indirect coupling in multilayers. The fundamental difference between the electronic structure of a thin film and of a three dimensional crystal is due to the dimensionality and to the boundary conditions at the interfaces. In the ideal case of an isolated and crystalline film composed by n layers the finite size of the system imposes a quantization of the levels so that n discrete levels En = En(kij) are created for every two-dimensional wavevector kij. The energy level spectrum changes with the film thickness, as indicated in Fig. 6, in analogy with the problem of a particle in a one-dimensional potential well.

The number of states increases with the film thickness while their energy separation decreases. The electronic

Fig. 6. Schematic representation of the electronic states in thin films. The electronic wavefunctions are represented as solutions for a one-dimensional potential well with infinite walls at the interface between substrate and vacuum. Localization of the wave function corresponds to quantization of the energy states.

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structure converges to that of the bulk material with increasing film thickness. For a film on a substrate, or part of a multilayer, the boundary conditions are determined by the interaction with the other material. The energy levels hybridize with a continuum of states, loose their discrete character and give rise to resonance states with a finite band-width. In the limiting case in which the discontinuity of the potential at the interface is negligible,

Fig. 7. Photoemission spectra from Cu thin films deposited on Co(100).

The spectral structures outlined have a binding energy which depends on the film thickness. They derive from the quantization of the energy levels due to the finite dimensions of the film and are observable for thicknesses up to 50 atomic layers (90 Å).

the wave function is completely delocalized in the semiinfinite system.

Due to its considerable surface sensitivity, photoemission spectroscopy is traditionally used in thin films to investigate the effects both of the interaction at the interface and of the finite size of the film. In practice, however, the complexities of the growth-modes often makes the observation of quantization effects impossible. Photoemission experiments are often able to follow finite size effects on the electronic structure only in the first stages of growth (up to about 5 monolayers) and only rarely

are able to resolve quantized levels in films of greater thickness.

As an example of a favorable case, we report in Fig. 7 the results of photoemission experiments of Cu grown on Co.12 Photoemission spectra show a series of structures, derived from electronic states of ∆1 symmetry, whose binding energy varies with the film thickness. Quantization of levels is clearly observable up to 50 monolayers.

The dependence of the binding energy on the thickness has a simple behaviour: these states cross the Fermi level at regular intervals of 6 monolayers (Fig. 8).

This regularity corresponds to the period of oscillation of the indirect exchange interaction through layers of Cu(100).

Moreover, it is observed that the boundary conditions imposed by the ferromagnetic substrate induce a magnetic character to these two-dimensional states. Spin polarization analysis shows that these spectral features originating from the quantization of ∆1 conduction states of the Cu film, have predominantly minority spin character (Fig. 9a). States with majority spin develop, on the other hand, a band structure similar to that of bulk Cu. In

Fig. 8. The binding energy of quantized levels has a simple dependence on the thickness of the Cu film. The energy dispersion of 13 series demonstrate the appearance of states near the Fermi level with a regular interval of 6 monolayers, corresponding to the period of indirect coupling through Cu(100).

other words, both quantized and bulk states coexist in the same thin film, with opposite spin character. In qualitative terms this can be easily understood because Co acts as a spin-dependent potential barrier (Figs. 2 and 9b). Majority spin electrons, which have very similar energy in the two films, are delocalized in the two metals. Minority spin electrons, for which the potential is very different in the two metals, are strongly confined in

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the Fermi level due to the quantized levels determines period, phase and intensity of the indirect coupling.

At present, magnetic coupling in noble metal multilayers is well understood. Systematic spectroscopic studies show complete series of quantized and polarized states for many systems. These investigations have contributed in developing an accurate and quantitative description of the indirect interaction which finds support in theoretical analysis. Open problems still exist in the detailed description of phenomena in the more complex cases in which the interaction is mediated by transition metals with partially filled 3d shell. In these cases, a novel contribution from photoemission studies in experimentally identifying the branches of the Fermi surface and the electronic states which determine the oscillation period can be expected.13

Fig. 9. Polarization analysis decomposes the conventional spectrum (ü + ÿ) of Cu thin films on Co(100) in the two spin components, majority (ÿ) and minority (ü). We show that (a) the quantized levels which give rise to the structures outlined in the spin-integrated spectra have predominantly minority spin character. The magnetic substrate acts as a potential barrier which depends on the orientation of the electronic spin; a different degree of localization of the two spin components follows (b).

The future: expectations and perspectives.

The examples described demonstrate how the use of spectroscopic techniques permits the identification of the microscopic mechanisms which determine the magnetic behaviour of thin films. The range of magnetic phenomena is, however, enormous and it is certain that the application of spectroscopy in this field is in its infancy. For example, apart from multilayers, other materials show strong links between transport and magnetic properties and are potentially of great technological interest. They are, however, complex systems such as, for example, colossal magneto-resistance (CMR) oxides or ternary Heusler alloys. Experimental information on their electronic structure is extremely scarce at present. This is due to the difficulty in obtaining thin films of alloys and compounds, and even more to the difficulty in preparing

the Cu layers. The electronic structure of a paramagnetic thin film in contact with a ferromagnetic substrate - in this case Cu in contact with Co - has acquired a strong dependence on the orientation of the spin.

Fig. 10 illustrates how these magnetic effects on the electronic structure give rise to indirect coupling. The potentials for electrons with opposite spin character are schematically illustrated for a system of two magnetic layers separated by a non-magnetic film. Electronic states depend on the relative orientation of the magnetization in two adjacent layers: quantum wells with magnetic character persist only for parallel magnetization. The total energy of the system varies with the thickness of the layers. When one of these levels crosses the Fermi level the variation of the total energy of the system - which is gained or lost in filling a new state - periodically modifies the relative energy of the two configurations, alternatively favoring the parallel or anti-parallel magnetization configuration. In this way the oscillatory behaviour of the indirect interaction can be explained. In other words, the periodic oscillation of the density of states at

Fig. 10. Schematics of the one-dimensional potential wells (thick black lines) which describe the indirect coupling between two magnetic layers (Co) mediated by a non-magnetic layer (Cu). The top left (right) panel represents the potential seen by a spin-up (-down) electron in the case of parallel magnetization alignment in the two ferromagnetic layers. The bottom left (right) panel represents the potential for a spin-up (-down) electron in the case of anti-parallel alignment. Only in the parallel alignment case and only for spin-down electrons are localized levels formed and the energetic levels are consequently quantized.

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compositionally and structurally well defined surfaces of these materials.

A very significant contribution to the preparation of films of magnetic compounds has come recently as a side effect of the discovery of high temperature superconductors. The Pulsed Laser Ablation technique has been revived and strongly developed while trying to produce monocrystalline thin films of these complex ceramic materials. By means of this technique it is now possible to obtain thin films (from a few tens to a few thousand Ă…) of alloys and of numerous stoichiometric compounds, many of which even in crystalline form.14 Outstanding examples of magnetic materials obtainable with this technique are oxides and in particular CMR oxides. Typical CMR materials are complex oxides of Mn derived from anti-ferromagnetic LaMnO3 by means of partial substitution of La with Ca or Sr. Their name derives from the fact that magnetoresistance effects are extremely big (∆R/R > 100 000). Physical properties of these materials are absolutely exceptional and very complex. For example, they display combined electrical and magnetic phase transitions such as insulator-paramagnet / metal-ferromagnet. Clearly there is a very close relationship between magnetic and transport properties in these materials. It is then easy to understand that these compounds are among the most promising for the realization of magnetoelectronic elements. We must recall, however, that colossal magnetoresistance effects are only observed under colossal magnetic fields (on the order of a few Teslas). On the contrary, GMR effects in multilayers require fields on the order of only a few Oesterds.15 An example of magnetic alloys currently studied and whose characteristics have similarities with CMR oxides are Heusler alloys, also based on Mn. The peculiar characteristics of these alloys is exclusively electronic. Various theoretical calculations predict a half-metallic electronic structure for some of these alloys (for example NiMnSb and PdMnSb): bands of one spin character have a gap (semiconductor) while bands of the opposite spin character cross the Fermi level (metal).16 This type of material is not only interesting from a fundamental point of view as it allows to study the conditions in which this peculiar electronic structure is obtained (few theoretical studies suggest that the half-metallic electronic structure is due to the particular symmetry of these ternary alloys) but also because it should allow the development of new magnetic devices. In the ideal case of a pure half metal there should be perfect conditions to construct magnetoelectronic circuit elements. Electronic currents being strongly polarized (charge carriers all with the same spin) should be coupled to the macroscopic magnetization of the material. With appropriate combinations of these materials one could therefore foresee the possibility of controlling electrical currents with

magnetic fields. It is surprising that even though the first theoretical predictions of half-metallic alloys in Heusler ternary systems are more than 10 years old there is no experimental proof of their existence. A big step forward has been recently made, however. Spin resolved photoemission experiments have clearly demonstrated the existence of half-metallic solids. In fact a 100% polarization at the Fermi level has been observed in the photoemission spectrum of a thin film of the CMR material La0.7Sr0.3MnO3.17 As in the case of simple thin films, effects due to reduced coordination and dimensionality can also be found in thin films of compounds and alloys. Moreover, the fact that these materials contain different atomic species opens new and interesting perspectives as demonstrated, for example, by the recent discovery of so-called surface magnetic alloys. In some cases it is possible to directly link microscopic structure and chemical composition to the formation of the magnetic moment. Recent studies show that some well ordered structures of surface alloy exist because they are magnetically stabilized.18 It is clear that there are important aspects of thin film magnetism which are practically unexplored from the microscopic point of view. Our comprehension of the microscopic aspects of magnetism - for example the close relationship between microscopic structure and magnetic properties - has greatly improved in the recent past. New and important advances can be expected, however, both because of new emerging techniques - in particular microscopy techniques which use dichroism effects to provide magnetic contrast - and because many of the methods already developed are constantly evolving and improving. For example, if spectroscopic studies haven't yet provided contributions corresponding to the full level of their potential in the field of magnetic anisotropies or of critical phenomena, this is essentially due to the fact that the variations of the magnetization state of a system are accompanied by very modest variations of the energies of electronic states.

The continuous improvement of the resolution of the instruments (both of the light sources and of the electron analyzers) is such that we are now very close to the required performance and in the near future it is extremely likely that high resolution spectroscopy studies will allow significant progress in the understanding even of these microscopic aspects of magnetic phenomena. Advances in this field take place continuously and the possibility of tackling new and more complex problems appears to be at hand.

References 1. "Ultrathin Magnetic Structures" eds B. Heinrich, J.A.C. Bland, Springer-Verlag (1995).

2. R. Klaesges, D. Schmitz, C. Carbone, W. Eberhardt, to be published.

3. D. Li, M. Freitag, J. Pearson, ZQ Qui, and S.D. Bader, Phys. Rev. Lett.

72, 3112 (1994); S. Muller, P. Bayer, C. Reischl, K. Heinz, B. Feldmann,

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H. Zillgen, and M. Wuttig, ibid. 74, 765 (1995).

4. V. L. Moruzzi, P.M. Marcus, K. Schwarz, Phys. Rev. B34, 1784 (1986).

5. D. Li, J. Pearson, S.D. Bader, D.N. McIlroy, C. Waldfried, and P.A. Dowben, Phys. Rev. B51, 13895 (1995).

6. E. Vescovo, C. Carbone, and O. Rader, Phys. Rev. B48, 7731 (1993).

7. G. A. Prinz, in "Ultrathin Magnetic Structures" eds B. Heinrich, J.A.C. Bland, vol.2, Springer-Verlag (1995).

8. P. Grunberg, R. Schreiber, Y. Pang, M.B. Brodsky, and H. Sowers, Phys. Rev. Lett. 57, 2442 (1986).

9. P. Bruno and C. Chappert, Phys. Rev. Lett. 67, 1602, 2592 (1991).

11. J. Ortega, and F.J. Himpsel, Phys. Rev. Lett. 69, 844 (1992); K. Garrison, Y. Chang, and P.D. Johnson, ibid. 71, 2801 (1993); C. Carbone, E. Vescovo, O. Rader, W. Gudat, W. Eberhardt, ibid. 71, 2805 (1993).

12. C. Carbone, E. Vescovo, R. Klaesges, W. Eberhardt, Sol.State Comm.

100, 749 (1996).

13. D. Li, J. Pearson, S. Bader, E. Vescovo, D.-J. Huang, P.D. Johnson, B. Heinrich, Phys. Rev. Lett. 78, 1154 (1997).

14. D.H. Lowndes et al., Science 273, 898 (1996).

15. see for example, G.A. Prinz, Physics Today, 48, 58 (1995).

16. H. van Leuken and R. A. de Groot, Phys. Rev. Lett. 74, 1171 (1995).

10. D.M. Edwards, J. Mathon, R.B. Muniz, and M.S. Phan, Phys. Rev.

17. J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, and T. Venka-

Lett. {\bf67}, 493 (1991); M.D. Stiles, Phys. Rev. B48, 7238 (1993);

tesan, "Observation of a Half-Metallic Ferromagnet" submitted to Na-

L.Nordstrom, P. Lang, R. Zeller, and P.H. Dederichs, Europhys. Lett.

29, 395 (1995).

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ture (1997).

18. M.Wuttig, Y. Gauthier, and S. Bluegel, Phys. Rev. Lett. 70, 3619 (1993).

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Articolo ricevuto in redazione nel mese di Marzo 1998

STRUCTURE DETERMINATION OF LOW-MELTING POINT MOLECULAR CRYSTALS BY NEUTRON POWDER DIFFRACTION R M Ibberson ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, Neutron powder diffraction is a particularly effective technique for the structure solution and refinement of molecular crystals, especially those with low melting points. In this article the efficacy of the techniques is described by recent examples using high -resolution time of flight neutron powder diffraction and future prospects are discussed.

Introduction The feasibility of determining crystal structures from powder diffraction has improved substantially over recent years. Moreover it is an area of study that is gaining increasing prominence, since many new and important classes of crystalline materials are initially, or sometimes can only be, prepared as powders There is however an old adage in powder diffraction that “neutron powder diffraction is the technique of choice for structure refinement while X-rays should be used for structure determination”. Whist this is an over-simplification, it is generally true that the essentially constant nuclear scattering lengths for neutron powder diffraction that are beneficial for refinement are also detrimental for structure solution since the effective number of visible atoms is higher for neutrons than for X-rays. Accordingly the vast majority of crystal structure determinations are carried out using X-ray diffraction data, however, there are a number of circumstances in which it can be advantageous to use neutron diffraction data.

One such case is in the field of molecular crystallography where the success and pre-eminence of X-ray powder diffraction is diminished and much has been achieved using neutron powder diffraction. Molecular solids are an interesting and well studied class of materials in which weak van der Waals interactions represent the primary inter-molecular bond nature. These interactions are weak, much weaker than the intra-molecular bonding, and so the molecular properties often significantly influence the properties of the crystal and give rise to novel structural behaviour. In the last five years those crystal structures determined solely from neutron powder data are all molecular, namely: acetaldehyde,1 dimethyl sulphide,2 methyl fluoride,3 dimethyl acetylene,4 trichlorofluoromethane,5 rhenium heptafluoride,6 trifluoroiodomethane,7 malonic acid,8 tribromofluoromethane,9 trifluorobromomethane10 and cyanamide.11 This success is due to a number of factors. In general, molecular crystals have low symmetry and comprise atoms, in the case of organic structures, with weak scat-

tering factors for X-rays. The high density of Bragg peaks and form-factor fall-off, which is exacerbated by a large Debye-Waller effect in these materials, yields reduced intensity over the high-angle region of an X-ray powder pattern and it is these data which are important in a Direct Methods structure solution. Naturally, the use of high resolution (synchrotron) instrumentation and data collection at low temperature can compensate for these factors to a limited degree. Nevertheless, the low melting point of the majority of the examples cited above gives additional technical problems for a study using X-rays both in producing and handling small single crystals and similarly with small sample volumes exhibiting a good powder average.

High resolution neutron powder diffractometers operating at pulsed sources such HRPD at ISIS permit the routine collection of data with a resolution, ∆d/d, better than 10-3 and which is effectively constant across the whole diffraction pattern. As a direct result of this inherently high resolution, high quality powder diffraction patterns containing a large number of reflections and consequently high information content may be recorded.

These high-resolution diffraction data may be collected for Bragg reflections at d-spacings of well below 1Å as there is no form-factor fall off with point scattering of neutrons from the nucleus. Also, low temperature data collection is technically much simpler for neutron powder diffraction than when using X-rays. This combination of high sinθ/λ and high resolution is the principal reason that has enabled HRPD to provide an alternative to single crystal neutron diffraction for obtaining both accurate and precise structural parameters in moderately complex crystal structures. The technique is particularly appropriate in cases of crystals that undergo reconstructive first-order phase transitions, and for lowmelting point materials for which growing single crystals is technically difficult. The larger sample volume, of the order 5 cm3, and bulk sampling in a neutron powder diffraction experiment also significantly reduces systematic errors, for example preferred orientation, that can be problematic in the comparable X-ray powder study.

Sample Preparation The preparation of a suitable powder sample from lowmelting point materials requires care if particle size and preferred orientation effects are to be avoided.

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be introduced. The mortar is small enough to be loaded into a standard glove box through which cold dry nitrogen gas is passed during sample preparation. This gas, along with natural boil-off from liquid nitrogen in the mortar reservoir, provides an inert, cold and dry atmosphere inside the glove box. Apart from the requirement for all tools to possess insulated handles and to be periodically cooled, again using liquid nitrogen, the preparation method is otherwise similar to that of an air-sensitive sample that is solid at room temperature.

The method naturally allows the experimentalist to observe the powder which, coupled with the ability to grind by hand rather than mechanically, enables crystallite habit and size to be monitored and controlled.

Figure 1a - Schematic drawing of the experiment setup for vapour deposition. (Temperatures shown correspond to those during sample preparation of CD3F).

In the majority of cases, simply freezing the sample, from the liquid phase, in-situ, in a container appropriate for use during the diffraction measurement, promotes the growth of large crystallites resulting in gross preferred orientation effects. The inclusion of an appropriate material, for example glass wool, to facilitate the aggregation of a fine grained sample is unreliable and typically of only limited success. Previous workers12 have suggested the 'snow method' to be most promising. This method requires a fine spray of the liquid to be directed on to a tray partially filled with liquid nitrogen, the snow thus produced is collected following the evaporation of the liquid nitrogen. The whole experiment set-up must necessarily be fully insulated and kept at temperature well below the melting point of the sample. Practical constraints therefore limit this method to working temperatures no lower than that of "dry ice" (~195 K).

A technique involving freezing of the sample and then hand-grinding using a chilled mortar and pestle13 (Figure 1) has been used in the present studies and by other workers.9 The apparatus and method, utilising liquid nitrogen as a coolant, enables highly crystalline powder samples of low-melting point materials to be routinely prepared. The stainless-steel mortar is cooled using an integral liquid nitrogen reservoir into which the sample holder, used during the diffraction experiment, may also

14

For materials with very low melting points - thus having low boiling points also - powder samples are best produced using a vapour deposition technique. Samples are prepared in a standard liquid-helium “orange” cryostat using a gas inlet line based on a design by Langel, Kollhoff and Knözinger.14 The gas is deposited at a controlled low rate (20-60cm3 min-1 at STP) in a standard cylindrical vanadium sample tube (diameter 15mm, height 40mm). A low rate of deposition is essential in order to keep the heat dissipation during condensation to a minimum and thus prevent the powder from annealing or melting. The vanadium tube has a copper disc at its base (see Figure 1a) and a copper finger is mounted on the disc inside the tube to serve as a cold point within the lower part of the can to initiate condensation of the gas. The inlet line comprises three coaxial stainless steel tubes. The can is connected, via an indium seal, to the outer tube of the inlet line and cooled by He exchange gas in the cryostat. The central capillary line, through which the gas is passed, is thermally isolated from cold points of the cryostat by means of a vacuum to the middle tube and from the sample by the vacuum (low gas pressure) above the sample during condensation conditions. The capillary may be heated using thermocoaxial wires and the temperature gradient along the inlet line monitored using type-K NiCr-Ni thermocouples.

