Third-generation Synchrotron Radiation Sources Research Articles

Investigation of structural defects and inhomogeneities in Al-Pd-Mn icosahedral quasicrystals by combined synchrotron X-ray topography and phase radiography

Abstract Phase-sensitive radiography, made possible by the lateral coherence of the beams from third-generation synchrotron-radiation sources, has been combined with monochromatic X-ray topography in an investigation of structural defects and inhomogeneities in the bulk of Al-Pd-Mn icosahedral single quasicrystal grains. Loop-shaped defects previously observed by X-ray topography are thus related with holes and second phase precipitates revealed by phase radiography. The evolution of these defects after annealing provides clues on their nature and formation.

Mechanisms of Lichen Resistance to Metallic Pollution

Some lichens have a unique ability to grow in heavily contaminated areas due to the development of adaptative mechanisms allowing a high tolerance to metals. Here we report on the chemical forms of Pb and Zn in the metal hyperaccumulator Diploschistes muscorum and of Pb in the metal tolerant lichen Xanthoria parietina. The speciation of Zn and Pb has been investigated by powder X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy using the advanced third-generation synchrotron radiation source of the European synchrotron radiation facility (ESRF in Grenoble). This study reveals that in both lichens cells are protected from toxicity by complexation of heavy metals, but the strategies differ: in D. muscorum, Pb and Zn are accumulated through an enhanced synthesis of oxalate, which precipitates toxic elements as insoluble salts, whereas in X. parietina, Pb is complexed to carboxylic groups of the fungal cell walls. We conclude that hyperaccumulation of metals results from a reactive mechanism of organic acid production, whereas metallo-tolerance is achieved by a passive complexation to existing functional groups.

Pure nuclear resonant scattering of synchrotron radiation by a multilayer structure: Theoretical analysis for 169Tm

The possibility of observing pure nuclear resonant scattering of synchrotron radiation by a multilayer structure containing the 169Tm isotope is analyzed theoretically. The main problem is the need to suppress the enormous background of radiation scattered by electrons. Two methods for the destructive interference of a synchrotron radiation beam in reflection at grazing incidence by a layered system containing Tm nuclei in one of the layers are considered, and their efficiency as applied to the conditions of third-generation synchrotron radiation sources, such as in the European Synchrotron Radiation Facility (ESRF), is calculated. An electron scattering suppression efficiency parameter is formulated as the ratio of the integrated nuclear scattering intensity (with a time delay) to the total prompt electron scattering intensity in assigned ranges of angles and energies. In the first method thin films of a special type on a substrate, viz., GIAR films, are used. In the second method a new effect, which is termed the Bragg antipeak effect and involves the destructive interference of a wave that is Bragg-diffracted in a multilayer superlattice and a wave reflected on the upper boundary of the sample, is employed. The physical properties of the Bragg antipeak effect are considered, and it is found that its efficiency is sufficient for practical use.

Bragg and Fresnel Diffraction Imaging Using Highly Coherent X-Rays

Abstract X-ray imaging started over a century ago. For several decades its only form was absorption radiography, in which contrast is due to local variations in beam attenuation. About forty years ago, a new form of X-ray imagery, Bragg-diffraction imaging or X-ray topography, developed into practical use. It directly reveals crystal defects in the bulk of large single crystals, and paved the way to microelectronics by leading to the growth of large, practically perfect, crystals. The advent of third-generation synchrotron radiation sources of X-rays such as ESRF and APS is now making possible, through the coherence of the X-ray beams, a novel form of radiography, in which contrast arises from phase variations across the transmitted beam, associated with optical path length differences, through Fresnel diffraction. Phase radiography and its three-dimensional companion, X-ray phase tomography, are providing new information on the mechanics of composites as well as on biological materials.