The temperature is also monitored using RhFe sensors mounted at the base of the can and at the heat exchanger inside the cryostat.

The temperature at which vapour deposition occurs greatly affects the quality of the powder sample produced. Preferred orientation is to be avoided if at all possible but line broadening due to strain and particle size effects in the powder sample must also be considered.

Condensation close to the melting point typically produces a less strained sample but can produce large crystallites and give rise to preferred orientation effects especially if recrystallisation occurs. The converse is true for condensation temperatures well below the melting point. The difference between condensing and annealing a sample at a given temperature should also be noted.

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Figure 1b - The assembled mortar and pestle apparatus.

Schematic cross-section through the chilled mortar apparatus.

The sample becomes more polycrystalline when condensing rather than annealing at a given temperature since, with the former, any reconstruction requires mobility only at a (2-D) surface which occurs easier than in the (3D) bulk sample necessary in the later case.

The relative ease of producing good-quality bulk powder samples using the two techniques outlined above has enabled detailed structure solution and refinement of moderately complex molecular crystal structures to be performed routinely using high-resolution neutron data.

On the basis of these low-temperature crystal structures transferable pair interaction potentials, used to describe and predict quantum rotor states of molecules, may be derived and their validity tested. Accurate crystal structures are also of fundamental importance for the correct interpretation of spectroscopic data.

In the following sections three examples are described: i) the structure determination of methyl fluoride (m.p. 131 K, b.p. 195 K); ii) the structure determination and anomalous line broadening in dimethylacetylene (m.p. 241 K) and iii) the structure determination and phase behaviour in acetaldehyde (m.p. 152 K). These compounds are the simplest members of the respective halomethane, dimethylalkyne and aldehyde families and so are ideal candidates from which transferable pair interaction potentials can be established.

In all cases however the expected model nature of the materials belies the structural complexity exhibited and demonstrates the need for careful structural study in order to develop a fundamental understanding of the behaviour of this class of material.

I Methyl fluoride The methyl halides, CH3X, represent some of the simplest polar molecular solids and therefore, not surprisingly, have been intensively studied in recent years by a va-

riety of spectroscopic techniques (for a review see 15). In the case of X=I, Br, Cl, the spectroscopic data can be reliably interpreted on the basis of known low-temperature crystal structures. The known methyl halide crystal structures are closely related. The structures of CH3I16 and the low-temperature β-phase of CH 3 Br 16,17 are orthorhombic, space group Pnma. The chloride structure18 and bromide high temperature a-phase structure17 corresponds to the orthorhombic space group Cmc21.

Both structures may be described in terms of a quasitwo-dimensional motif defined by C-H•••X bonds. In the α-phase structure these layers of molecules are aligned head-to-head whereas in the β-phase structure there is an inversion of every second molecule resulting in a head-to-tail sequence within each layer.

The crystal structure of CH3F was previously unknown.

However, a number of inferences have been made from spectroscopic data. Systematic behaviour observed in the crystal structure studies of the methyl halides naturally led to the interpretation of spectroscopic data on CH3F following these assumptions. However, inelastic incoherent neutron spectroscopy19 shows that the systematic increase of rotational potentials breaks down with CH3F. This indicates that the crystal structure is fundamentally different.

A powder sample of perdeuterated methyl fluoride, CD 3 F, supplied by Cambridge Isotope Laboratories (CIL), Massachusetts, USA, was produced using the vapour deposition technique described above. The high vapour pressure of methyl fluoride making a thermal short-circuit to the warm nozzle hindered the controlled condensation and was an important factor in selecting an appropriate deposition temperature. The final sample of some 3cm3 was condensed at 80K (~0.6Tmp; negligible vapour pressure of CD3F) over a period of some

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two hours and neutron powder diffraction data recorded at 5 K using HRPD at ISIS for a period of some 12 h.

Ab initio structure determination using Direct Methods was routine3 and the structure (shown in Figure 2) was found to be fundamentally different to the other halomethanes. It is monoclinic, space group P21/n, unit cell volume 187.16 Å3 with one molecule on a general position in the asymmetric unit. Methyl fluoride is a polar molecule with a large permanent dipole moment, µ=1.85 D.15 The effects of dipole-dipole coupling are therefore expected to be reflected by structural features and

Figure 3 - Schematic illustration, drawn using ORTEP35, of the monoclinic structure of DMA-d 6 at 5K. Schematic view of the structure sectioned parallel to the (4 0 1) plane.

The solid lines represent symmetry equivalent (4 0 1) planes and are seen to be perpendicular to the molecular axis. Dashed lines represent the unit cell axes.

the weakest rotational potential of all methyl halides19 and the departure from the anticipated systematic structural behaviour.

Figure 2 - Schematic illustration, drawn using ORTEP35, of the structure of CD3F at 5K. Thermal ellipsoids are drawn at the 50% probability level.

this is found to be the case. The intramolecular C-F bond length clearly shows evidence of bond polarisation. The equilibrium value determined by gas-phase electron diffraction20 is 1.382Å as compared with 1.399(4)Å in the present study. However, the average C-H bond length in the gas-phase study of 1.095Å and average D-C-D bond angle of 110.2º compare well with values determined in the present study of 1.070(4)Å and 110.1(4)º respectively.

Ab initio molecular orbital calculations on clusters of methyl fluoride have been carried out using the Gaussian-70 program with the minimal STO-3G and the 431G basis sets[21]. Whilst there is a broad agreement between dipole orientations in the observed structure and the cluster calculations, the optimum C-D•••F configuration in any of the modelled clusters is calculated to be linear and at a much shorter distance of 2.1Å. This configuration determined by ab initio calculations is therefore more consistent with the crystal structure of CH3Cl and also indicative of a stronger C-H•••X interaction in methyl fluoride than that for methyl chloride.21 The weak hydrogen bonds observed in the CD3F structure may well account for methyl fluoride showing

16

II Dimethylacetylene (m.p. 241 K) Dimethylacetylene (DMA), CH3-C≡C-CH3, is a simple linear molecule composed of only two different elements.

It is, therefore, a model compound for comparing ab initio calculation of methyl group rotation potentials from structural data with potentials derived from tunnelling and librational transitions. DMA melts at 241 K and specific heat measurements reveal an anomaly at 154 K. Severe technical problems are therefore encountered with single crystal techniques and the previous attempt at low-temperature structure determination22 is based on data from only two oscillation photographs after which the crystal shattered, presumably as a result of the reconstructive nature of the phase transition.

Data were recorded on HRPD using a 2 cm3 sample of fully deuterated DMA that was prepared by the coldgrinding techniques described earlier. Above 154 K DMA-d 6 is found to adopt a highly disordered rhombohedral structure which undergoes a structural phase transition to a C-centred monoclinic cell below that temperature.4 Neither structure is consistent with the earlier X-ray studies.22 The low-temperature monoclinic structure, space group C2/m, with a unit cell volume of 184.97 Å3, was solved

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routinely using Direct Methods[4] and is shown in Figure 3. The structure solution is simplified since there is only one half-molecule in the asymmetric unit; the full molecular conformation is achieved through an inversion operation. However the diffraction data show hkldependent line broadening (see Figure 4a). Such high-resolution data clearly are more sensitive to line broadening effects and, although complicating structure refinement using standard Rietveld techniques, an effective analysis will provide additional structural information.

In this study, a modification of the Rietveld technique was utilised to analyse peak broadening in a model-independent manner.23,24 Instead of accounting for the peak width in an isotropic manner by using a smooth functional variation that is parameterised in time-of-flight, the Gaussian and Lorentzian components of each peak were separately refined. This results in a multi-parameter Rietveld problem but the approach lead to a substantial improvement in the profile fitting of the data (Figure 4a) and has the distinct advantage that no assumptions are made about a particular line-broadening model.

The accuracy and precision of the structural parameters obtained illustrate well the efficacy and success of the multi-parameter profile fitting procedure. Carbon-carbon triple and single bonds were determined to be 1.201(4) Å and 1.468(4) Å respectively, carbon-deuterium bond lengths in the methyl group are 1.091(6) Å and 1.075(3) Å. As shown in Figure 3, the molecular axis and one deuterium atom of the methyl group lie in the crystallographic a-c plane. There is clearly a layering of the molecules within this plane, the precise orientation of the molecules defines the direction along which the structure may shear and provides the key to the origin of the anisotropic line broadening. The sharpest peak in Fi-

Figure 4a - Observed (points) and calculated (line) and difference/e.s.d profiles for DMA-d 6 at 5K. The vertical tick marks represent the calculated peak positions. Note the sharp peak at 1.51Å is the (4 0 1) reflection.

gure 4a, indeed of the whole diffraction profile, is the (4 0 1) reflection at a d -spacing of 1.51 Å. Line broadening is typically attributed to strain (and particle size) effects in the crystallites and the anomalously narrow peak width thus may be rationalised in terms of the relative insensitivity to the effects of shearing and stacking faults, giving rise to strain effects, along this plane. Figure 3 illustrates the low-temperature structure sectioned parallel to symmetry equivalent (4 0 1) planes. It is evident that not only is the (4 0 1) plane effectively perpendicular to the molecular axis, it cuts the axis at a near optimum point so as to be relatively insensitive to any along-axis shearing of the structure. Moving away from this plane invokes directions that intersect or are in close proximity to the methyl groups, hence they are likely to be highly disrupted by the suggested stacking fault mechanism and so exhibit line broadening effects.

Acetaldehyde (m.p. 152 K) Acetaldehyde, CH3CHO, is the simplest member of the aldehyde family and has a melting point of 152 K. No crystal structure information was previously available but heat capacity measurements25 on the protonated material showed a smooth variation of Cp between 5 K and the melting point. Preliminary low-temperature measurements on HRPD using a 5 g sample of fully deuterated acetaldehyde could not be indexed. This is surprising since lattice parameter determination from first principles, using programs such as ITO26 and DICVOL27, is typically routine and gives solutions with high figures of merit from time-of-flight powder diffraction experiments when the highest d -spacing information is available. This is because in time-of-flight measurements the zero-point error is not only small but becomes progressively less important the larger the d-spacings which have longer times of flight, in contrast to the situation in constant-wavelength measurements. To attempt to resolve the problem, half-hour data sets were then recorded as a function of temperature between 5 K and the melting point. These suggested the formation of two crystalline phases in acetaldehyde (Figure 4b) and prompted new heat capacity measurements to be undertaken.28 These determined that perdeuteroacetaldehyde has two crystalline phases with similar melting points. A metastable phase is obtained (containing some stable phase) on crystallisation from the liquid. The Cp data show a large anomaly around 140 K corresponding to an irreversible transition to the stable phase. Annealing the sample around this temperature and then slowly cooling yields a sample in the stable phase.

Neutron powder diffraction data were recorded using HRPD at 5 K from both annealed and quenched acetaldehyde samples that were initially prepared by hand grinding. Annealing yielded a well crystalline and single phase sample in the thermodynamically stable phase (Figure 5). The diffraction pattern was indexed on a mono-

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Discussion The examples cited illustrate the effectiveness of high-resolution neutron powder diffraction for both the structure solution and refinement of low-melting point molecular compounds. In each case the degree of complexity associated with the structural studies, for example phase behaviour or anomalous line broadening, is unexpected in view of the assumed model nature of the simple compounds studied but reinforces the requirement for careful investigation using neutron data.

Figure 4b - Thermodiffraction patterns of aceteladehyde-d 4. The arrows highlight the main peaks corresponding to the stable monoclinic phase.

Peaks corresponding to the metastable phase disappear below 150K.

clinic cell, Vc=257Å3, in space group P21/c. The structure which contains one molecule (7 atoms) in the asymmetric unit was solved ab initio using standard Direct Methods techniques.1 The study yielded accurate and precise intramolecular bond lengths and angles, C1-C2 1.519(3) Å, <C2D>methyl 1.033(3) Å, C1-O1 1.204(3) Å, C1-D1ald1.104(4) Å. Significantly, the refined molecular conformation is not Cs symmetry, with a rotation of 3.7(4)° along the C1 - C2 axis between the aldehyde and methyl groups.

Attempts to solve the crystal structure of the metastable phase are being hampered by the inherent two-phase nature of the diffraction pattern. At 5 K the metastable phase is pseudo-tetragonal with a unit cell volume similar to that of the stable monoclinic phase.

Figure 5 - Section of the observed (points) and calculated (line) profiles for stable-phase monoclinic acetaldehyde-d 4 at 5 K. The vertical tick marks indicate calculated peak positions.

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Such ‘routine’ results are by no means the norm in genuine ab initio structure solution attempts from powder data. The collapse of three dimensions of diffraction data on to the one dimension of a powder diffraction profile leads to an inevitable peak overlap. The consequent loss of information, particularly at short d -spacings, makes structure solution from powder data inherently difficult.

The extraction of reliable structure factor amplitudes from powder data is therefore of crucial importance, assuming a good quality powder sample. Instead of performing a traditional Rietveld profile refinement in which the unit-cell, atomic co-ordinates and peak shape parameters are varied, in the examples described, the method involves the least-squares refinement of unit-cell constants, peak width parameters and individual peak intensities. This technique was originally pioneered by Pawley29 and is carried out using an in-house suite of programs.24 The method is highly successful in regions of no peak overlap, but the refined intensity values can become highly correlated when substantial overlap occurs. It should be noted that whilst the use of high resolution powder diffractometers such as HRPD serve to minimise peak overlap, in cases like the present, intrinsic sample-dependent line broadening leads to unavoidable severe overlap. Under these circumstances the Pawley method will still permit a summed group of Bragg peaks to be well determined, although the individual intensities can have extreme negative and positive values, often larger in magnitude than the overall "clump" intensity.

In the present work, this fundamental weakness of the least-squares method, has been overcome using a Bayesian approach formulated by Sivia and David30 to extract the structure factor amplitudes. The Bayesian analysis imposes a positivity constraint to the extraction of structure factor amplitudes that reduces the effects of correlation. Assuming an arbitrary value of 70% correlation above which intensity information cannot be reliably extracted, the Bayesian approach permits the extraction of a substantially greater number of structure factor amplitudes, |F(hkl)|, compared with a standard leastsquares analysis. Moreover, and of crucial importance for a Direct Methods solution, the range over which reliable intensity information could be extracted was extended to much lower d -spacings. In all cases, attempts at structure solution without recourse to the Bayesian data processing procedure yielded highly ambiguous results.

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The complexity of structure solution tenable by neutron powder diffraction remains modest even with recourse to sophisticated intensity extraction software outlined above. The monoclinic structure of acetaldehyde, Vc=257Å3, with 7 atoms in the asymmetric unit and the triclinic structure of dimethylsulphide2, Vc=179 Å3, with 9 atoms in the asymmetric unit represnet the most difficult examples tackled to date. To improve on this following a Direct Methods route, the ability to extract reliably more integrated intensities, especially at short d spacings - in the region of high peak overlap, is required.

One option to further improve the method of powder pattern decomposition is to harness the anisotropy of the unit cell expansion, a commonly observed phenomenon with organic materials, by using data recorded at different temperatures in order to separate overlapping Bragg reflections. Data sets at different temperatures are combined to yield peak intensities corresponding to some average temperature, account having being taken of the Debye-Waller effect. In addition to reducing the correlation of overlapping peaks, by combining data sets the method naturally improves on the statistical quality of intensity information obtained from the single-temperature runs. This method has already been successfully applied to improve integrated intensity extraction using both neutron2 and X-ray31 powder data. The need to extract quasi-single-crystal integrated intensities from powder data will remain a limiting factor.

Returning to the adage that neutron powder diffraction is the technique of choice for structure refinement perhaps offers the best route toward increasing the complexity of structure solution tenable for molecular crystals.

Techniques using Monte Carlo, 32,33 simulated annealing33 and genetic algorithms34 methods enable trial structures to be postulated initially without recourse to the diffraction data. These trial structures, which incorporate known geometry of the molecule or of a molecular fragment, in the case of the first two methods are then compared directly with the whole diffraction data and assessed using an appropriate fitness function based on the profile R factor used in a standard Rietveld refinement. The genetic algorithms34 method the trial structure is compared against extracted intensities and the covariance matrix from a Pawley refinement. In general, these methods adopt a more global refinement strategy which lessens the reliance on extracting individual structure factor amplitudes from powder data. To date these new approaches have been restricted to examples using X-ray data, the natural extension to molecular crystal structures using neutron powder diffraction data promises much.

References 1. Ibberson, R.M., Physica B (1997) in press.

2. Ibberson, R. M., McDonald, P. J. & Pinter-Krainer, M., J. Mol. Struct.

(1997) in press.

3. Ibberson, R. M. & Prager, M., Acta Cryst. B52 (1996), 892-895.

4. Ibberson, R. M. & Prager, M., Acta Cryst B51 (1995), 71-76.

5. Cockcroft, J.K & Fitch, A.N., Z. Kristallogr. 209 (1994), 488-490.

6. Vogt, T., Fitch, A.N. & Cockcroft, J.K, Science 263 (1994), 1265-1267 7. Clarke, S.J., Cockcroft, J.K & Fitch, A.N., Z. Kristallogr. 206 (1993), 8795.

8. Delaplane, R.G., David, W.I.F., Ibberson, R.M. & Wilson, C.C., Chem.

Phys. Lett. 201 (1993), 75-78.

9. Fitch, A.N. & Cockcroft, J.K, Z. Kristallogr. 202 (1992), 243-250.

10. Jouanneaux, A., Fitch, A.N. & Cockcroft, J.K, Molecular Physics 71 (1992), 45-50.

11. Torrie, B.H., Von Dreele, R. & Larson, A.C., Molecular Physics 76 (1992), 405-410.

12. Jeffrey, G.A., Ruble, J.R., Wingert, L.M., Yates J.H. & McMullan, R.K. (1985). J. Am. Chem. Soc. 107, 6227-6230.

13. Ibberson, R.M., J. Appl. Cryst. 29 (1996), 498-500.

14. Langel, W., Kollhoff, H. & Knözinger, E. (1986). J.. Phys. E: Sci. Instrum. 19, 86-87.

15. Anderson, A., Andrews, B. & Torrie, B.H. (1985). J. Chim. Phys. 82, 99-109.

16. Kawaguchi, T.K., Hijikigawa, M., Hayafuji, Y., Ikeda, M., Fukushima, R. & Tomie Y. (1973). Bull. chem. Soc. Japan 46, 53-56.

17. Gerlach, P.N., Torrie, B.H. & Powell, B.M. (1986). Mol. Phys. 57, 919930.

18. Burbank, R.D. (1953). J. Am. Chem. Soc. 75, 1211-1214.

19. Prager, M. (1988). J. Chem. Phys. 89, 1181-1184.

20. Duncan, J.L. (1970). J. Mol. Struct. 6, 447-457.

21. Oi, T., Sekreta, E. & Ishida, T. (1983). J. Phys. Chem. 87, 2323-2329.

22. Miksic, M.G., Segerman, E. & Post, B., Acta Cryst. 12 (1959), 390-393.

23. David, W.I.F., Mat. Res. Soc. Symp. Proc. 166 (1990), 203-208.

24. David, W.I.F., Ibberson, R.M. & Matthewman, J.C., Rutherford Appleton Laboratory Report, RAL-92-032.

25. Lebedev, B.V. & Vasil’ev, V.G., Zhur. Fiz. Khim. 62 (1988), 3099-3102.

26. Visser, J.W., (1988). Autoindexing program - ITO. Technisch Physische Dienst, P.O. Box 155, Delft, The Netherlands.

27. Louër, D. (1991). Autoindexing program DICVOL91. Laboratoire de Cristallchemie, Université de Rennes, avenue de Général Leclerc, 35042 Rennes CEDEX, France.

28. Yamamuro, O.Y., & Ibberson, R.M., unpublished data (1997).

29. Pawley, G.S., J. Appl. Cryst. 14 (1981), 357-361.

30. Sivia, D.S. & David, W.I.F. (1994) Acta Cryst. A50, 703-714.

31. Shankland, K., David, W.I.F., & Sivia, D.S., J. Mater. Chem. 7 (1997), 569-572.

32. Andreev, Y.G., Lightfoot, P. & Bruce, P.G., J. Chem. Soc. Chem. Commun. (1996) 2169-2170.

33. Harris, K.D. & Tremayne, M., Chem. Mater. 8 (1996), 2554-2570.

34. Shankland, K., David, W.I.F. & Csoka, T., , Z. Kristallogr. 212 (1997), 550-552.

35. Johnson, C.K., ORTEPII. Report ORNL-3794. Oak Ridge National Laboratory, Tennessee, USA.

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Articolo ricevuto in redazione nel mese di Novembre 1997

HARD X-RAY WAVEGUIDES AS A NEW TOOL IN X-RAY MICROSCOPY S. Lagomarsino, A. Cedola Istituto di Elettronica dello Stato Solido (IESS) - CNR Roma, Italy

C. Riekel European Synchrotron Radiation Facility - Grenoble Cedex, France

S. Di Fonzo, W. Jark Sincrotrone - Trieste, Italy The x-ray waveguides are a novel optical component for hard x-rays able to produce submicrometer beams. In this article we review the operating principle and describe the characteristics of the exiting beam, with particular emphasis on the coherence properties. We will then give example of applications in microdiffraction and in phase contrast imaging where we achieved an unprecedented spatial resolution of 140 nm in one direction.

Introduction The interest in studying matter with ever improving resolution has always pushed scientists to develop new and more powerful methods to artificially extend the limited resolution power of the human eye. To this purpose the optical microscope, then the Transmission Electron Microscope (TEM) with its extreme form - the high energy physics particle accellerator -, the Scanning Electron Microscope (SEM) and more recently the Scanning Tunnelling Microscope (STM) and the Atomic Force Microscope (AFM) have been developed. The application of these imaging techniques with atomic resolution has become a standard in the most advanced laboratories. If such performances have been reached, why to look for other microscopy tools? In fact every technique has its limitations: for example SEM, AFM and STM can only be used to visualize surfaces, while the analysis with the TEM is destructive, requires very thin samples and complex preparation procedures. Moreover, all microscopy techniques involving electrons or other charged particles for sample probing need vacuum conditions. These limitations are usually not encountered with x-rays (and in particular “hard” x-rays , i.e. with energies from few keV to few tens of keV) which can penetrate deeply into condensed matter and are not absorbed in air permitting to investigate the sample in its natural environment at atmospheric pressure. Moreover, a number of different techniques furnishing complementary information can be used with x-rays: diffraction, spectroscopy, radiography, etc. It is therefore not surprising that recently considerable effort has been dedicated to the development of new concepts for the production of high intensity x-ray microspots. The unprecedented source brightness found at the newest so called third generation high-

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brilliance X-ray synchrotron radiation facilities has stimulated many new developments. As a consequence submicron-size monochromatic x-ray beams can now be obtained by use of reflection mirrors,1 multilayer mirrors,2 Fresnel zone plates,3 tapered glass capillaries,4 Bragg-Fresnel optics5 and X-ray waveguides.6-10 As the characteristics of the focal spots differ from technique to technique the simultaneous application of more than one of these objectives will allow for a desirable complementary sample analysis. In this report we will describe the properties of microbeams produced in x-ray waveguides with special emphasis on their diffrence from other microspots. Even though the first attempt to fabricate and study waveguides for hard x-rays was already made in 1974 by Spiller and Segmuller,6 only very recently at the beginning of 1995 the interest in these objects was revived with the success of two collaborations 7,8 which could make an x-ray beam exit from the end face of a waveguide with submicron thickness in one direction. In the meantime the efficiency could be greatly improved and the submicron beam size could be verified.9 The coherence properties of the beam exiting from the waveguide have been used in a prototype of a projection microscope by the use of which phase contrast images could be recorded with a resolution an order of magnitude better than previously possible.10 In what follows we will describe the basic principles of x-ray waveguides and the characteristics of the exiting beam and we will conclude with some applications.

X-ray reflectivity, standing waves and resonances In the hard x-ray range it is convenient to write the refractive index n for matter in the form n = 1 - δ - iβ, where β is related to the absorption and δ is given by:

δ=

λ2 Z + ∆f ' e r Nρ 2π A

(1)

λ is the incident wavelength, Z the atomic number, f’ the real part of the anomalous scattering factor, A the atomic weight, re the classical radius of the electron, N is Avogadro’s number and ρ is the density. In the hard x-ray

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regime δ and β are usually small positive quantities for which one finds 1 >> δ > β.

X-rays impinging at a grazing angle a smaller than the critical angle

α c = 2δ

with a very thin (5 nm ) metal film.

Fig. 2 shows the e.m. field intensity calculated in such a thin C film for the first three resonances for the structure described above and an incident photon beam energy of 13 keV. It is immediately obvious that the different resonance modes have the antinodes positioned at different depths in the C film. These standing wave fields are thus

are totally reflected. The interference between the incident field and the reflected field of equal intensity produces standing waves above the surface with iso-intensity lines parallel to the surface.10 The standing wave field periodicity is simply given by

D=

λ 2 sin α

At the surface the intensity changes smoothly, when a is increased from 0 to αc , from 0 to E02 , where E0 is the incident electric field.

If a film of a light material is deposited onto a heavier substrate, an interesting phenomenon takes place: the critical angle is smaller for the light film than for the heavier substrate, therefore the incident x-ray beam can penetrate the film above its critical angle, it undergoes refraction in the film and will impinge onto the substrate below its critical angle. Consequently the x-rays will be totally reflected from the substrate (see fig. 1) with the subsequent formation of a standing wave extending into the film. If now the standing wave periodicity is equal to an integer fraction of the film thickness, a constructive interference of the electromagnetic waves takes place inside the film. The reflectivity in this case drops to zero and the e.m. field intensity inside the film will be stron-

Fig.2. Calculated electric field intensity vs. depth for the first three resonances Transverse Electric (TE0, TE1 and TE2), in a structure composed of a glass substrate, a Cr film 22 nm thick, a C layer 136.9 nm thick and a Cr cover 4.4 nm thick. Incident photon energy: 13 KeV.

effective tools for localizing contaminants in a film with high accuracy.11,12

Fig. 1. Sketch of the x-ray beams transmitted and reflected at the different interfaces in a 2-layer system. If the critical angle for total reflection a 2 c for medium 2 is larger than a1 c for medium 1, then an x-ray beam incident at an angle a1 > a1 c penetrates in medium 1 where it undergoes refraction. If a2 < a2 c then the beam is totally reflected at the surface of medium 2 and standing waves with periodicites D = l / (2 sin a 2 ) are formed in medium 1. If D is an integer fraction of the film thickness T, then a resonance takes place and the e.m. field intensity inside medium 1 can reach tens or hundreds times the value of the incident e.m. field intensity.

gly enhanced with respect to the incident intensity. For example, a 30 fold enhancement is easily obtained in a structure composed of a glass substrate covered with an opaque metal layer (for ex. Cr) 20 nm thick and a C film 130 nm thick. The resonance enhancement can exceed values of 140 or even more if the C layer is overcoated

Waveguides The above described resonance effect gives origin to xray waveguiding in the thin film. In fact the e.m. standing wave field, which is established in the light material layer, can propagate in the direction parallel to the substrate plane. This is in analogy to mode excitation and propagation in hollow resonators for guiding microwaves. The first demonstration of the possibility of propagation of x-rays through a waveguide was reported in 1974 by Spiller and Segmuller using a standard xray generator.6 They used Al2O3 cladding layers and a BN layer to guide the x-ray beam. About 20 years later, in 1993, Feng et al. repeated the experiment with synchrotron radiation in a waveguide of polyimide sandwiched between a Si substrate and a SiO2 overlayer.14,15 In all these cases the x-rays were coupled to the internal standing waves in an input resonant beam coupling section (RBC), they were passing through a guiding section and then they were detected after they were decoupled in an output Resonant Beam Coupling section (RBC), identical to the input RBC. At the beginning of 1995 independently Lagomarsino et al.8 and Feng et al.7 measu-

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red for the first time the beam exiting the terminal part of the waveguide, demonstrating that high intensity submicrometer beams can be obtained with these optical components for photon energies between 10 and 20 keV. Fig. 3 shows a layout of these waveguides: the beam impinges on the Resonant Beam Coupler and propagates through the guiding section covered by a thick and thus opaque layer. At the end of the guiding section two wave fields emerge from the waveguide, with the inclination between them being twice the internal angle of reflection. We will see this point later in more detail.

tion. A high precision goniometer or goniometer sector for the angular adjustment of the waveguide was used in both cases. Several different detectors (pin diodes and CCD cameras) were used depending on the experiment.

Generally unfocused monochromatic radiation from a Si(111) double crystal monochromator at 13 keV was used, however, we also used in one experiment a W/Si double multilayer monochromator with reduced resolving power (bandwidth about 10-15%).

Reflectivity and waveguided beams: coherence properties Fig. 4 shows as points the reflectivity versus incidence angle obtained at ID13 from one of the waveguides. Superimposed as a line is the reflectivity calculated on the basis of the recursion technique of Parratt16 applied for a total of 3 layers. From the measured modulation the geometrical parameters of the structure have been derived, resulting in 22 nm thickness of Cr in the base layer, 136.9 nm for the carbon guiding layer and 4.4 nm for the semitransparent Cr cover layer. A surface and interface microroughness of 0.8 nm rms and a beam divergence of σy'= 8.2 µrad was also included into the calculations.

An exploded view of the reflectivity spectrum covering the first six resonances and the intensity measured at the wave-

Fig. 3. Schematic view of the waveguide. The incident beam couples with the waveguide in the Resonant Beam Coupling (RBC) section, is guided through the guided section and exits at the waveguide end where it splits into two beams, with the inclination between them being twice the internal angle of reflection.

Waveguide preparation and experimental set-up Our waveguiding structures were produced in the lowpressure triode assisted sputtering chamber of the SINCROTRONE TRIESTE with relatively small deposition rates of around 1 nm/min. The thickness and size of the different coatings were optimized for good feeding and guiding efficiency. Chromium was chosen for the base and the cover layer, however, other metals (like Nickel) will work as well. For opaque layers we deposited more than 10 nm, while the coupling section was covered with only 4 to 5 nm of Cr. For the guiding layer between 130 and 150 nm carbon were deposited. Best results were obtained using as substrates ultra smooth plane Zerodur slides, but satisfactory efficiency was obtained even with optically flat pieces of float glass with 0.6 nm rms surface roughness. The Cr base and thin cover layer and the graphite layer were deposited without vacuum interruption. Instead the extra Cr covering a 2 mm long and 20 mm wide guiding section was added in a subsequent cycle.

The experiments were performed at the European Synchrotron Radiation Facility (ESRF), either on the ID13 undulator beamline or on the BM5 bending magnet sta-

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Fig. 4. Comparison between the measured (points) and calculated (solid line) reflectivity vs. incident angle for the same structure described in fig.

2. Photon beam energy: 13 KeV. A surface and interface microroughness of 0.8 nm and a beam divergence of 20 µrad FWHM have been included in the calculations.

guide exit in the same angular scan are shown in fig. 5.

Due to the refraction in the carbon layer the resonances do not occur at constant angular increments.

This happens instead in terms of the internal angle,

α int = α 2 ext − 2δ which, however, is not accessible to a measurement.

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For the different orders this internal angle aint is increasing approximately as an integer multiple of a fundamental angle, which is as small as αint = 0.0185o (or 0.32 µrad) for the chosen photon energy of 13 keV.

Fig. 5. Exloded view of the reflectivity of fig. 4 for the first six resonances together with the intensity measured at the exit of the waveguide. The spectra are normalized to the incident signal. The points are guide for the eye.

Fig. 3 presents schematically the operating principle of the waveguide, applying arguments from geometrical optics. In this picture finally two waves exit from the waveguide with an inclination angle of twice aint between them. The dimensions in the waveguide actually require to apply arguments from wave optics. Consequently we need to consider in the thin film a standing wavefield traveling along the waveguide. In order to calculate the intensity distribution which one will find far from the waveguide exit, one needs to consider the field amplitude of the standing wave confined between the two carbon interfaces as the source for a Fraunhofer diffraction pattern.6 In agreement with the geometrical optics picture one finds diffraction maxima in the directions of the two internally traveling beams.

A low-noise CCD camera with 14 bit resolution and fast read-out has been used to visualize the exit beams and to measure their intensity profiles.

Fig. 6 shows the waveguided beams for the first 5 resonance orders together with the reflected beam and a spurious one, which has the direction of the incident beam (probably the third harmonic from the Si(111) monochromator, which is transmitted with little losses through the waveguide substrate). Superimposed as a white line is the intensity profile obtained integrating the CCD pixel contents in a few pixels in the horizontal direction.

All the resonance orders except the first one display two peaks whose angular separations are multiples of 2 αint = 0.037° (or 0.64 µrad). Some oscillations with smaller amplitude and higher spatial frequency are evident between the main peaks. A special situation is found for the first (transverse electric) mode TE0, where the separation of the two beams is so small that the broadening due to diffraction at the waveguide exit is sufficient to superpose the two exiting beams in such a way that only

Fig. 6. Waveguided beams for the first 5 resonances as measured by a low-noise CCD camera placed at 840 mm. from the waveguide. The reflected beam and a spurious one which has the direction of the incident beam (probably the third harmonic from the Si(111) monochromator) are also visible. Superimposed is the intensity profile obtained integrating the CCD pixel contents in a few pixels in the horizontal direction. The acquisition time was 2 s for TE0, TE1 and TE3, 5 s for TE5 and 40 s for TE2.

On the left in the small insert is an optimized view of TE0 with an acquisition time of 60 s.

one Gaussian-shaped maximum can be seen in the direction tangential to the film.

In fig. 7 these measured profiles are confronted with the simulations for the structure parameters of the waveguide.

The intensity distribution for the far field Fraunhofer diffraction pattern is given here by: 2 ∞ ∞  I (ϑ 0 ) = C  E( z)e i ( zk sin θ ) dz  dθ   −∞ −∞ 

∫ ∫

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where θ is the observation angle in the plane containing the incident and the reflected beams, C is a constant, E(z) is the field amplitude of the standing wave in the guiding layer at the waveguide terminal and z the coordinate perpendicular to the waveguide surface. k = 2π/λ is the wavevector and λ the wavelength. The standing wavefield is considered in this case a completely coherent

Fig. 7. Comparison of the intensity profile of fig. 6 with the simulation as calculated from eq. 2. The source in the simulation has been considered as completely coherent.

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source. The good agreement between the calculations and the experimental results, in particular in the zones with small intensities between the principle peaks, points onto a very high degree of coherence in the registered radiation. With this property the beam exiting from the waveguide is of particular importance in applications requiring a divergent and coherent beam, e.g. in the field of microscopy, as will be discussed later. It is plausible that the divergent beam from an x-ray waveguide will contain only little content of incoherent radiation. This radiation can have its origin in the interface roughness. Feng et al [14,15] pointed out that the scattering at the interface irregularities will produce a mode mixing, i.e. the scattered radiation will arrive with a high efficiency at the waveguide exit only in case it can excite internally another permitted mode. On the other side the most efficient modes are the low-order ones, therefore the scattered intensity will be coupled to modes with better traveling efficiency only if a reduction in internal angle will occur in the scattering process, otherwise the intensity will be lost rapidly. Consequently together with e.g. the exiting TE4 mode also intensive modes TE0 to TE3 can be found, while the exiting TE0 is accompanied by very weak other modes. The more regular intensity distribution in the latter mode TE0 mode favors its use for experiments. For this mode it becomes clear from an inspection of fig. 7, that even if intensity is coupled into higher modes, this will not contribute unwanted radiation in the direction of the TE0 intensity maximum.

Indeed all higher modes send little (even numbered modes) or no (odd numbered modes) intensity in this direction. The central region of the fundamental mode will thus be virtually free of mode mixing and should therefore be completely coherent. The tails of TE0 instead can contain significant incoherent contributions. With the same arguments we will find for the higher modes the following: in the direction of the maxima of the principle peaks only small incoherent contributions can be directed, however, depending on the interface roughness significant incoherent intensity may be found between the principle peaks. This is not observed in our waveguides, and points thus onto a very good interface quality.

Beam size, efficiency and flux The e.m. field intensity inside the waveguide has, for the fundamental mode, a cos2 (2πz/D) profile, where z is the spatial coordinate perpendicular to the surface and D is the standing wave periodicity, with nodes at the two interfaces. If one would like to assign a beamsize to the beam exiting from the waveguide, one needs to take the full-width-at-half-maximum (FWHM) of this intensity distribution, which is then approximately only half of the C film thickness, i.e. less than 100 nm. For higher order modes the corresponding beam size will grow to at most the layer thickness. These results cannot be obtained from an experiment. The beam size is only mea-

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surable at a certain minimum distance from the exit where it will already have undergone a broadening due to diffraction. It was measured at approximately 0.1 mm distance by moving a test pattern composed of gold bars with 0.1 micron step size through the beam [9]. Three different patterns of 1 mm long parallel lines of 2.8 micron thick gold bars were produced with lithographic techniques: the repeat periods were 1, 2 and 6 microns.

The fit to the curves with the real bar pattern shapes yields a FWHM for a gaussian beam of 0.2 microns, which is consistent with the expectation for the beam size at the bar pattern position. The recovery of the transmitted signal in the bar pattern allows for only very little intensity in eventual tails of the beam profile. This is actually an advantageous property of the waveguided microbeam.

Fig. 8 reports a comparison of its theoretical angular beam profile (137 nm thick resonator layer at 13 keV photon energy) with the profile expected from a classical slit with the same dimension and illuminated with a plane wave. The FWHM of the profile of the waveguided beam is slightly larger than the profile for the slit (0.91 mrad instead of 0.68 mrad). However, while we find as much as 9.4% of the intensity in the secondary peaks (tails) observed for classical slit diffraction, we will find only 0.51% in the secondary peaks of the waveguided beam. Consequently a very clean beam is produced by the waveguide.

The efficiency of the waveguide depends strongly on the extent and the divergence of the incident beam. Losses in the waveguide are mainly due to absorption in the C layer and to scattering and uncomplete reflexion at the interfaces. From a theoretical point of view in a simple ray-tracing approach, a beam of zero size impinging just at the limit of a guiding section 2 mm long, has 0.53 probability to exit the guide. The same beam vertically translated of 5 mm (which means, at a grazing angle of about 0.132°, translated on the waveguide surface by about 2.2 mm) will have half this probability to exit.