A State-of-the-art X-Ray Microprobe and its Applications at the Advanced Photon Source

Abstract We have developed a state-of-the-art x-ray microprobe with many unprecedented capabilities for a broad range of scientific applications. The combination of the high brilliance third-generation synchrotron radiation sources and the development of high performance x-ray focusing optics have led to the construction of a x-ray microprobe at Argonne National Laboratory with a FWHM focal size less than 0.25 μm and photon flux exceeding 5×l010 photons/cm2/sec/0.01% BW. The microprobe is very flexible and versatile and can be used simultaneously to perform x-ray diffraction microscopy, fluorescence microscopy, spectromicroscopy, and 3D tomography with submicron spatial resolution. It has been used for a broad range of applications since its commissioning in December of 1996. For example, it was used in the study of the symbiotic relation between plant root and fungi, platinum-based anticancer agents, integrated laser/modulator microdevices, electromigration in Al wires in semiconductor devices, and submicron spatial resolution threedimensional tomography of a single cell. The capabilities of the microprobe are described below, and applications using these capabilities will be presented.

Surface morphology by reflectivity of coherent X-rays

Coherent X-ray diffraction (CXD) is a new technique made possible by the enhanced brilliance specifications of hard X-ray wigglers and undulators, especially at third-generation synchrotron radiation sources. CXD differs from conventional diffraction in that it uses a microscopic beam that is close to fully coherent one. The resulting diffraction pattern is related to the Fourier transform of the entire object illuminated by the beam, and hence is sensitive to any fluctuations within it, whether these are in space or in time. We have observed CXD effects in the near-specular reflectivity from silicon wafers. We introduce a simple theoretical formalism for explaining the origin of the coherent diffraction signal in a reflectivity experiment, in which we find a slow but distinct evolution of the pattern with the perpendicular component of momentum transfer, q z . We demonstrate a practical energy dispersive method of measuring this q z -dependence.

On a spatially resolving USAXS instrument for operation at a third-generation synchrotron radiation source.

A point-collimation USAXS system based on the Bonse-Hart-diffractometer concept is proposed which takes advantage of a CCD detector to rapidly obtain two-dimensional SAXS data at both high spatial and angular resolution for a line of points on the sample. The method might typically be employed using Bonse-Hart crystal optics in the horizontal plane, encompassing the high-angular-resolution scan direction, and a condensing monochromator in the out-of-diffraction-plane direction as the basis for a pinhole SAXS camera in that plane. The new system is well suited to operation at an undulator beamline on a third-generation synchrotron radiation source. Other applications of the instrument include (i) both absorption and phase-contrast imaging/tomography, (ii) polycrystalline topography, (iii) high-resolution triple-crystal measurements and (iv) diffuse scattering measurements.

A novel approach to the control of experimental environments: the ESCA microscopy data-acquisition system at ELETTRA.

An efficient control system is today one of the key points for the successful operation of a beamline at third-generation synchrotron radiation sources. The high cost of these ultra-bright light sources and the limited beam time requires effective instrument handling in order to reduce any waste of measurement time. The basic requirements for such control software are reliability, user-friendliness, modularity, upgradability, as well as the capability of integrating a horde of different instruments, commercial tools and independent pre-existing systems in a possibly distributed environment. A novel approach has been adopted to implement the data-acquisition system of the ESCA microscopy beamline at ELETTRA. The system is based on YASB, a software bus, i.e. an underlying control model to coordinate information exchanges and networking software to implement that model. This 'middleware' allows the developer to model applications as a set of interacting agents, i.e. independent software machines. Agents can be implemented using different programming languages and be executed on heterogeneous operating environments, which promotes an effective collaboration between software engineers and experimental physicists.

SPring-8 Program.

SPring-8 is a third-generation synchrotron radiation source operating in the soft and hard X-ray region. It consists of an injector linac of 1 GeV, a booster synchrotron of 8 GeV and a storage ring with a natural emittance of 5.9 nm rad. The storage ring can accommodate 61 beamlines in total, and 26 of them are under construction. The project has been carried out jointly by JAERI and RIKEN and construction of the facility started in 1991. Commissioning of the injector linac was started in August 1996 and an 8 GeV electron beam was injected into the storage ring in March 1997. The first synchrotron radiation from a bending magnet was observed at the front end of the beamline on 25 March and radiation from an undulator was observed on 23 April. On-beam testing of seven beamlines, four of them from in-vacuum undulators and three from bending magnets, started in July. The maximum stored current is currently fixed at 20 mA and the lifetime at maximum current is longer than 20 h. The dedication is scheduled for October 1997.