Thus the width of an incident beam which effectively contributes to the outgoing flux is approximately 15 µm, resulting in an integrated efficiency of about 25%. Concerning beam divergence the waveguide acceptance angle for a monochromatic beam is very small: about 2.6 µrad. If the optical elements of a beamline will not increase the opening angle under which an observer will see the source from the waveguide position, the divergence of the beam as produced by the source and by the limited spatial acceptance (15 µm) will be given by (s + S)/L, where s is the spatial acceptance, S the source size and L the source distance. With s = 15 µm, S = 24 µm and L = 30 m the effective incoming vertical divergence is 1.3 µrad, i.e. smaller than the waveguide acceptance angle. This points onto almost perfect waveguide matching to the source. On the other hand, the experimental FWHM of the smallest resonance peaks is of the order of

Fig. 8. Angular profile calculated for the beam at the exit of the waveguide described in fig. 2 compared with the profile calculated for a classical slit having the same dimension of 137 nm and illuminated by a plane wave.

15 µrad, giving indications that some optical element in the beamlines contributes to an increase of the incoming beam divergence. Incidentally it is interesting to note that the waveguide is an exceptionally sensitive tool for the determination of the incident beam divergence. This is also due to the fact that the angular dispersion related to the relative bandwidth of the order of 10-4 for commonly used crystal monochromators is always negligible. The beam size and divergence in the horizontal direction does not affect the waveguide performance. In the best performing waveguides we measured an overall efficiency (output flux/input flux) of 0.01 with a vertical beam size of about 45 µm. It is worth to note that in this case the output flux is about three times higher than the flux passing a hypothetical slit (not considering here its feasibilty) with the same vertical size of the waveguide resonator (0.13 µm). Concerning absolute flux, we measured on ID13 beamline about 7x108 ph/sec in a monochromatic beam of nearly 130 nm x 600 mm with a band pass of about 2x10-4. This flux can be considerably increased (at least an order of magnitude) if instead of a crystal monochromator a multilayer monochromator with a larger band pass is used. In fact, as has been recently demonstrated,17 a beam with a 10% bandpass can propagate in the waveguide without losses in bandpass.

However, in order to not excite simultaneously the first two modes, which have the smallest separation, at most a relative bandwidth of 0.02 is acceptable. This cannot be achieved with the multilayer monochromator and consequently one will always simultaneously excite several modes, resulting in a subsequent loss of coherence. As long as one does not need the coherence this is an interesting alternative for getting the highest possible flux, as

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for example microfluorescence experiments.

It has to be noted that the theoretical efficiency of an ideal waveguide has not yet been reached, thus considerable gain in efficiency can still be obtained. Moreover better experimental conditions and proper optics before the waveguide could also contribute to significant increase in absolute flux.

Applications: microdiffraction We tested the capability of the waveguided beam to produce measurable diffraction patterns. In order to increase flux, the beam from the monochromator was focused by an ellipsoidal mirror18 to a spot with about 30 micron diameter. A 30 µm diameter collimator at the focal position removed any spurious beam. For these measurements a nitrogen cooled CCD camera, in which the diffraction pattern at a fluorescence screen is transferred by optical fibres to the CCD, was placed as close as possible at about 85 mm from the sample. Particular care has been devoted to background suppression. To this purpose the waveguide was covered with a metal protection in order to absorb the reflected beam and to eliminate the scattering and fluorescence radiation excited by the primary beam. Between the sample and the camera a beam stopper was placed to block the direct waveguided beam and to allow only the diffracted beams to reach the detector. It is clear that the major interest in using a microbeam in fiber studies stays in the possibility to obtain diffraction pattern from different positions in one fiber.

We tested this possibility with a single Kevlar fiber, a very interesting material characterized by its exceptional strength (is used to manufacture, for example, the best sails for sailing boats). Fig. 9 shows the diffraction pattern from a single Kevlar fiber 12 µm thick, after background subtraction, placed perpendicular to the larger beam dimension. The characteristic fiber texture pattern of Kevlar can be easily recognized, with the most intense equatorial reflections (011), (200), (211), and even some meridional reflections. Just as a simple observation, a scan of the fiber showed marked differences in the ratio of the intensities for (200) and (011) reflections for different positions in the fiber. These aspects are the subject of further studies. This example clearly shows the potentialities of submicrometer beams in the study of materials like fibers.

Applications: phase contrast The coherence properties of the beam coming out from the waveguide can be advantegeously exploited in imaging experiments based on phase contrast. In radiography contrast with hard x-rays is generally based on absorption. However light materials, such as carbon based or biological samples, are very weakly absorbing, and consequently only poor contrast can be achieved.

Much better results can be obtained by imaging the phase modulation induced by an object in a coherent beam.

Several techniques are used to this purpose: pioneering

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Fig. 9. Diffraction pattern, after background suppression, of a single Kevlar fiber 12 µ thick perpendicular to the larger beam dimension. The beam from the monochromator was focused by an ellipsoidal mirror to about 30 µ x 30 µ at the waveguide entrance. The fiber axes were perpendicular to the larger beam dimension. The diffraction pattern was recorded with a nitrogen cooled CCD camera placed at about 85 mm from the sample.

work was carried out by Bonse and Hart with an x-ray interferometry19 where two beams coming from the same diffraction process were allowed to interfere. Interference fringes were observed when an object (for example a plastic wedge) was placed in one of the two beams introducing a variable optical path length. Momose et al.20 applied this technique to obtain 3-D phase-contrast images of biological fissues. In a simpler experimental setup the refraction effect created by an object illuminated by an x-ray plane wave is detected with high-resolution by an analyzer crystal.21-22 An alternative technique similar to optical in-line holography23 can be used both with standard x-ray generators24 and with synchrotron radiation.25-28 In this case propagation in free space transforms the phase modulation introduced by the sample into an intensity modulation at the detector.

A key requirement for this kind of technique is the high spatial coherence of the beam.

Let’s consider an e.m. wave incident on an object.

The field u(x,y) just after the object is related by the field just before u0(x,y) by the simple relation: u(x,y) = F(x,y) u 0 (x,y), where F(x,y) is the transmission function given by:

F( x , y ) = M( x , y )e iϕ ( x , y ) (3) with:

 1  M( x , y ) = exp − µ( x , y , z)dz  2 

∫

ϕ (x, y) =

2π δ ( x , y , z)dz λ

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(4)

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M is related to absorption and j to phase modulation. In (4) δ is given by eq. (1) and is of the order of 10-5 - 10-6.

A variation in phase implies a deviation in the propagation direction. If after the object the e.m. field is allowed to propagate, interference takes place between undisturbed waves and waves modulated by the object. In this way phase modulation is transformed into intensity modulation at the detector plane. In the standard geometry 24-28 the interference field is strongly dependent on the defocusing distance D, given by:

D=

z1 z2 z1 + z2

where z1 and z2 are the source-object distance and the object-detector distance, respectively. For synchrotron radiation experiments with unfocused radiation, where z1 >> z2, D coincides practically with z2. The field at the detector can be calculated either by using the Kirchoff integral27 or by developing the e.m. field into plane waves and applying Fourier transformations.26 In any case the contrast is significant only in points of the sample where the second derivative of its optical density is different from zero. Thus with phase contrast mainly the boundaries are evidenced. The resolution is essentially limited by the finite source size, by the detector and by vibrations. In the case of third generation x-ray synchrotron radiation sources the main limitation is due to the detector which, in the best conditions, has a resolution of 1 mm. Use of the waveguide allows to circumvent this limitation and to obtain a resolution at least an order of magnitude better. The experimental arrangement is sketched in fig. 10. The sample is put at a distance z1 from the waveguide exit and the detector at the distance z2 from the sample. As mentioned above, the waveguide provides not only a coherent but also a divergent beam that can project a magnified image of an object onto a detector.

The magnification M is given by:

M=

z1 + z2 z1

and can easily reach values from 100 to 1000. As in the standard geometry, the defocusing distance

D=

z1 z2 z1 + z2

determines the interference field. Now z2 is generally much larger than z1, therefore the defocusing distance D coincides practically with z1. Considering only one dimension and a pointlike source, the field amplitude at the detector can be calculated as the convolution of the

Fig.10. Schematic experimental set-up for phase-contrast microscopy.

The waveguide delivers a coherent and divergent x-ray beam which projects on the detector placed at a distance z2 from the sample the modulations in amplitude and phase produced by the sample which is at a distance z1 from the waveguide exit. Propagation in free space allows interference between disturbed and undisturbed waves. The magnification M is given by (z1+z2)/z1.

transmission function (eq. 3) with a propagation function given by:

K(X / M ) =

1

λz1 z2 i

e

[

iπ ( X / M ) / λD 2

]

(5) Where X refers to the detector coordinates.

An important consideration is that due to the coherence properties of the exiting beam, the spatial resolution is not limited to the size of the source. Moreover, because of the magnification, the limit in resolution given by the detector is bypassed. Therefore a considerable improvement in resolution with respect to the standard geometry is achieved by use of the waveguide. To prove this point, we performed phase contrast experiments using the waveguide as virtual source at the microfocus beam line ID13 of ESRF. An unfocused beam about 45µ(V) x 600 µ (H) from a Si(111) monochromator was used. The photon energy was again 13 keV. The detector was a low-noise CCD camera with fast read-out.29 The waveguide-sample distance ranged from a few micrometers to a few millimeters, while the distance waveguide-detector z1+z2 was kept constant at 0.99 m. Each pixel of the CCD camera corresponded to 6.5 µm on the converter screen . The sample was scanned vertically across the beam with a high resolution piezoelectric transducer.

Fig. 11 presents three images obtained from a test pattern consisting of five 0.3 µ wide gold stripes separated by 0.2µ. Their thickness was 0.3 µ. Each image corresponds to a different waveguide-sample distance z1 (5.3, 2.8 and 1.3 mm) and therefore different magnifica-

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tion values M (187, 354 and 762). We recall here that z1 corresponds in practice to the defocusing distance. Its dominating effect for the image formation can be seen from the fact that the number of interference fringes changes significantly in the pictures with different z1.

These images were corrected for spatial variations in incident beam intensity and for detector efficiency inhomogeneities with a flat-field procedure, consisting of a background subtraction and a normalization to the incident beam intensity. The exposure time was 5 s. for each image, but a detectable contrast was already visible for 0.1 s exposure time. The intensity profile extracted from the images is shown in fig. 12 a)-c) together with simulations d)-f) based on the procedure described above. The present agreement is obtained by convoluting the calculated profile with a resolution function of gaussian form.

The full width at half maximum for this function was

order to prove that visible contrast can be obtained not only in artificial structures but also on real samples, a nylon fiber 12 µ thick was illuminated by the beam exiting the waveguide at a distance z1 of 4.1 mm, corresponding to a magnification value M = 241 (see fig. 13 a).

At this distance the beam illuminates only about 5 µ, and thus only part of the sample. The full image in fig. 13 a) was obtained as superposition of images independently recorded during a vertical scan of the fiber (note that for reasons of space the present image is rotated by 90o clockwise. While the right fibre edge (corresponding to the bottom edge in the sample) is straight, and presents

Fig. 12. a-c: vertical intensity profiles integrated horizontally over 6 pixels for the images represented in fig. 11. d-f: corresponding simulations calculated according to eq. 5 with the structural parameter of the gold stripes as obtained from Scanning Electron Microscopy (SEM) images. In the simulations a resolution function of Gaussian shape has been introduced with a FWHM of 0.14 µ for (e) anf (f) and of 0.2 µ for (d).

Fig. 11. Images taken at the ID13 beamline of a test pattern composed of five gold stripes 0.3 µ wide separated by 0.2 µ for three different values of the distance waveguide-sample z1 (practically coincident with the defocusing distance D) and therefore three different values of magnification M. In all the cases the images were recorded by a low-noise CCD detector placed at a constant distance z1 + z2 = 0.99 m. The exposure time was 5 s. and the photon energy 13 KeV. a) z1 = 5.3 mm; M = 187. b) z1 = 2.8 mm; M = 354. c) z1 = 1.3 mm; M = 762.

0.14 µ for magnifications M=354 and M=762, and 0.2 µ for the magnification M= 187. In this last case the beamline vacuum system introduced additional vibrations. In

28

almost perfectly sharp Fresnel fringes, the left edge is deformed. Indeed an inspection with an SEM showed a damage in the surface of the fibre at this position.

Fig 13 b) shows the vertical intensity profile of the straight edge represented in fig. 13a). Fig. 13c) presents the corresponding simulation calculated according to eq. 5 with a resolution function of 0.14 µm Both, the test pattern and the nylon fiber images, were obtained at an undulator beamline.

To prove that also the beam from a bending magnet could provide enough flux to record phase contrast images with submicrometer resolution using the waveguide as optical element, we carried out experiments at the BM05 optics beamline of ESRF. The photon energy was also in this case 13 keV monochromatized by a doublecrystal Si(111) monochromator. The same CCD camera of the previous experiment was used, adjusted for a spatial resolution of about 10 µ.

Figs. 14 and 16 show a sequence of images with different magnification values of a cellulose fiber and a bone sec-

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ges is evident, with increasing deformation towards larger magnification values: in fact in the present configuration magnification is achieved only in the vertical direction. Work is in progress to extend the beam focusing to both dimensions.

Fig. 13. a) Image of a nylon fiber 12 µ thick for z1 = 4.1 mm and M = 241 at a photon energy of 13 KeV. At this distance the vertical beam dimension is nearly 5 µ, therefore the image is the superposition of different images taken during a vertical scan. For layout reasons the image has been rotated of 90° clockwise. The right edge (top in the original figure) appears deformed due to some defects in the fiber, confirmed by SEM examination.

Conclusions The properties and applications of submicrometer beams generated by hard x-ray waveguides have been reviewed. Beam sizes of 130 nm x 600 µ with a flux of about 7 x 108 ph/sec have been achieved with a monoch-

Fig. 14. Images taken an the bending magnet beamline BM5 of a cellulose fiber 50 µ thick at different magnification values, indicated on the images.

Fig. 13. b) Vertical intensity profile of the straight edge represented in fig. a).

Fig. 15. Images taken an the bending magnet beamline BM5 of a bone section at different magnification values, indicated on the images. The contrast is due in this case to osteocites.

Fig. 13. c) corresponding simulation calculated according to eq. 5

tion, respectively. In the cellulose fiber (about 50 m thick) one “V” shaped edge is evidenced. In the bone section the contrast is due to osteocites.

In both cases the strong astigmatic character of the ima-

romatic beam at an undulator beamline at ESRF. Higher fluxes can be reached with larger band-pass monochromators and/or with proper conditioning optics in front of the waveguide. Clear diffraction patterns from a single Kevlar fibre have been obtained. The beam exiting from the waveguide is highly coherent, and this allowed an unprecedented resolution in phase contrast microscopy. We believe that this innovative x-ray optics has good potentialities as a diagnostic tool for several applications in material sciences and in life sciences.

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Acknowledgements P. Cloetens is gratefully acknowledged for his unvaluable contribution in the phase contrast experiments. We want also to thank A. Freund and his group for the assistance at BM5 and E. Di Fabrizio of IESS for providing the test patterns. Finally, A.C. acknowledges INFM for her grant.

References 1. A. Iida, K. Hirano, Nuclear Instr. and Meth., B114, 149 (1996) 2. A.C. Thompson, K. Chapman, Applications of Synchrotron Radiation

13. S. Di Fonzo, S. Lagomarsino, W. Jark, A. Cedola, B. R. Müller and J. B. Pelka, Thin Solid Films 287, 288 (1996) 14. Y. P. Feng, S. K. Sinha, H.W. Deckman, J. B. Hastings and D. P. Siddons, Phys. Rev. Lett. 71, 537 (1993) 15. Y. P. Feng, H.W. Deckman and S. K. Sinha, Appl. Phys. Lett. 64, 930 (1994) 16. L.G. Parratt, Phys. Rev. 95, 359 (1954) 17. A. Cedola, S. Lagomarsino, S. Di Fonzo, W. Jark, C. Riekel and P. Deschamps, to appear on J. Synchr. Rad. 18. S. Fiedler, P. Engstrom, C. Riekel, Rev. Sci. Instr., 66, 1348 (1995)

Techniques to Materials Science III, L.J. Terminello, S.M. Mini, H.

19. U. Bonse and M Hart, Appl. Phys. Lett., 6, 155 (1965)

Ade, D.L. Perry eds, Mat. Res. Soc., Symp. Proc. Vol. 437 (1996)

20. A. Momose, T. Takeda, Y. Itai, and K. Hirano, Nature Medicine 2, 473

3. Y. Suzuki, N. Kamijo, S. Tamura, S. Handa, A. Takeuchi, S. Yamamoto, Sugiyama, K.Ohsumi, M. Ando, J. Synchr. Rad., 4, 60 (1997).

4. D.J. Thiel, D.H. Bilderback, A. Lewis, E.A. Stern, Nuclear Instr. and Meth. A317, 597 (1992) 5. S.M. Kuznetsov, S.M., Snigireva, I.I., Snigirev, A.A., Engström, P., Riekel, C., Appl. Phys. Lett., 65, 827-829 (1994) 6. E. Spiller and A. Segmüller, Appl. Phys. Lett. 24, 60 (1974) 7. Y. P. Feng, S. K. Sinha, E. E. Fullerton, G. Grübel, D. Abernathy, D. P. Siddons and J. B. Hastings, Appl. Phys. Lett. 67, 3647 (1995) 8. S. Lagomarsino, W. Jark, S. Di Fonzo, A. Cedola, B. R. Müller, C. Riekel and P. Engstrom, J. Appl. Phys. 79, 4471 (1996) 9. W. Jark, S. Di Fonzo, S. Lagomarsino, A. Cedola, E. Di Fabrizio, A. Brahm, and C. Riekel, J. Appl. Phys. 80, 4831 (1996) 10. S. Lagomarsino, A. Cedola, P. Cloetens, S. Di Fonzo, W. Jark, G. Soullié and C. Riekel, Appl. Phys. Lett., 71, 18 (1997) 11. M.J. Bedzyk, G.M. Bommarito and J.S. Schildkraut, Phys. Rev Lett., 62, 1376 (1989)

(1996) 21. E. Förster, K. Goetz, and P. Zaumseil, Krist. Tech. 15, 937 (1980) 22. T.J. Davis, D. Gao, T.E. Gureyev, A.W. Stevenson, and S.W. Wilkins, Nature 373, 595 (1995) 23. D. Gabor, Nature (London) 161, 777 (1948) 24. S.W. Wilkins, T.E. Gureyev, D. Gao, A. Pogany, A.W. and Stevenson, Nature 384, 335 (1996) 25. A. Snigirev, I. Snigireva, V. Kohn, S. Kuznetsov, and I. Schelokov, Rev. Sci. Instrum. 66, 5486 (1995) 26. P. Cloetens, R. Barrett, J. Baruchel, J.P Guigay, M. and Schlenker, J. Phys. D: Appl. Phys. 29, 133 (1996) 27. C.Raven, A. Snigirev, I. Snigireva, P. Spanne, A. Souvorov and V. Kohn, Appl. Phys. Lett., 69, 1826 (1996) 28. P. Cloetens, M. Pateyron-Salomé, J.Y. Buffière, G. Peix, J. Baruchel, F. Peyrin, and M. Schlenker, J. Appl. Phys. 81, 5878 (1997) 29. J.C. Labiche, D. Segura-Puchades, Van Brussel, and J.P. Moy, ESRF Newsletter 25, 41 (1996)

12. J. Wang, M.J. Bedzyk and M. Caffrey, Science, 258, 775 (1992)

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Articolo ricevuto in redazione nel mese di Aprile 1998

STRUCTURAL CHARACTERIZATION OF SEMICRYSTALLINE POLYMER BLENDS: SAXS AND SANS COMPLEMENTARITY A. Triolo and R. Triolo Dipartimento di Chimica Fisica, Università di Palermo, Italy Introduction Structural characterisation of semicrystalline polymer blends is of primary importance both from a technological and academic point of view. Most of the macroscopic mechanical and physical properties of polymers depend on the amount of crystalline phase in the sample and the peculiar arrangement of this phase can be detected by means of diffraction techniques, such as x-ray, neutron or electron diffraction. As a consequence of a delicate balance between entropic effects and enthalpic ones, polymer chains tend to arrange themselves in a lamellar fashion, thus leading to a peculiar sequence of crystalline and amorphous layers. 1 Amorphous and crystalline layers differ in the molecular arrangement of the polymer chain, thus showing a different density (e.g. volumetric or electronic or scattering length density). The amount of crystalline phase strongly influences macroscopic properties such as the strength, the elasticity or the stress modulus of the sample.

Moreover other properties such as the permeability to gases and aromas are strongly influenced by the presence of a crystalline phase.

It is then of outstanding importance to relate macroscopic properties to mesoscopic molecular arrangement: xray and neutron scattering techniques do allow to investigate this very size range and these "particles" are then ideal probes to understand the nature of polymer morphology in a non destructive way.

In the following we present some results we obtained studying SAXS and SANS data from semicrystalline polymer blends.2,3 Macroscopic properties of polymers can be largely improved by means of blending them.

It has been found that iPP permeability to O2 and aromas can be drastically reduced by blending this polymer with HOCP. This result makes iPP-HOCP blends extremely suitable for industrial applications in the packaging field. iPP-HOCP blends have been largely studied in the past and a review of these studies will be presented in the following.

The iPP-HOCP phase diagram shows interesting features, as a miscibility gap can be entered as the temperature changes in a well defined composition range.4,5 In figure 1, we present a schematic representation of iPPHOCP phase diagram.

It can be noticed that the miscibility gap is defined in the composition range 75-40 % w/w iPP/HOCP and the

Figure 1. Schematic representation of the iPP-HOCP phase diagram.

Note the presence of a miscibility gap.

temperature range between 90 and 250°C. The occurrence of the miscibility gap implies that in a well defined thermodynamic regime, two different kinds of amorphous layers can intercalate between the crystalline ones, iPP being the only crystallizable component in the blends. The amorphous phases do differ in composition, each containing a different amount of the additive HOCP. It can be foreseen2 that blends, whose thermodynamic state lies outside the miscibility gap (hereinafter called out of gap blends), are characterized by just one periodicity, reflecting the occurrence of a periodic alternating of crystalline and amorphous layers. On the other side,

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blends whose thermodynamic state lies inside the miscibility gap (hereinafter called in gap blends), should be characterised by the occurring of two different periodicities, as the amorphous phase composition should influence the amorphous layers' sizes.

An optimal probe should then allow to determine the existence of just a periodicity in out of gap blends, while it should detect the occurrence of two different so called Long Periods in in gap blends.

HDPE-HOCP blends6 show far more complex phase diagram, as it was not possible, up to now, to characterise the composition range delimiting the miscibility gap. To our knowledge just a 10% w/w addition of HOCP is sufficient to demix the corresponding blend.

Experimental section Materials Binary blends of iPP (Moplen T30S, Mw=300,000; Montedison) and HDPE (Eltex A1050, Mw=3.15 105; Exxon Co.) with saturated cyclopentadiene (Escorez, Mw=630; Esso Chemical) were prepared in a microextruder. iPP/HOCP blends were melted at ~280°C. After extrusion blends were cooled to room temperature. Samples were obtained melting the granulated blends at ~260°C and then quickly quenching at -20°C. The 40/60 %w/w HDPE/HOCP blend was mixed at 210°C and then granulated. The mixed material was then compression-molded in a press at 200°C. Finally the samples were quenched in water at room temperature.

SANS and SAXS Measurements SANS data were collected at the W.C. Koehler 30m SANS facility at the Oak Ridge National Laboratory (ORNL).7 The neutron wavelength was 4.75 Å. The sample-detector distance was 9.0 m. SAXS data were collected at the 10 m SAXS camera of ORNL.8 The system uses pin-hole collimation and is equipped with a two-dimensional position-sensitive detector. Sample-detector distance varied between 2 and 5 m (mostly 3.12 m) and Cu Kα radiation, produced with a 12 KW rotating anode, was used. Samples were contained in stainless steel cells equipped with Kapton windows.

SANS and SAXS scattering intensities were isotropic and radial average was performed on data, after correction for background, scattering of the cell, transmission and thickness. The net intensities were converted to absolute differential cross sections per unit sample volume (cm-1) by comparison with secondary standards.9 Result and discussion Small Angle X-ray Scattering is extensively employed in polymer science. Statistically averaged information on morphological details can be extracted from Lorentz corrected SAXS patterns. In figure 2, we present Lorentz corrected SAXS patterns as obtained from iPP/HOCP

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Figure 3. Schematic representation of the analysis approach applied to the Fourier inverted Lorentz corrected SAS pattern (after G. R. Strobl and M. Schneider; J. Polim. Sci.: Phys. Ed. 18, 1343 (1980))

95/5 %w/w blends, as a function of temperature. The occurrence of an interference peak is classically attributed to the existence of a characteristic periodicity in the sample at a mesoscopic scale length. It is well known that this interference peak is a consequence of the presence of a lamellar morphology which commonly characterizes semicrystalline polymers. Classically SAX diffraction patterns are interpreted in terms of Bragg law, thus deriving an indication of the average size of long period. On the other hand, this information is quite limited, as it does not allow to infere any indication about the mutual relationship between crystalline and amorphous layers. Since the pioneering work of Vonk and Kortleve10 , it has been pointed out that Fourier inversion of scattering patterns arising from lamellar morphologies can be interpreted in terms of a fairly detailed structural model, thus allowing to get at least averaged information on the lamellar morphology structural parameters. The theoretical background involved in this kind of real space analysis is well established . Let us assume that the local electronic density in the sample is described by the function ρ(r). ρ(r) does necessarily reflect the occurrence of a lamellar periodicity in the sample. X-ray scattering technique (as well as neutron scattering one) allows to investigate periodicities in ρ(r). As a matter of fact, Vonk and Kortleve pointed out that Lorentz corrected SAXS (or SANS) patterns are defined as: ∞

∫

K(r) = ℑ(Q)Q 2 cos 2( πQr)dQ

(1)

0

where ℑ(Q) is the experimentally derived scattering pattern and K(r) is the electron density autocorrelation function, defined as:

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∞

K( r ) = 1/ ρ 2

∫ [ρ(r ) − 0

]

ρ Iρ(r − r0 ) − ρ dr0

(2)

0

with <ρ> defined as the sample averaged electron density. It can then be argued from a comparison between 1. and 2. that I(Q) contains information on the electron density function, in an averaged way. Moreover from simple considerations arising from the theory of autocorrelation functions, it can be noted that the occurrence of a periodicity in a given function ρ(r) automatically implies the occurrence of a maximum in its autocorrelation function, K(r). As a consequence, the Fourier inverted scattering pattern can then be analized, aiming to get information on the periodicity occurring in the sample.

In figure 3, we present the Fourier inversion of Lorentz corrected SAXS pattern arising from a 95/5 %w/w iPP/HOCP blend. The geometrical features of this curve have been interpreted in terms of the above mentioned structural parameters of the lamellar morphology.11 It can be appreciated that a separate indication of the average sizes of both crystalline and amorphous layers can be derived, Rc and Ra respectively. Moreover other parameters can be derived, such as the thickness of the transition layer between the amorphous and crystalline ones, Rd, the interface specific surface between these layers, Os, the thickness of the long period, LP, the contrast between the layers, ∆η2, and the invariant, Q. In figure 4, we present a schematic representation of the lamellar morphology occurring in semicrystalline poly-

Figure 4. Description of the periodic alternating of crystalline and amorphous layers in a semicrystalline polymer.

mers. It can be understood that the parameters that can be derived from the analysis of figure 3, describe in an almost complete way this kind of morphology, only the size distribution functions lacking as a result of the analysis.

In figure 5, the Fourier inverted Lorentz corrected SAXS

patterns are reported for the same sample described in figure 2. It can be appreciated that the occurrence of the peak in the Q-domain translates into a relative maximum in the r-domain, thus indicating the existence of a structural periodicity.

A FORTRAN code has been developed which allows to

Figure 6. Temperature evolution of Long Period, Rc, Ra and Rd for the various compositions: 100/0 (h), 95/5 (g), 85/15 (5), 80/20 (6) % w/w iPP/HOCP.

Fourier invert Lorentz corrected SAS data, according to equation 1.

Moreover the code analyses the K(x)'s geometrical features in order to automatically extract the whole set of structural parameters reported in figure 3. A large series of SAXS data has been collected, in order to acquire a fairly detailed picture of the temperature-composition dependence of polymer morphology.

The whole data set has been analised in terms of the above described approach and in figure 6 and 7 we report the temperature and composition dependence of Rc, Ra, LP and Rd. The observed increase of the crystalline layer thickness with temperature is mainly due to the reorganisation of the crystalline lamellae as a consequence of annealing phenomena, in agreement with the chain folding theory.12,13 The composition dependence of crystalline layer is a decreasing one when HOCP content increases and this behaviour can be rationalised in terms of a disturbance effect played by the HOCP to iPP crystallisation.5 As shown by Ruland,14 the second derivative of K(x) gives the distribution of distances between interfaces, in the case of an alternating layers’ array.

According to this approach, it is possible to derive information on the statistical distribution of thicknesses (i.e.

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amorphous, crystalline and total (long period) thicknesses). Let us call these size distribution functions ha(x), hc(x), and hLP(x), respectively. According to Ruland’s approach,14 K”(x) = 0.5 Os ∆η2 [ha(x)+h c(x)-2hLP(x)+.......]

(3)

where O s is the specific interfacial surface between amorphous and crystalline layers, ∆η is the contrast between the two phases and we truncated the expression for K(x) to the most important terms, disregarding contributes arising from interference among far more distant interfaces.

In figure 8 we present the temperature evolution of K”(x) for plain iPP. It is evident the raising of significative features in the range 7-10 nm as the temperature increases. It is the amorphous layers’ thickness which in-

Figure 2. Temperature dependance of the Lorentz corrected SAXS pattern as obtained from a 95/5 %w/w iPP-HOCP blend.

Figure 7. Composition dependence of Long Period, Rc, Ra and Rd at the various investigated temperatures: 25 (h), 50 (g), 75 (5), 95 (6), 105 (u), 120 (Y), 130 (l) °C.

creases in average size, thus separating from the crystalline layer thickness distribution. In figure 9, we present a fit of a typical K”(x), in terms of the model indicated in Eq. 3. The hi(x)’s are described in terms of gaussian functions. The whole set of data was analysed in terms of the fitting procedure illustrated in figure 9. The resulting structural parameters are in agreement with the corresponding quantities as derived from the analysis presented in figure 3.

The Ruland’s method is an obvious improvement over the simple approach of figure 3, as it leads to a knowledge of the hi(x) functions, thus giving an indication of the evolution of the thicknesses populations and, therefore, to the maximum amount of information that one can extract from scattering data.

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Figure 5. K(x) temperature evolution as obtained from Fourier inversion of Lorentz corrected SAXS patterns from a 95/5 %w/w iPP-HOCP blend.

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Figure 8. Temperature dependence of the second derivative of K(x), K"(x), for the case of an out of gap blend (90/10 %w/w iPP-HOCP blend).

Figure 12. Temperature evolution of K(x) as obtained for a 60/40 %w/w iPP-HOCP blend.

Figure 10. Composition dependence of the Lorentz corrected SAXS pattern as obtained from iPP-HOCP blend at 105°C. As soon as the miscibility gap is entered, a new feature appears.

We can conclude that a wise combination of the two analysis approaches we outlined can lead to a fairly complete description of the polymer morphology, in the case of the schematic structure reported in figure 4.

On the other hand, we previously anticipated that the very same kind of analysis could not be so much easy when two different amorphous layers can alternate with the crystalline ones.2 The picture we have in mind to describe the morphology of in gap blends is the following: two different kinds of mesodomains do coexist, differing just on the composition of the amorphous phase (and consequently on the structural parameters!). If this picture is correct we should expect that the SAS patterns present evidences of the occurring of two different periodicities. In order to describe the morphology of in gap blends we run both SAXS and SANS measurements on these HOCP-rich blends. In figure 10, we present the composition evolution of the Lorentz corrected SAXS pattern as obtained for a temperature treatment at 105°C. Out of gap blends do show a simple behaviour like the one presented in figure 2. On the other side, as soon as the composition enters the range inside the miscibility gap a peculiar behaviour can be observed, as a new feature appear in the lower Q range. This is a strong clue of the presence of a new periodicity, appearing as soon as the miscibility gap is entered.

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Anyway SAXS derived K(x)'s are not directly interpretable in terms of the model inherent in equation 4: in figure 11, a comparison is reported between SAXS and SANS derived K(x) for the very same sample (60/40 %w/w iPP-HOCP blend aged at 105°C).

In figure 12, the SANS derived K(x) temperature dependence is reported for a 60/40 %w/w iPP-HOCP blend. It is apparent that as soon as the temperature enters the immiscibility range, a new feature appears and experimental K(x) can be interpreted as built up by two different contributes: Kexp(x) = Ca * Ka(x) + Cb * Kb(x) where Ka(x) and Kb(x) are the electronic density autocorrelation functions of each of the two coexisting mesodomains. It appears that as soon as the miscibility gap is entered, a new relative minimum sharpens more and more as the temperature raises. At the moment, to our knowledge, no analysis approach has been found for distinguishing the two contributes to K(x) in the real space. Following we present a novel approach which can lead to this kind of information.2 We assumed that the LC SANS pattern can be separated into two different contributes arising from the constructive interference between radiation diffused by lamellae belonging to the two different domains. In this context, we fitted the LC data with two gaussians, each corresponding to the scattering from a different lamellar domain:  (q − q 2 ) 2   (q − q 1 ) 2  q 2 I(q ) = A 1 exp − + A 2 exp −  2 2c 1  2c 22   

(4)

The result of this approach is presented for a particular case in figure 13.

Results of comparable fitting quality have been obtained for the other cases.

The resulting gaussian curves have been Fourier inverted to derive the two K(x)’s (Ka(x) and Kb(x)) building up the experimental one. In figure 14 we present the result of this procedure and a comparison between experimental K(x) and Kfit(x)=Ka(x)+Kb(x). At this stage, the Ki(x) functions can be analysed in terms of the out of gap model presented in figure 3 and the lamellar parameters for the two different domains are extracted, thus leading to a complete insight into the bimodal lamellar populations in the samples. In figure 15 some of the structural parameters evolutions are presented.

For sake of completeness, we must cite that the very same kind of analysis that we presented above, based on the Ruland’s analysis approach, could not be applied to extract the very same amount of information in the case of in gap blends. This is a consequence of the complexity of experimental K(x), whose second derivative could not have been interpreted in terms of a simple combination

36

Figure 9. Fitting of K"(x) with three gaussian functions, according to the Ruland's approach (see the text for details).

Figure 11. Comparison between SAXS and SANS derived K(x) for the very same 60/40 % w/w iPP-HOCP blend at 105°C.

of gaussian functions: as too many kinds of layers have similar thicknesses, K”(x) geometrical features are not reliably interpretable in terms of the simple relation such as equation 3.

The analysis approach that we previously outlined can also be applied to HDPE-HOCP blends. As we noted, the HDPE-HOCP phase diagram is different from the iPP-HOCP one, as there are evidences that the miscibility gap is far wider. We collected SANS data for one HDPE/HOCP blend composition and a further study describing the morphology composition dependence is currently running. In the following we present the result of the application of the above described analysis method to SANS data from a HDPE/HOCP blend.6 40/60 %w/w HDPE/HOCP blends show more complex LC SANS patterns than the iPP ones. In fact, the main peak is built up of three Gaussian contributions (see Figure 16-a for a 40/60 %w/w HDPE/HOCP blend at 105°C). As only two of them can be rationalized in terms of interference peaks arising from different lamellar morphologies (the thermodynamic state

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transform K(x)’s. As a result, lamellar, crystalline and amorphous layers average thicknesses were derived and their temperature dependance is shown in Figure 18. The results shown in Figure 18 seem to indicate that on a mesoscopic scale blend’s morphology is not appreciably influenced by temperature (at least in the studied temperature range: 40°<T<120°C), all the structural parameters remaining practically unaltered.

Further studies are in progress in order to rationalize these observations.

Moreover other HDPE/HOCP compositions are going to be analysed to investigate any composition effect on blends’ morphology.

Figure 13. Example of the fitting procedure used to analyse Lorentz corrected SANS pattern from an in gap blend (60/40 %w/w iPP-HOCP at 105°C). See the text for details.

lying inside the miscibility gap), one must conclude that the third one corresponds to a further structure function peak. We argue that peaks corresponding to the same structural periodicity should be characterized by the same value of the contrast between amorphous and crystalline phases, ∆η2. Analysis of K(x)’s leads to values of this parameter and so we are able to determine whether some of the peaks building up the LC SANS data correspond to the same periodicity. In Figure 17 we show the temperature evolution of the parameter ∆η2, as derived from the analysis of the three K(x)’s, Fourier transform of the Gaussians buiding up

Figure 14. Result of the Fourier inversion of the gaussian functions obtained from the fitting procedure described in figure 13. The agreement between experimental data and the model is fairly good.

the interference peak in Figure 16-a (see Figure 16-b).

It can be noticed that two K(x)’s are characterized by the same value of contrast η(ETA), the third K(x) having a substancially different value for this parameter. This result seems to indicate that the structural information on lamellar domains can be gained by simply analyzing two Gaussians in Figure 16-a (strictly speaking the first and the third Gaussians), and their Fourier

Conclusion In this paper we presented detailed structural investigation of the mesoscopic morphology occurring in semicrystalline polymer blends (iPP/HOCP and HDPE/HOCP). This polymer blends show complex phase diagrams, as they present miscibility gaps, where two different amorphous phases can intercalate between the crystalline phases.

Blends lying outside the miscibility gap show Lorentz corrected SAXS patterns which can be interpreted in terms of a pseudo two-phase model, consisting of alternating crystalline and amorphous layers. In the case of iPP-HOCP blends, structural parameters for the lamellar morphology have been presented as functions of both composition and temperature.

As soon as the mixture enters the miscibility gap, a demixing occurs and the out of gap lamellar morphology becomes bimodal. As a consequence far more complex LC SAXS patterns are obtained. Blends lying inside the miscibility gap were then studied by means of SANS technique. In order to rationalise the experimental results we presented a novel analysis approach leading to a separation of the different contributes to the scattering pattern.

Each of these separate contributes has then been interpreted in terms of the aforementioned pseudo two phase model, thus leading to a description of the structural parameters of the two coexisting lamellar mesodomains.

Moreover, out of gap data were analysed in terms of the Ruland’s approach which leads to a statistical description of the layers’ thicknesses populations. This approach led to a substantial agreement with the results obtained from K(x) analysis. On the other hand, the same kind of analysis was not possible on in gap blends data, as a consequence of the substancial superposition of contributes arising from different layers.

HDPE-HOCP blends present far more complex SANS patterns, as further scattering orders could be appreciated due to the high regularity in the lamellar morphology. Notwithstanding with this complexity, we were able to subtract the contribute arising from the successive scattering order and derive the structural parameters for the two coexisting lamellar domains.

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Figure 18. Temperature dependence of the structural parameters for a 40/60 %w/w HDPE-HOCP blend, as derived from K(x)'s analysis (see the text for details).