Inelastic scattering energy loss in CaF2 observed with soft X-ray fluorescence spectroscopy

Abstract The advent of third-generation synchrotron radiation sources has stimulated interest in the study of X-ray emission using photon excitation. Soft X-ray fluorescence (SXF) provides a means of measuring the element and angular momentum selective valence band density of states in complex materials. Tenability of the excitation source allows a detailed investigation of the dynamical behavior of the relaxation channels and the coupling between the excitation and emission process that occurs near a resonance or an absorption edge. As an example of resonant inelastic scattering studies in solid systems by SXF spectroscopy at the insertion device beam line BL 8.0 at ALS, results from CaF 2 will be presented. When the energy of exciting photons is tuned to the Ca 2p–3d threshold absorption peak in CaF 2 , the SXF produced by recombination of the excited electrons with core holes exhibits remarkable resonances for both the 2p 3/2 and 2p 1/2 levels. When the resonant 2p–3d SXF peaks are excited, additional broad peaks appear in the spectra below the normal valence band spectrum, which are displaced from the resonant peaks by about 17 eV and track the changes in excitation energy. We identify this as evidence of a coherent, inelastic scattering process, analogous to resonant Raman scattering, in which incident photons excite a localized electronic transition and are re-radiated at an energy reduced by the energy of the excitation. Evidence is also presented which indicates that the 2p–3d absorption transitions lie about 1 eV above a weak Ca 4s absorption threshold.

Surface Atomic Structure of KDP Crystals in Aqueous Solution: An Explanation of the Growth Shape

With this study on KDP, we present an interface atomic structure determination of a crystal in contact with its growth solution. Using x-ray diffraction at a third-generation synchrotron radiation source, the structure of both the ${101}$ and ${100}$ faces has been determined. We found that the ${101}$ faces are terminated by a layer of ${\mathrm{K}}^{+}$ ions and not by ${\mathrm{H}}_{2}{\mathrm{PO}}_{4}^{\phantom{\rule{0ex}{0ex}}\ensuremath{-}}$ groups, resolving a long-standing issue that could not be predicted by theory. This result leads to an atomic-scale explanation of the influence of metal impurities on the macroscopic growth morphology.

Single-energy x-ray absorption detection: a combined electronic and structural local probe for phase transitions in condensed matter

We describe the general principle and prototypical applications of a largely unexploited x-ray absorption technique based on single-energy detection during temperature scans. The present developments take advantage of the highly automated experimental set-ups and unique x-ray radiation characteristics available at third-generation synchrotron radiation sources. The sensitivity of the x-ray absorption cross-section to the local structural order makes it possible to investigate the occurrence of phase transitions. We show that by tuning the photon energy to a spectral feature that is highly sensitive to the sample phase it is possible to follow the occurrence and the characteristics of the phase transition. The sensitivity to the atomic fraction of the sample in a certain phase can be in the range. The energy tunability can be used to enhance the sensitivity to structural or electronic transformations. Several applications to prototypical cases are discussed in detail, including examples of solid-solid phase transitions, melting and undercooling, melting of binary alloys, impurity melting, and non-bulk phenomena.

Self-Similar Porosity In Quasicrystals

AbstractSingle grains of real quasicrystals have been investigated using X-ray imaging techniques at a third-generation synchrotron radiation source (ESRF). Facetted microholes have been observed. The experimental results are discussed with reference to a description of the quasicrystal structure in terms of overlapping atomic clusters (self-similar packing or random covering modifications). Relations between holes and quasicrystal formation are also discussed.

Submicrometre Beams from a Hard X-ray Waveguide at a Third-Generation Synchrotron Radiation Source

The use of an X-ray waveguide for scattering experiments at an undulator of a third-generation synchrotron radiation source is discussed. The performance with a perfect crystal monochromator, multilayer monochromator and focusing mirror is explored. A maximum flux of 8 x 109 photons s(-1) at lambda = 0.083 nm was obtained for a 0.15 (V) x 600 (H) micron(2) beam at the exit of the waveguide with a multilayer monochromator. The combination of an Si (111) monochromator and ellipsoidal mirror resulted in a flux of approximately 10(9) photons s(-1) but with a horizontal compression of the beam to approximately 30 micron. The use of the waveguide in diffraction experiments is addressed.