Figure 15. Temperature evolution of some structural parameters as obtained for in gap iPP-HOCP blends. See the text for details.

The method we have been presenting since now allows to distinguish between different coexisting structural mesodomains. In principle the method is applicable to any kind of morphology (it is not limited to the lamellar one) and allows to derive the structural parameters for the assumed model.

References 1. See for example: A. Ryan, J. L. Stanford, W. Bras e T. M. W. Nye, Polymer, 759, 38 (1997) 2. For a more detailed study refer to the following papers: a) A. Triolo et al., Polymer, 39, 1697 (1998); b) A. Triolo et al., Macromolecules submitted for pubblication 3. A. Triolo, PhD Thesis, 1998 4. a) Cimmino S.; Di Pace E.; Karasz F. E.; Martuscelli E.; Silvestre C. Polymer 1993, 34, 972; b) Cimmino, S.; Martuscelli, E.; Silvestre, C. Macromol. Symp. 1994, 78, 115 5. Caponetti E.; D. Chillura Martino D.; Cimmino S.; Floriano M. A.;

Figure 16. Example of the fitting approach used to analyse 40/60 %w/w HDPE-HOCP Lorentz corrected SANS patterns. In the inset the corresponding K(x)'s are reported. See the text for details.

Martuscelli E.; Silvestre C.; Triolo R. J. Mol. Struct., 383, 75 (1996) 6. A. Triolo, J. S. Lin and R. Triolo, Physica A 249, 362 (1998) 7. W. C. Koehler; Physica (Utrecht) 137B, 320 (1986) 8. G. D. Wignall, J.S. Lin and S. Spooner; J. Appl. Cryst. 241, 23 (1990) 9. G. D. Wignall and F. S. Bates; J. Appl. Cryst. 28, 20 (1986) 10. C. G. Vonk and G. Kortleve; Kolloid-Z. u. Z. Polymer, 220, 19 (1967) 11. G. R. Strobl and M. Schneider; J. Polim. Sci.: Phys. Ed. 18, 1343 (1980) 12. L. Mandelkern, ‘Crystallization of Polymers’, McGraw-Hill, New York, 1964 13. C. Silvestre, S. Cimmino, E. Di Pace in ‘The Polymeric Materials Encyclopedia: Synthesis, Properties and Applications’, J. C. Salamone (Edt.), CRC Press, Inc., 1996 (pgg. 689-703) 14. W. Ruland, Colloid Polym. Sci., 255, 417 (1977)

Figure 17. Temperature dependence of the contrast as derived from K(x)'s analysis.

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Articolo ricevuto in redazione nel mese di Aprile 1998

STATUS OF THE TOSCA PROJECT: THE BACK-SCATTERING PORTION TOSCA-1 M. Zoppi Consiglio Nazionale delle Ricerche, Istituto di Elettronica Quantistica - Firenze, Italy M. Celli Consiglio Nazionale delle Ricerche, Istituto di Elettronica Quantistica - Firenze, Italy C. Checchi L.O.T.O. - Firenze, Italy D. Colognesi Consiglio Nazionale delle Ricerche, ISIS - Rutherford Laboratory-Chilton - Didcot, U.K.

V. Rossi Albertini Consiglio Nazionale delle Ricerche Istituto di Struttura della Materia - Roma, Italy S. Contarini, L. De Angelis, C. Gariazzo e P. Nataletti ENIricerche, Monterotondo - Roma, Italy M. Baciocchi, E. Difabrizio e M. Gentili CNR-IESS - Roma, Italy

Introduction TOSCA-1 is the back scattering section of the TOSCA spectrometer. This instrument will substitute the TFXA spectrometer at ISIS till year 2000, when the sample position will be removed from the present 12 meters distance from the moderator to the planned 17 meters. In that occasion, the forward scattering section of the instrument, TOSCA-2, will be installed.

In these notes, we give a detailed technical description of the basic construction characteristics of the instrument and of its mechanical and vacuum components that were build and assembled in Italy. A detailed description of the acquisition electronics and the final instrument characteristics will be given in a different note.

TOSCA-1 is the result of the joined work of the Tosca Project Committee. That is a mixed team of scientists from Italy and the United Kingdom.

On the Italian side, the Committee is chaired by F.P. Ricci (Università di Roma III, Italy).

This is composed by: R. Caciuffo (Università di Ancona), O. Moze (Università di Parma), M. Celli and M. Zoppi (CNR-IEQ, Firenze), C. Petrillo and F. Sacchetti (Università di Perugia), F. Cilloco and V. Rossi (CNR-ISM, Roma).

On the British side, the Committee is chaired by A.D. Taylor (ISIS, Rutherford Laboratory, U.K.) and is composed by: C.J. Carlile, J. Tomkinson, S. Parker, Z. Bowden, M. Krendler and T. Pike (ISIS, Rutherford Laboratory, U.K.), and R.J. Newport (University of Kent, U.K.).

The instrument has left Italy on February 16, 1998 and is actually at ISIS, for the mounting of the data acquisition electronics and the final wiring, under the direct supervision of the British components of the Project Committee and of D. Colognesi (CNR on duty at ISIS).

The commissioning is expected to start in mid April 1998 and the first neutrons should be seen before the end of this April cycle.

The instrument is expected to be fully operative starting on the October 1998 cycle.

Fig. 1. Front view of the instrument and its designer (C. Checchi). The sample chamber, in the center of the instrument, is surrounded by the ten monochromator modules.

General Description A picture of the TOSCA-1 is reported in Figs. 1 and 2 (front and back view, respectively).

The instrument has an inverse geometry design with the energy selection placed on the secondary path.

This is composed by ten monochromator modules, placed radially with respect to primary neutron beam.

The middle point of the large aluminium chamber, placed in the centre of the instrument, determines the sample position. This will be held at the end of the cold finger of a specially designed cryogenic equipment that will be fixed to the (vacuum-tight) flange that is visible on the top of the instrument.

The shape of the sample chamber has been specially designed in order to minimise the amount of air that the scattered neutrons find on their path to the detectors.

Each monochromator module is composed of a pyrolitic

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ne of the instrument, sitting on the same kinematic mountings, has been constructed.

How it works.

A schematic drawing of the instrument core is reported

Fig. 2. Back view of the instrument. The ten monochromator modules are connected by the center flange so that the graphite single crystals are all in the same plane. The closed circuit helium refrigerators are used to cool down the beryllium filters.

graphite crystal and a beryllium filter cooled at a temperature below 50K by a closed circuit helium refrigerator.

The neutrons that have been selected by the monochromator module are detected by fourteen 3He tubes placed beyond the output window of the module (see Fig. 1).

The sample chamber is fixed to the instrument frame in three ways. On the top side, the aluminium flange of the chamber is held (vacuum tight) by a stainless steel counterflange that is attached to the main frame. The large front flange of the chamber is also fixed by three steel legs to the instrument frame (see Fig. 1). Finally, a flange in the back, that is fixed to the chamber, is also connected to the frame by means of the ten steel covers of the monochromator modules (see Fig. 2).

On the back side of the instrument (Fig. 2) we can observe the ten monochromator modules and the close circuit helium refrigerators used to cool down the beryllium filters. Also visible, in the picture, are the windows holding the graphite single crystals that are placed on the same plane perpendicular to the primary neutron beam.

The machined surfaces on the external frame have been provided to give good mechanical reference points for the future installation of TOSCA-2.

The body of the instrument is obtained from a massive steel frame that has been carefully machined, after construction, in order to give precise mechanical reference points for the other instrument components. The whole instrument sits on a three-point basis (kinematic mounting) whose corresponding parts are placed on the steel basement. This, in turn, will be fixed to the ground in place of the former TFXA spectrometer. In order to obtain a rather good alignment of the instrument base, a dummy structure that mimics the longitudinal centre li-

40

Fig. 3. Schematic drawing of the instrument core (see text for a full explanation).

in Fig. 3. The vertical arrow indicates the primary (white) neutron beam while the sample position is in the center of the circle. The neutrons, scattered by the sample, enter the aluminium window (2 mm thickness) and hit the graphite single crystal. Here, only those neutrons satisfying the Bragg condition will be reflected by the graphite while the others will be absorbed by a thin (1 mm thickness) layer of cadmium placed behind the crystal. As the interplane distance of graphite is 3.354 Å and the scattering angle on the graphite is 45°, the Bragg condition selects those neutrons whose wavelength satisfies the equation Nλ = 4.743 Å (where N is an integer). The following filter (a massive beryllium block, cooled below the liquid nitrogen temperature) will remove the higher order harmonics from the secondary beam. Therefore, only the fundamental reflection (N=1) will travel freely through the filter and will reach the 3He detectors placed beyond the thin aluminium output window (2 mm thick).

The detector section is made of fourteen 3He tubes (squashed to increase the time of flight resolution) covering the whole effective surface of the output window and placed on a plane that is parallel to that of the graphite crystals (cf. Fig. 3). Since both, the plane of the analysing crystals and that of the detectors, are perpendicular to the primary beam direction, the system fulfils a time-focusing configuration. This is such that neutrons arriving on any detector, at the same time, are characterised by the same transit time on the sample target, i.e. the same incident energy. Therefore, a Time Of Flight (TOF) analysis becomes equivalent to a sweep in the va-

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lue of incident energy, with an energy resolution of the secondary neutrons that is determined, in the end, only by the crystallographic quality of the graphite analysers.

Thus, the peculiar characteristic that has to be fulfilled in the whole instrument is the parallelism between the plane of the graphite single crystals and that of the 3He detectors. As a consequence, a very good mechanical precision was necessary in building the whole instrument.

This should be better than a few fraction of a mm, over an overall size of more than one meter. This mechanical accuracy was seeked, during the construction, and tested in the final configuration of the instrument.

The sample chamber(s) Due to the need of increasing the count rate of the scattered neutrons to the maximum available level, the sample

Fig. 4. The non-standard sample chamber. All the ten detector modules can be used with this chamber.

chamber of TOSCA could not be build following the standard size adopted by the Sample Environment Division of RAL. In fact, to obey the standard configuration (sample chamber top flange of 400 mm diameter, 300 mm apart on the vertical of the beam) it would have implied removing the two top detection modules, i.e. loosing 20% of the designed count rate. This was a hard decision to take. Therefore, it was agreed to build two sample chambers. The decision was dictated by the need of fulfilling two distinct and conflicting requirements. In order to exploit the whole instrument potential, as far as the count rate is concerned, a smaller flange chamber was designed. This allows to use all the ten detection modules and to take advantage of the whole solid angle that is available for TOSCA-1. On the other hand, in order to avoid renouncing definitely to the possibility of using the standard equipment that is available from the Sample Environment Division, it was decided to leave an open option by building the second sample chamber with a standard flange. In case of need this can be used upon removing the two highest detector modules of the instrument.

The two chambers are qualitatively of the same shape, with a vertical cylindrical well welded into a horizontal cylindrical chamber (cf. Figs. 4 and 5). A large flat flange, on which the primary neutron output pipe is adapted, closes this chamber on one side (front end). On the back side, a cut pyramidal structure accommodates the output windows for the scattered neutrons and the flange where the primary neutron input pipe is adapted.

Both chambers are made of aluminium alloy and are shaped to optimise the efficiency of the detection system and to leave sufficient flexibility for future developments. The walls of the chambers have a minimum thickness of 6 mm. Only in correspondence with the output windows (in the cut pyramidal structure) the thickness has been reduced to 2 mm to reduce the absorption of neutrons in the secondary path. On the interior of each chamber, a number of aluminium rods have been welded to the walls to improve the mechanical stability.

In addition, a number of threaded holes have been provided on these rods to allow easy fixing of the absorbing material inside the sample chamber.

The monochromator module A not negligible source of problems was represented by the need to compact as much as possible the whole structure in order to obtain a large solid angle in the collection of the scattered neutrons. This was a very demanding task as we had to provide a large space inside the monochromator modules while trying to keep the external size of the modules as small as possible, leaving a sufficient space in the interior to avoid thermal leakage between the cold surfaces and the walls of the modules.

Fig. 5. The standard sample chamber. Only eight detector modules can be used with this chamber.

A picture of three assembled monochromator modules, waiting for the beryllium filters, is shown in Fig. 6. The Closed Circuit helium Refrigerator (CCR) is welded to

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The beryllium filter (a block of 121.4x121.4x151.4 mm3) is fastened to the cold surface of the CCR using a copper plate (Fig. 8). Two relatively wide edges, on the lateral sides, are used to clamp the beryllium using a steel plate, placed on the opposite face, using steel threaded rods.

Since the thermal dilatation of beryllium is very similar to that of steel, no loosing of the clamp is expected upon thermal cycling. However, since the beryllium is wrapped in a cadmium sheet, this possibility cannot be excluded a priori . Therefore, on the longitudinal sides, two small edges were provided, both on the copper and the steel plates, to keep the beryllium block from sliding.

Two thermal sensors (Pt100) have been placed in the lower (copper) and upper (steel) plates to monitor the beryllium temperature. The wires are connected to a Fig.6. Three monochromator modules waiting for the beryllium filters.

Fig. 8. Fixing the beryllium filter in good thermal contact with the cold finger of the Closed Circuit Refrigerator.

Fig. 7. The graphite single crystals are fixed to the inspection window using cadmium clamps.

the stainless steel cover of the vacuum-tight aluminium chamber. This is composed by aluminium walls (6 mm thick) welded together. The connection with the steel cover is obtained using an O-Ring seal. The box is vacuum tight and contains the monochromating devices (cf. Fig. 3). The graphite single crystals, composed by 4 pieces 50x50x2 mm3 are fixed to an aluminium inspection window using cadmium clamps. The crystals have been cut to a trapezoidal shape (with the two basis of 100 and 80 mm and height of 100 mm) to save space in the region close to sample chamber. The four single crystals are kept together in the centre by an aluminium screw.

Behind the graphite, a cadmium sheet of 1 mm thickness absorbs the neutrons that do not satisfy the Bragg condition (see Fig. 7).

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standard vacuum feedthrough connector.

The inside walls of the modules have been fully covered with a 1 mm sheet of cadmium. Only the input and the output windows were left free of cadmium. The sheets of cadmium are fixed to the walls using a composite method. On the top of the aluminium box, i.e. on the massive border close to the cover, we used eight aluminium screws. On the bottom and on the side walls, the thinness of the material prevented us to use the screws and we had to glue the cadmium sheet to the walls using a small amount of a two-components glue. Finally, in order to avoid plastic relaxation of the cadmium sheets on the components of the monochromator modulus, the various sheets of cadmium covering the internal walls were connected together, using tin soldering1 (see Fig. 9).

The inside wall of the steel plate cap, as well as the interior of the cylindrical wall of the CCR and of the vacuum port, were also covered with cadmium sheet. This was fixed to the steel cap using six aluminium screws. In addition, it was tin-soldered to the cylindrical shaped sheets of cadmium that were placed around the walls of

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the CCR and the vacuum port. The resulting structure is believed to remain in place for rather long periods.

Due to the reduced available space, the walls of the vacuum chamber had to be designed very close (~10 mm) to the cold edge of the beryllium block. The subsequent

Fig. 9. The tin soldering of the cadmium sheets was carried out wearing a scuba-diving equipment.

covering of the inside walls of the modules with cadmium sheet (1 mm thickness) made the problem almost dramatic. Luckily, a test carried out on a prototype has shown that the left distance (6 mm) is large enough to avoid heat losses from the beryllium filter to the aluminium wall. The temperature of the beryllium block was checked using the two platinum sensors. The measured temperatures were 35 and 45 K respectively and no particular cooling of the external wall, in correspondence with the beryllium block, was observed. Also, the relatively small temperature difference of 10 K makes us confident that no thermal leak is present.

The Vacuum System The monochromator modules need to be evacuated. This is obtained by connecting the various modules in parallel, and then connecting the line to a suitable vacuum port that is placed in the bottom centre of the frame. In order to keep the vacuum impedance of the line within reasonable limits we used Pneurop KF40 flanges for all the vacuum connectors. The vacuum source is obtained using a turbomolecular pump placed on the instrument frame as close as possible to the modules. A primary pump with a pumping speed of 75 l/m completes the vacuum circuit.

It is important to keep the vacuum system of the modules separated from that of the sample chamber. This, in order to avoid changing the temperature of the beryllium filter on each sample change. The vacuum system of the sample chamber is obtained fitting a turbomolecular pump on one of the primary neutron beam vacuum pipes.

The primary pump for this system has a pumping speed of 170 l/m.

In both cases the vacuum level is measured using a combination of two gauges, namely a thermocouple and a cold cathode type. Two pairs are placed on either end of the vacuum chain of the modules thus giving a measure of the pressure in the worst situation. A third pair will be placed on the primary beam vacuum pipe. In either case, the reading instrument is provided with an RS232 interface for easy connection to the control electronics.

The Detection Section The detector modules are composed by ten groups of fourteen 3He tubes. Each modulus is mechanically and electronically independent from the others and includes the preamplifier electronics, so as to reduce the electronic noise. The detector modules are fixed to the machined surfaces that are provided on the instrument frame.

This allows obtaining a good coplanarity among the various modules and the required parallelism with the plane of the graphite single crystals. However, this requirement is not crucial as each single 3He detector will be calibrated independently (see the following sections).

Each 3He detector has an effective area of 14.5x100 mm2.

Therefore, the effective detector surface covers an area of 203x100 mm2. As it was mentioned before, the 3He tubes are squashed in order to reduce the indetermination on the TOF of the collected neutrons. The actual effective thickness of the active gas is 2.5 mm. Therefore, care should be taken in planning the sample thickness if high resolution measurements need to be carried out.

The calibration procedure The actual calibration of TOSCA-1 will be carried out during the commissioning period and is not available yet. However, preliminary steps have been carried out to make this procedure as smooth as possible.

In an inverted geometry inelastic machine, like TOSCA, the energy transfer from the scattered neutron to the sample, E, is a highly non-linear function of four main variables: the flight path from the moderator to the sample, L0, the flight path from the sample to the detector, L1, the final energy, E1, and the total TOF of the neutrons from the moderator to the detector,t . It is fairly simple to measure precisely the first quantity, L0, using surveying methods, while the data acquisition electronics gives t. It should be pointed out that both, E1 and L1, can be rather different passing from one detector tube to another and their direct determination cannot be very accurate, especially E1 which is function of the incidence angle on the graphite analyser. This is the reasons why an indirect calibration procedure is more effective.

The indirect calibration procedure that will be used makes use of a calibrant sample. For an inelastic spectrometer the ideal calibrant is a substance showing a large set of clear and narrow peaks, corresponding to values

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of the energy transfer that are independent of the scattering angle. The relevant energy range of TOSCA is between 100 and 3500 cm-1. After a long and accurate study, an organic molecular crystal, the 2,5 diiodothiophene, was chosen as calibrant because of a number of advantages: -the substance shows several strong and sharp vibrational features (involving proton motions) extended from 22 to 3110 cm-1; -the heavy molecular weight implies a small amplitude of the phonon wing associated to each vibrational peak; -the substance has a relatively low cost; -the molecular structure is relatively simple, making a computer simulation possible and reliable; -good and well resolved optical spectroscopic data, both infrared and Raman, provide a precise knowledge of the energy associated to the main internal molecular vibrations.

A preliminary study was carried out on the formal aspects of the calibration procedure. The old TFXA instrument was originally calibrated making use of only two spectrum features, i.e. the elastic line and one inelastic peak. In this way, the determination of E1 and L1 is quite simple and direct but, as a drawback, the irregular and broad shape of the elastic line and the possible errors in the determination of the inelastic one, make this procedure rather unstable and not highly accurate, especially if the contributions coming from the different detectors are added together. On the contrary, using several different inelastic peaks (together with the elastic one) appears as a better way to achieve an accurate, stable and reliable calibration procedure. This is specially needed for TOSCA, owing to its large numbers of detectors and its better energy resolution.

The effect of using a superabundant set of measurements, all affected by statistical errors, was carefully analysed. The uncertainty originated from these errors and propagated to E1 and L1 was estimated, paying special attention to the strong non-linear relations between measured quantities and calibration parameters. It was shown, for instance, that a precise determination of E1 requires, above all, low energy transfer peaks.

On the contrary, L1 is rather insensitive to the value of the energy transfer, E. At the end of this preliminary study, a software code was prepared for practical aims.

This procedure allows the inclusion of a large number of peaks, together with their experimental errors (in TOF) and the respective energy values and uncertainties coming from the optical spectroscopy data.

At the moment the procedure has been tested only using old TFXA data and we were able to reproduce, and improve, the evaluation of the calibration parameters for this instrument. However, during these tests, some difficulties emerged and subsequent steps were undertaken to improve further the procedure. In fact, a careful analysis of the optical data showed the existence of a small splitting for most of the vibration peaks.

The splitting is likely attributable due to the in-phase and

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anti-phase motion of the two molecules contained in the elementary cell of the lattice. Of course, the existence of twin peaks causes an additional uncertainty on the attribution of the energy transfer. In fact, the low resolution neutron spectra do not show splittings and the two modes are merged together. However, the measured amplitude of the lines could sensitively differ from the Raman optical data.

This problem was not particularly important in the case of TFXA. However, it is expected to become sensible in the case of TOSCA, owing to the increased resolving power.

Thus we used simulation and fitting code, named CLIMAX, that is specially designed for molecular vibrational studies. In this way this last uncertainty on the calibration procedure has been removed and all the tools are ready for the calibration of TOSCA that is expected to be carried out before the end of this April cycle.

We conclude, showing in Fig. 10 the calculated energy resolution of TOSCA-1. This is evaluated by assuming that the sample position is 17 meters apart from the moderator and that the sample thickness is of the same size of the detectors. We are confident that the final figures will agree with the expectations.

Fig.10. Expected energy resolution of TOSCA-1 in its final configuration.

As a comparison, we report the energy resolution of the former TFXA spectrometer.

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CONVEGNO “HIGHLIGHTS IN X-RAY SYNCHROTRON RADIATION RESEARCH” In occasione del cinquantesimo anniversario della prima osservazione della radiazione di sincrotrone, effettuata negli Stati Uniti nei laboratori della General Electric, la European Synchrotron Radiation Facility (ESRF) ha organizzato una conferenza a Grenoble che si è tenuta nei giorni 17-20 novembre 1997 ed è stata intitolata "Highlights in Synchrotron Radiation Research" alla quale hanno partecipato circa 200 ricercatori.

Il saluto iniziale è stato data da Y. Petroff che ha annunziato l'intenzione di ESRF di organizzare questo tipo di convegno con scadenza triennale. Subito dopo B.W. Batterman (CHESS, Cornell Univ., USA) ha ricordato le tappe fondamentali del progresso della ricerca basata sulla Radiazione di Sincrotrone (RS) in questi cinquanta anni; tuttavia va detto che solo dalla platea (in particolare F. Sette) è arrivato il giusto riconoscimento a B. Touschek per il suo lavoro a Frascati con ADA. Le prospettive per il futuro sono state illustrate da J.M. Filhol (ESRF, Grenoble) che si è soffermato sulle caratteristiche di emittanza e della lunghezza dei pacchetti di elettroni delle macchine di quarta generazione e, naturalmente, sull'utilizzo di LINAC per la messa a punto di laser ad elettroni liberi.

La prima giornata del convegno è stata dedicata al magnetismo, dove la RS ha rappresentato, e rappresenta tuttora, un elemento fondamentale per il progresso delle conoscenze. In questa sessione sono stati presentati contributi teorici (M. Blume, BNL, USA; P. Carra, ESRF) e sperimentali (C.T. Chen, SRRC, Taiwan; Kisker, Dusseldorf, Germania; J.B. Goedoop, ESRF, Grenoble). G. A. Sawatzky (Groningen, Olanda) ha parlato di materiali caratterizzati da forti interazioni "spin-orbit". La giornata è terminata con la prima delle tre ricchissime sessioni poster.

La seconda giornata è stata dedicata a studi di diffusione e di diffrazione e si è aperta con contributi dedicati ancora al magnetismo da parte di D. Gibbs (BNL, USA) e di S. Ferrer (ESRF, Grenoble) che ha illustrato il metodo della diffrazione risonante per lo studio del magnetismo di superficie.

Gli aspetti connessi con la coerenza e/o la non- coerenza della radiazione sono stati illustrati da S.K. Sinha (ANL, USA). E’ stata quindi presentata una serie di contributi sullo studio delle fasi condensate che hanno illustrato la potenzialità della RS nello studio della dinamica

(G. Grubel e F. Sette, ESRF, Grenoble), delle superfici e delle interfacce (J. Daillant, CEA Saclay, Francia), della cristallizzazione e della microstruttura dei polimeri (A.J. Ryan, Sheffield, UK).

Infine, E. Burkel (Rostock, Germania) ha parlato sullo scattering anelastico da parte di fononi.

La giornata si è conclusa con la seconda sessione poster.

La terza giornata è stata organizzata in due sessioni parallele in modo da dare spazio ad un simposio sulla struttura dei sistemi biologici.

Per introdurre questo simposio parallelo, il convegno principale si è aperto con una conferenza di W.A. Hendrickson (Columbia Univ, N.Y., USA) sulla diffrazione anomala a più lunghezze d'onda (MAD) applicata allo studio di macromolecole biologiche. Nel seguito della giornata, il convegno principale è proseguito con contributi dedicati ad argomenti per lo più "esotici". J. Trumper (Garching, Germania) ha illustrato come la RS può aiutare gli studi di astrofisica nel campo dei raggi x soprattutto per la messa a punto di equipaggiamenti da montare su satelliti. J.R. Schneider (DESY, Amburgo) ha parlato sull'uso della RS ad alta energia per lo studio delle proprietà interne di campioni relativamente grandi. E. Gerdau (Amburgo, Germania) ha introdotto le nuove potenzialità della spettroscopia Mossbauer.

R.M. Martin ha intrattenuto i partecipanti parlando di "idrogeno metallico" mentre H.K. Mao (Washington, USA) e D. Andrault (Parigi, Francia) hanno illustrato in cosa può contribuire la RS per la modellizzazione dell'interno del pianeta Terra. La conferenza seguente (M. Parrinello, Stuttgart, Germania) è stata di carattere teorico ed ha avuto come soggetto il "legame idrogeno" il cui studio ancora propone contributi interessanti in studi chimico-fisici e biochimici sulla struttura e la dinamica dell'acqua. Infine P. Loubeyre (Parigi, Francia) ha parlato di esperimenti di diffrazione ad alta pressione su solidi contenenti elementi leggeri mediante l'incudine a diamante.

Il simposio parallelo sulla struttura delle molecole e dei sistemi biologici ha visto una larga partecipazione: almeno metà dei partecipanti all'intero convegno hanno affollato la saletta dedicata al simposio. Senza entrare nel dettaglio delle singole presentazioni, quello che ha colpito è stato il livello di complessità delle strutture che sono state mostrate (virus, fagi, ribosomi e nucleosomi) per le quali sono state messi in evidenza i dettagli strutturali di interazioni che hanno un notevole interesse biomedico (p. es. tra virus e recettori, tra virus e composti

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antivirali, tra proteine ribosomiali e molecole di RNA) e di importanti meccanismi biochimici come l'espressione genica in eucarioti.

La giornata si è chiusa con una visita a ESRF ed una cena servita in un castello del XVIII secolo a Le Touvet a metà strada tra Grenoble e Chambery.

La quarta ed ultima giornata è stata ancora strutturata in due sessioni parallele: la prima era dedicata alla microscopia, mentre la seconda era la continuazione del simposio sulla struttura delle macromolecole biologiche.

Prima delle sessioni parallele, S. Doniach (Standford, USA) ha illustrato le possibilità dello scattering a basso angolo risolto in tempo applicato allo studio del ripiegamento e dell'associazione di sistemi proteici.

Nella sessione sulle microscopia, U. Bonse (Dortmund, Germania) e S. Wilkins (Melbourne, Australia) hanno parlato sulla microscopia in contrasto di fase, mentre A. Momose (Hitachi Ltd, Giappone) ha illustrato i vantaggi di questa tecnica per lo studio di tessuti biologici molli.

A. Snigirev (ESRF, Grenoble) ha commentato la possibilità offerta dalle macchine di terza generazione per quel che riguarda lo sviluppo di un'ottica coerente nel campo delle alte energie, mentre P. Spanne (ESRF, Grenoble) ha parlato dell'ottenimento di immagini in campo medico.

Infine, J Gronkowski (Varsavia, Polonia) e J. Baruchel (ESRF, Grenoble) si sono soffermati su alcuni aspetti della topografia.

L'ultima parte del simposio paralello sulle molecole biologiche ha riguardato problemi strutturali legati a membrane ed alla cristallografia risolta in tempo.

Il convegno si è concluso con la terza sessione poster e con l'assegnazione del Premio della Società Europea di Radiazione di Sincrotrone (ESRS), assegnato al giovane M.I. McMahon, Royal Society Research Fellow presso il Dipartimento di Fisica dell'Universtità di Liverpool, per i suoi studi sulla struttura delle fasi condensate ad alta pressione.

In conclusione, esprimendo il mio parere personale, il convegno è stata un'ottima occasione per avere una panoramica generale su quello che si riesce a fare oggi con la RS e quello che si potrà fare in un futuro prossimo. Le sessioni poster, inoltre, hanno rappresentato bene il luogo dove discutere di problemi scientifici specifici.

(Roberto Rizzo)

test della installazione di lenti rifrattive al Be dentro i "front-end"; - lo spostamento della linea dedicata al MAD da magnete curvo a sezione dritta; la costruzione della linea "MEDEA" dedicata ad analisi in traccia su wafer di semiconduttori; la proposta di costruzione di una nuova beamline per risonanza nucleare; - la possibilità di fare richiesta di tempo macchina sulla base di progetti a lungo termine (2 anni): a riguardo è disponibile in rete un modulo specifico; - il programma di accesso rapido per misure XAFS sulla linea BM 29.

Il programma è proseguito con tre relazioni scientifiche su invito. A. Hiess ha presentato misure di diffusione magnetica risonante su film sottili (1000 A) di UPd2Al3, discutendo il rapporto tra magnetismo e superconduttività. Lo studio del danneggiamento in materiali microeterogenei mediante tomografia in contrasto di fase è stato descritto da J.Y. Buffiere; egli ha spiegato com questa tecnica permetta di ottenere informazioni nuove sulle fratture indotte dalla deformazione plastica in compositi quali Al/SiC. Infine M.J. Zwanenburg ha illustrato le proprietà di una guida d'onda per raggi X coerenti con gap di aria variabile, sottolineando come sia possibile selezionare i diversi modi nella cavità variando la spaziatura tra le superfici delimitanti la guida stessa.

Come ogni anno è stato assegnato il "Young Scientist Award"; quest'anno il vincitore è stato Michael Thoms, per lo sviluppo di un image plate con tempo di lettura di 12 secondi, realizzato nell'ambito delle ricerche in alta pressione.

Lo Users’ Meeting è stato anche l'occasione della presentazione del nuovo Council della Users’ Organization di ESRF. Sono stati eletti in questo organismo: M. Cooper (Chairman), J. Cockroft, D. Nicholson, A. Kaprolat, P. Armand, F. Boscherini e M. Frey.

Nel pomeriggio si sono svolti tre mini-workshops i cui titoli illustrano alcuni degli argomenti di maggior interesse al momento ad ESRF: imaging (organizzatore: J. Baruchel), tecniche di assorbimento e diffusione con risoluzione temporale (M. Wulff), applicazioni industriali (J. Doucet).

Il prossimo Users’ Meeting si terrà nel febbraio 1999.

(Federico Boscherini)

ESRF USERS’ MEETING 1997 Il 21/11/97 si è svolto a Grenoble lo Users’ Meeting 1997. Questa edizione ha avuto la durata di solo un giorno in quanto preceduta dalla conferenza "Highlights in SR Research".

Lo Users’ Meeting è iniziato con gli interventi dei direttori della macchina e scientifici (J.M. Filhol, C. Kunz e P. Lindley) e di R. Mason per lo Users’ Office. Di particolare interesse per gli utenti: - l'installazione prevista di camere da vuoto per ondulatori di gap minimo 8 mm ed il

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EUROPEAN SPALLATION SOURCE (ESS) In questi ultimi anni un consorzio di istituzioni Europee e di Enti di ricerca delle singole nazioni interessate ha condotto, grazie ad un finanziamento comunitario, uno studio preliminare sul progetto di una nuova sorgente neutronica europea di terza generazione (European Spallation Source, ESS) che dovrebbe, alla fine del prossimo decennio, affiancarsi e successivamente sostituire le sorgenti attualmente esistenti, la maggior parte delle quali cesserà di essere operativa tra il 2010 ed il 2020.

Questa attività di studio si è conclusa nel 1996 ed ha prodotto un progetto di fattibilità per la nuova ‘facility’, con una valutazione dei costi approssimata al 20%; inoltre ha individuato le aree di ricerca che richiedono ancora uno sforzo di R&D per arrivare alla formulazione di un progetto definitivo che possa essere presentato ai governi dei paesi della UE e costituisca l’ultima fase di studio prima dell’avvio della fase di realizzazione nel caso di una decisione positiva da parte delle autorità politiche.

Per arrivare alla formulazione di un tale progetto i rappresentanti di cinque tra le più rilevanti Istituzioni Scientifiche Europee che operano nel campo della neutronica: - Commisariatá l’Energie Atomique (Francia) - Council for the Central Laboratory of the Research Councils (Gran Bretagna) - Forschunszentrum Jülich, GmbH (Germania) - Paul Scherrer Institute (Svizzera) - Risø National Laboratory (Danimarca) hanno siglato, nel corso del 1997, un “Memorandum of Understanding” (MoU) dando vita ad una Commissione denominata “ESS R&D Council”. Obiettivo del Council è quello di portare a compimento gli studi necessari per definire un progetto tecnico dettagliato della nuova sorgente neutronica europea. L’attività del ESS R&D Council, dovrebbe concludersi nel 2002 con la presentazione di un progetto che sia ancora indipendente dal sito di costruzione del laboratorio, ma che definisca tutti i punti e le opzioni che lo studio preliminare, concluso nel 1996, ha lasciato ancora aperti. In particolare le aree ove è necessaria ancora un’azione di R&D sono state individuate in: • Area 1: Linac (acceleratore di protoni), anelli di accumulazione e linee di trasferimento dei protoni; • Area 2: Targhette e moderatori • Area 3: Strumentazione

Le istituzioni che hanno sottoscritto il MoU si sono impegnate a svolgere ricerche tra loro coordinate in questi settori con l’obiettivo di arrivare alla formulazione del progetto operativo entro il 2002. Gli attuali membri del ESS R&D Council hanno inoltre invitato le istituzioni scientifiche che svolgono nei vari paesi attività di ricerca in campo neutronico e tra queste, per quanto riguarda l’Italia, il CNR e l’INFM, ad aderire al MoU rafforzando così l’azione intrapresa dai primi firmatari Va ricordato che nell’ambito delle attività del progetto ESS, concluso come detto nel’96, l’attività italiana è stata consistente e qualificata soprattutto se si tiene conto che gli investimenti italiani nella neutronica sono nettamente inferiori a quelli di altri paesi europei il cui PIL è confrontabile col nostro o anche inferiore. Ricercatori delle diverse Istituzioni Italiane operanti nel campo (CNR, INFM ed in misura minore INFN) hanno attivamente contribuito alla stesura del progetto ESS in particolare nei settori degli acceleratori (INFN - Area 1) e della strumentazione (CNR, INFM - Area 3).

Durante il meeting di Risø è stato anche definito un livello minimo delle dimensioni dell’attività di ricerca per essere ammessi a partecipare all’ESS R&D, e tale livello è stato valutato dell’ordine di 8-10 uomini/anno.

E’ stato inoltre previsto che più istituzioni di uno stesso paese possano far parte all’ESS R&D stabilendo però un numero massimo di due rappresentanti per nazione nel Council.

Dimensioni dell’attività italiana per attività di ricerca in ambito ESS R&D Nell’ambito delle attività del progetto ESS, concluso come detto nel’96, l’attività italiana è stata consistente e qualificata. Ricercatori di diverse Istituzioni (Università, CNR, INFN) hanno attivamente contribuito alla stesura del progetto ESS. In particolare i contributi italiani hanno riguardato il settore degli acceleratori (INFN) e della strumentazione (Università, CNR, INFM).

Le attività più recentemente sviluppate dal CNR (progetto TOSCA per la realizzazione di uno nuovo strumento per spettroscopia neutronica vibrazionale), a seguito della firma del nuovo accordo con il CLRC inglese per l’accesso alla sorgente ISIS, si inquadrano perfettamente nella tematica di sviluppo di nuova strumentazione (Area 3). In tale attività sono attualmente impegnati tre ricercatori dell’IEQ (Firenze), due ricercatori dell’ISM (Roma) ed un ricercatore, recentemente assunto con con-

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tratto ex. art. 36, che ha come sede di lavoro il Rutherford-Appleton Laboratory e che si occuperà specificamente del ‘commissioning’ di TOSCA. Parimenti le attività sviluppate in ambito INFM che riguardano la spettroscopia con neutroni epitermici (Progetto VESUVIO) e lo sviluppo di nuovi rivelatori (Network Europeo XENNI) sono perfettamente inquadrabili nel ESS R&D e coinvolgono un numero di ricercatori confrontabile con quello del CNR. Appare quindi possibile ed auspicabile una partecipazione italiana alla nuova iniziativa dell’ESS R&D tramite almeno i due enti maggiormente coinvolti nel settore della neutronica, vale a dire il CNR e l’INFM. I costi di partecipazione sono estremamente modesti in quanto non viene richiesto necessariamente l’avvio di nuove attività di ricerca nel settore, ma solo un coordinamento europeo delle attività in atto ed una loro ragionevole finalizzazione agli obiettivi del ESS R&D. L’attività complessiva dei due enti (CNR ed INFM) è stata definita molto positiva e sicuramente sufficiente a consentire l’ingresso italiano nel ESS R&D anche nel meeting del ESS R&D Council tenutosi a Riso (Danimarca) il 17/10/97.

Tale ingresso, senza comportare la necessità di ulteriori investimenti rispetto a quelli attuali, consentirebbe ai ricercatori italiani di mantenere il contatto con i colleghi stranieri che si occupano delle varie attività di interesse per il progetto ESS e di assicurare quindi al nostro paese il necessario know-how scientifico per partecipare alla sua realizzazione qualora, dopo le fasi di studio e progettazione, si passasse alla realizzazione di questa nuova facility europea.

Per quanto riguarda il CNR il Comitato Scienze Fisiche si e’ già espresso favorevolmente in tal senso, nella riunione del 7/11/97 invitando la Commissione di Spettroscopia Neutronica del CNR a predisporre un programma delle attività già in atto in ambito CNR ed inquadrabili nell'ambito dell'ESS, secondo le richieste del ESS R&D Council. Tale documento, preparato congiuntamente dalla Commissione di Spettroscopia Neutronica del CNR e dell’INFM (Allegato A) è stato trasmesso per l’approvazione agli Organi deliberanti del CNR. (Carla Andreani - Segretario Scientifico Comitato di Consulenza per le Scienze Fisiche del CNR)

Allegato A Italian Participation to Memorandum of Understanding ESS (European Spallation Source) CNR (Consiglio Nazionale delle Ricerche) and INFM (Istituto Nazionale per la Fisica della Materia) are the two major institutions which promote and co-ordinate activities in the field of neutron scattering, both providing, the major financial contribution for the Italian neutron scattering community to access the two major European neutron scattering facilities, respectively ISIS (UK) and ILL (France). Several accompanying measures are

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considered by the two institutions and each of them has its own development program as well as a collaboration development program.

In the present document we shall present the most important Italian development programs for the period ending in 2002 which are relevant to the ESS project. The Italian neutron scattering community is also involved in other development projects like for instance DIANE facility at Saclay and BRISP and IN13 at ILL. However these projects deal with steady sources and are of marginal interest to the Memorandum of Understanding (MoU).

1. CNR - TOSCA Project TOSCA (Time Of flight Spectrometer Crystal Analyser, see Appendix A) is an inverse geometry inelastic neutron scattering spectrometer to be installed at ISIS. It is composed of two main sections. The first section consists of a back scattering crystal analyser spectrometer based on the original idea of one of the ISIS spectrometer, TFXA, but with improved solid angle and better energy resolution. The second section consists of a cylindrical crystal analyser spectrometer based on a new 'venetian blind' crystal analyser which would allow for both a wide scattering angle range and for constant-Q scans in disordered systems. A development station is also planned to be built downstream. The TOSCA spectrometer will involve about 4.5 men/year on the CNR side in the period 1996-2002. The first part of the spectrometer will be installed and commissioned at ISIS during the first half of the year 1998, while the second part is planned to be installed during the first part of the year 2001. The total capital cost of the CNR contribution to TOSCA is expected to be of the order of 1.5 MECU. 2. INFM development programs: solid state detectors and epithermal neutron spectroscopy.

The INFM is involved in several development programs, however the most directly related to the ESS project are those involving the development of solid state neutron detectors and epithermal neutron spectroscopy. The first activity is performed within the XENNI network which is one of the European Large Scale Facilities action.

The development of large area fast Position Sensitive Detectors (PSD) is a key point to fully exploit the ESS. The use of VLSI technology will allow for the development of very fast neutron detectors with extremely good spatial resolution. At present one can anticipate a dead time of 0.1 ms and a spatial resolution even better than 0.1 mm. The feasibility of solid state neutron detectors having an efficiency for thermal neutron better than 2030 % has been already demonstrated and 1d and 2d PSD prototypes are under construction. This activity will last until the year 2000 and will involve 2.5 men/year and a total investment of about 0.5 MECU. The second INFM activity, namely the VESUVIO project,

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VARIE

is connected to an old standing problem of efficiently using the copious epithermal neutron flux available at the pulsed neutron sources. Until now the use of epithermal neutrons has been only partially successful because of the difficulty connected with the analysis of the scattered neutron energy. A partial success has been achieved by the eVS spectrometer installed at ISIS, but the still relatively poor resolution prevented to really exploit the experimental possibilities of the epithermal neutron scattering. In order to improve over the present eVS performance the possibility being tested is to employ the present 238U filter technique with improved resolution achieved by cooling the filter down to low temperature, T< 70 K. An improvement of the resolution has been recently obtained but further effort is necessary in order to go beyond the demonstration phase. In particular detectors with higher efficiency and shorter dead time are necessary and the Data Acquisition Electronics must be improved in order to work with very high counting rates. This project is expected to last until the year 2000 and will involve 2 men/year and a total investment of about 0.6 MECU. 3. Collaboration development program on crystal monochromators.

Both CNR and INFM are involved in a collaboration concerning the development of large area monochroma-

tors (or analyser) to be used both at steady and pulsed sources. This project is directly connected to the TOSCA spectrometer, whose forward scattering bank is based on a new design for the crystal analyser in order to use the high performance pyrolitic graphite crystals in a transmission constant-Q design. A test prototype of this design which has been called 'venetian blind' analyser will be tested at ISIS in order to define its real performance.

The project also involves the design of large area focusing assemblies which can be used in order to get large solid angles still maintaining a given optics or wavelength bandwidth. This project will last until the end of the TOSCA project, that is the year 2002, and will involve at least 2.5 men/year.

4. Conclusion The two major Italian institutions involved in neutron scattering, CNR and INFM, as discussed above, are directly involved in various instrumental work which is of interest for the development of ESS with particular emphasis on the exploitation of the new facility, that is Area 3 of the MoU. The total staff will be about 11.5 men/year and the investment will exceed 2.6 MECU. Part of this activity is already within international collaborations with other European institutions and can be easily connected to the existing collaborations discussed with the MoU.

Vol. 3 n. 1 Giugno 1998 â&#x20AC;˘ NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE

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CALENDARIO

CALENDARIO

6-14 maggio 1998 Luso, Portogallo ESRS Summer School: Applications of Synchrotron Radiation in Materials and Physics. esrs@spec.warwick.ac.uk http://fy.chalmers.se/esrs/luso.html

26 - 31 luglio 1998 Jerusalem, Israel 12th International Conference on Crystal Growth (ICCG12) PO Box 29313, Tel Aviv 61292, Israel fax: ++972 351 60604

1 - 6 giugno 1998 Rimini, Italia International School on Crystal Growth (ISSCG - 10) fornari@maspec.bo.cnr.it http://www.maspec.bo.cnr.it/CG/school_cg.html

3 - 7 agosto 1998 San Francisco, USA VUV XII - 12th International Conference on Vacuum Ultraviolet Radiation Physics,LBL-ALS, MS 80-101, Berkeley, CA 94720 USA E-mail : vuv12@lbl.gov

14 - 20 giugno 1998 Jaszowiec, Poland 4th International School and Symposium on Synchrotron Radiation in Natural Science synchro@ifpan.edu.pl http://info.ifpan.edu.pl/pelkay/issrns_98.html

9 - 15 agosto 1998 Toronto, Canada 17th General Mtng Int'l Mineralogical Association (IMA-98) ima98@quartz.geology.utoronto.ca

14 - 19 Giugno 1998 Stoccolma, Svezia NANO’98 Fourth International Conference on Nanostructured Materials Royal Institute of Technology S-10044 Stockholm, Sweden Tel. ++46 8 790 90 72 E-mail: nano98@kth.se http://www.kth.se/conferences/nano98

25 - 30 giugno 1998 Rimini, Italia Congresso Nazionale di Fisica della Materia (INFMeeting98) http://www.infm.it/annuncio.html 14 - 18 luglio 1998 Siena, Italia 6th European Conference on Molecular and Atomic Physics (ECAMP-VI) http://www.unisi.it/fisica/ecamp98/welcome.html

15 - 18 Luglio 1998 Paris, France Internationl Conference on “Strongly Correlated Electron Systems”

18 - 23 luglio 1998 Washington D.C., USA ACA'98 aca@hwi.buffalo.edu http://www.hwi.buffalo.edu/ACA

50

10 - 14 agosto 1998 Chicago, USA Tenth International Conference on X-ray Absorption Fine Structure (XAFS-X) http://ixs.iit.edu

16 - 20 agosto 1998 Prague, Czech Republic 18th European Crystallographic Meeting (ECM-18) kuzel@karlov.mff.cuni.cz

22-25 Agosto 1998 Budapest, Hungary 6th European Powder Diffraction Conf. (EPDIC-6) T. Ungar, Dept. of General Physics, Eötvös University, H-1445 Budapest, POB 323, Múzeum krt 6-8, Hungary.

Tel/Fax: +36 1 2669833 E-mail: ungar@ludens.elte.hu

25 -29 agosto 1998 Grenoble, France 17th General Conference of the Condensed Matter Division of the European Physical Society http://www.polycnrs-gre.fr/eps.html

10 - 12 settembre Lisbon, Portugal IAEA Technical Committee Meeting on Neutron Beam Research F.C. Carvalho, Nuclear and Technilogical Institute, EN10, P-2685 Sacavem, Portugal.

Tel: +351 1 9550825 Fax: +351 1 9551525 E-mail: @itn1.itn.pt

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20-23 settembre 1998 Grenoble, France Polarised Neutrons for Condensed Matter Investigations 1998 (PNCMI 98) http://www.ill.fr 26 set. - 4 ott. 1998 Palau, Italia 4ª Scuola di Spettroscopia Neutronica: Diffusione dei Neutroni dalla Materia dura Hotel Capo d’Orso, località Cala Capra, Palau (Sassari) E-mail: ianni@axcasp.caspur.it http://helium.mater.unimi.it 22 - 24 ottobre 1998 Grenoble, France Workshop on Particle Physics with Slow Neutrons Christel Kazimierczak, Institut Laure-Langevin, B.P. 156, F-38042 Grenoble Cedex 9 Tel: +33 476207291 Fax: +33 476207777 E-mail: kazim@ill.fr

25 - 28 ottobre 1998 International Rare-Earth Conference

Freemantle

9 12 dicembre 1998 Grenoble, France Neutrons & Numerical Methods B. Aubert, Institut Laue-Langevin, B.P. 156, F-38042 Grenoble Cedex 9 Tel: +33 476207008 Fax: +33 476483906 E-mail: n2m@ill.fr 11 - 13 febbraio 1999 Grenoble, France ESRF Users’ Meeting http://www.esrf.fr

22 - 27 maggio 1999 Buffalo, NY, USA ACA '99 aca@hwi.buffalo.edu http://www.hwi.buffalo.edu/ACA

4 - 13 agosto 1999 Glasgow, Scotland 18th IUCr Gen. Assembly and Int'l Congress of Crystallography http://www.chem.gla.ac.uk/iucr99

30 nov. - 4 dic. 1998 Boston, Massachusets, USA Materials Research Society Fall Meeting http://dns.mrs.org

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SCADENZE

SCADENZE

SCADENZE PER RICHIESTE DI TEMPO MACCHINA PRESSO ALCUNI LABORATORI DI NEUTRONI ISIS

La scadenza per il prossimo ‘call for proposals’ è il 16 aprile e 16 ottobre 1998

ILL

La scadenza per il prossimo ‘call for proposals’ è il 1 marzo 1999

LLB-SACLAY

La scadenza per il prossimo ‘call for proposals’ è il 1 ottobre 1998

BENSC

La scadenza è il 15 settembre 1998 e 15 marzo 1999

RISØ E NFL

La scadenza per il prossimo ‘call for proposals’ è il 1 aprile 1999

SCADENZE PER RICHIESTE DI TEMPO MACCHINA PRESSO ALCUNI LABORATORI DI LUCE DI SINCROTRONE

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ALS

Le prossime scadenze sono il 1 giugno 1998 e il 1 dicembre 1998

BESSY

Le prossime scadenze sono il 15 agosto 1998 e il 15 febbraio 1999

DARESBURY

La prossima scadenza è il 24 novembre 1998

ELETTRA

Le prossime scadenze sono il 31 agosto 1998 e il 28 febbraio 1999

ESRF

Le prossime scadenze sono il 1 settembre 1998 e il 1 marzo 1999

GILDA

(quota italiana) Le prossime scadenze sono il 1 maggio 1998 e il 1 novembre 1998

HASYLAB

(Nuovi progetti) Le prossime scadenze sono: 1 settembre , 1 dicembre 1998, 1 marzo 1999

LURE

La prossima scadenza è il 30 ottobre 1998

MAX-LAB

La scadenza è approssimativamente febbraio 1999

NSLS

Le prossime scadenze sono il 31 maggio 1998, il 30 settembre 1998 e il 31 gennaio 1999

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE • Vol. 3 n. 1 Giugno 1998

FACILITIES

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

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 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 ASTRID ISA, Univ. of Aarhus, Ny Munkegade, DK-8000 Aarhus, Denmark tel: +45 61 28899 fax: +45 61 20740 Tipo: PD Status: O BESSY Berliner Elektronen-speicherring Gessell.für Synchrotron-strahlung mbH Lentzealle 100, D-1000 Berlin 33, Germany tel: +49 30 820040 fax: +49 30 82004103 http://www.bessy.de Tipo: D Status: O BSRL Beijing Synchrotron Radiation Lab. Inst. of High Energy Physics, 19 Yucuan Rd.PO Box 918, Beijing 100039, PR China tel: +86 1 8213344 fax: +86 1 8213374 http: //solar.rtd.utk.edu/~china/ins/IHEP/bsrf/bsrf.html Tipo: PD Status: O

DAFNE INFN Laboratori Nazionali di Frascati, P.O. Box 13, I00044 Frascati (Rome), Italy tel: +39 6 9403 1 fax: +39 6 9403304 http://www.lnf.infn.it/ 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 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 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 ESRF European Synchrotron Radiation Lab. BP 220, F-38043 Grenoble, France tel: +33 476 882000 fax: +33 476 882020 http://www.esrf.fr/ Tipo: D Status: O EUTERPE Cyclotron Lab.,Eindhoven Univ. of Technol, P.O.Box 513, 5600 MB Eindhoven, The Netherlands tel: +31 40 474048 fax: +31 40 438060 Tipo: PD Status: C

CAMD Center Advanced Microstructures & Devices Lousiana State Univ., 3990 W Lakeshore, Baton Rouge, LA 70803, USA tel:+1 504 3888887 fax: +1 504 3888887 http://www.camd/lsu.edu/ Tipo: D Status: O

HASYLAB Notkestrasse 85, D-2000, Hamburg 52, Germany tel: +49 40 89982304 fax: +49 40 89982787 http://www.desy.de/pub/hasylab/hasylab.html Tipo: D Status: O

CHESS Cornell High Energy Synchr. Radiation Source Wilson Lab., Cornell University Ithaca, NY 14853, USA tel: +1 607 255 7163 fax: +1 607 255 9001 http://www.tn.cornell.edu/ Tipo: PD Status: O

INDUS Center for Advanced Technology, Rajendra Nagar, Indore 452012, India tel: +91 731 64626 Tipo: D Status: C

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FACILITIES

KEK Photon Factory Nat. Lab. for High Energy Physics, 1-1, Oho,Tsukubashi 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

SPring-8 2-28-8 Hon-komagome, Bunkyo-ku ,Tokyo 113, Japan tel: +81 03 9411140 fax: +81 03 9413169 Tipo: D Status: C SOR-RING Inst. Solid State Physics S.R. Lab, Univ. of Tokyo, 3-2-1 Midori-cho Tanashi-shi, Tokyo 188, Japan tel: +81 424614131 ext 346 fax: +81 424615401 Tipo: D Status: O

LNLS Laboratorio Nacional Luz Sincrotron CP 6192, 13081 Campinas, SP Brazil tel: +55 192 542624 fax: +55 192 360202 Tipo: D Status: C

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

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

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

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

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

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

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

NSRL National Synchrotron Radiation Lab. USTC, Hefei, Anhui 230029, PR China tel:+86 551 3601989 fax:+86 551 5561078 Tipo: D Status: O Pohang Pohang Inst. for Science & Technol., P.O. Box 125 Pohang, Korea 790600 tel: +82 562 792696 f +82 562 794499 Tipo: D Status: C Siberian SR Center Lavrentyev Ave 11, 630090 Novosibirsk, Russia tel: +7 383 2 356031 fax: +7 383 2 352163 Tipo: D Status: O

SURF B119, NIST, Gaithersburg, MD 20859, USA tel: +1 301 9753726 fax: +1 301 8697628 http://physics.nist.gov/MajResFac/surf/surf.html Tipo: D Status: O TERAS ElectroTechnical Lab. 1-1-4 Umezono, Tsukuba Ibaraki 305, Japan tel: 81 298 54 5541 fax: 81 298 55 6608 Tipo: D Status: O UVSOR Inst. for Molecular ScienceMyodaiji, Okazaki 444, Japan tel: +81 564 526101 fax: +81 564 547079 Tipo: D Status: O

D = macchina dedicata; PD = parzialmente dedicata; P = in parassitaggio.

O= macchina funzionante; C=macchina in costruzione.

D = dedicated machine; PD = partially dedicated; P = parassitic.

O= operating machine; C= machine under construction.

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FACILITIES

NEUTRONI NEUTRON SCATTERING WWW SERVERS IN THE WORLD (http://www.ISIS.RL.AC.UK/ISISpublic/neutron_sities.html) BENSC Berlin Neutron Scattering Center, Hahn-Meitner-Institut, Glienicker Str. 100, D- 14109 Berlin-Wannsee, Germany Rainer Michaelsen; tel: +49 30 8062 3043 fax: +49 30 8062 2523 E - Mail: michaelsen@hmi.de http://www.hmi.de BNL Brookhaven National Laboratory, Biology Department, Upton, NY 11973, USA Dieter Schneider; General Information: Rae Greenberg; tel: +1 516 282 5564 fax: +1 516 282 5888 http://neutron.chm.bnl.gov/HFBR/ GKSS Forschungszentrum Geesthacht, P.O.1160, W-2054 Geesthacht, Germany Reinhard Kampmann; tel: +49 4152 87 1316 fax: +49 4152 87 1338 E-mail: PWKAMPM@DGHGKSS4 Heinrich B. Stuhrmann; tel: +49 4152 87 1290 fax: +49 4152 87 2534 E-mail: WSSTUHR@DGHGKSS4 IFE Institut for Energiteknikk, P.O. Box40, N-2007 Kjeller, Norway Jon Samseth; tel: +47 6 806080 fax: +47 6 810920 telex: 74 573 energ n E-mail: Internet JON@BARNEY.IFE.NO ILL Institute Laue Langevin, BP 156, F-38042, Grenoble Cedex 9,France Peter Timmins; tel: +33 76207263 E-mail: TIMMINS@FR ILL Peter Linder; tel: +33 76207068; E-mail: LINDER@FR ILL Roland P.May;tel:+3376207047; E-mail: MAY@FRILL fax: +33 76 48 39 06 telex: ILL 320-621 http://www.ill.fr IPNS Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4814, USA P.Thiyagarajan,Bldg.200,RM. D125; tel :+1 708 9723593 E-mail: THIYAGA@ANLPNS Ernest Epperson, Bldg. 212; tel: +1 708 972 5701 fax: +1 708 972 4163 or + 1 708 972 4470 (Chemistry Div.) http://pnsjph.pns.anl.gov/ipns.html

ISIS The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxfordshire OX11 0QX, UK Richard Heenan; tel +44 235 446744 E-mail: RKH@UK.AC.RUTHERFORD.DEC-E Steve King; tel: +44 235 446437 fax: +44 235 445720; Telex: 83 159 ruthlb g E-mail: SMK@UK.AC.RUTHERFORD.DEC-E http://www.nd.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 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

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FACILITIES

NIST National Institute of Standards and Technology-Gaithersburg, 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 G.D. Wignall; tel: +1 615 574 5237 fax: +1 615 576 2912 E-mail: GDW@ORNLSTC http://www.ornl.gov/hfir/hfirhome.html

56

PSI Paul Scherrer Institut Wurenlingen und Villingen CH-5232 Villingen PSI tel: +41 56 992111 fax: +41 56 982327 RISØ EC-Large Facility Programme, Physics Department, Risø National Lab.P.O. Box 49, DK-4000 Roskilde, Denmark K. Mortenses; tel: +45 4237 1212 fax: +45 42370115 E-mail: CLAUSEN@RISOE.DK or SANS@RISOE.DK NFL-Studsvik in Sweden E-mail: mcgreevy@studsvik.uu.se

NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE • Vol. 3 n. 1 Giugno 1998