Fifty Years of Synchrotron Radiation

This issue marks the 50th anniversary of the first observation of synchrotron radiation light from the General Electric 70 MeV synchrotron and coincides with the third anniversary of the Journal of Synchrotron Radiation (JSR). The issue brings together review articles from some of the pioneers of the field and in our view forms a unique historical record. The interdisciplinary nature of synchrotron radiation research has been apparent from the early 1970's and the planners of the secondand third-generation sources have paid particular attention to the requirements of such an interdisciplinary community. It is clear that synchrotron radiation has removed the boundaries in otherwise somewhat isolated research areas. Indeed, JSR is playing an important part in furthering such interdisciplinary interactions. Over the last three years we have brought together the broad synchrotron radiation community by offering rapid communication of scientific and technical developments in the journal. We have not compromised on the quality of the contributions and have managed to combine this with a speed and high standard of production. In this period, more than half a dozen third-generation synchrotron radiation sources have come into operation and several more are now at an advanced state of planning or construction (e.g. SLS, DIAMOND and SOLEIL). Worldwide this is still a rapidly expanding field. It is now just over 40 years since the first synchrotron radiation experiments were performed by Tomboulin and Hartman at Cornell in the soft X-ray region. These experiments were immediately followed by those of Madden and Codling in 1961 at the National Bureau of Standards in the UV/VUV region. This early utilization demonstrated the wealth of physics that needed to be explored by these new continuum light sources. Initiatives in Europe (Frascati, DESY, NINA), the former Soviet Union (VEPP) and Japan (INS-SOR) began in the mid-1960's along with expanding scientific opportunities. The first biological experiments were performed at DESY by Holmes, Rosenbaum and Witz in 1970, opening synchrotron radiation to the biological community. We again reiterate the main objectives of JSR; namely, to provide a focus for the whole of the Synchrotron Radiation community, and to accept papers only of a high standard covering sources, instrumentation, methods and applications. Through 1997 our main aim has been to increase the size of each issue. We are grateful to the many authors who have responded enthusiastically to our call. We continue to urge you to always consider JSR as the first choice when you have new results, methods or instrumental facilities. It is only through your high-quality contributions that our community will be able to sustain a high-quality journal. We accompany this editorial with a photograph (Fig. 1) taken in August 1997 at the Synchrotron Radiation Instrumentation Conference in Himeji, Japan, featuring some of the pioneers of the synchrotron radiation field (papers from the conference will be published in the May 1998 issue of the journal).

Time-Resolved Small-Angle X-ray Scattering Combined with Wide-Angle X-ray Scattering

The high X-ray intensity of synchrotron radiation (SR) beamlines makes it possible to perform time-resolved small-angle X-ray scattering (SAXS) experiments. The information that can be obtained by collecting the wide-angle diffraction pattern simultaneously not only increases the information content of an experiment but also increases the reliability of the time-correlations between SAXS and WAXS (wide-angle X-ray scattering) patterns. This is a great advantage for experiments with a time resolution below the level of 1 s per frame. With appropriate instrumentation, this is a time domain that is routinely accessible for a large group of research fields. This has had a considerable impact upon the understanding of fundamental aspects of phase transformations. Not only fundamental processes but also more applied fields have benefited from these developments. In polymer research this has led to a situation in which it has become possible to simulate materials processing techniques on-line. With the advent of third-generation synchrotron-radiation sources (e.g. ESRF, APS, Spring8), it has become possible to develop SAXS/WAXS beamlines that will open up new research opportunities by utilizing the higher intensity, the tuneability and the higher collimation offered by these SR sources. However, some of the instrumentation limits in detector and sample environments that have become apparent in research on second-generation synchrotron-radiation sources still have not been appropriately addressed, which means that in some fields it will not be possible to take full advantage of the superior X-ray beam quality that third-generation synchrotrons can offer. A way in which these instrumentation limits can be overcome is discussed, and the instrumentation for a new bending-magnet beamline at the ESRF is used as an example.

Fractional Talbot imaging of phase gratings with hard x rays

Fractional Talbot images of optical gratings acting as periodic phase objects have been obtained by use of x rays of 0.069-nm wavelength from a third-generation synchrotron radiation source. Quantitative evaluation of the data obtained as a function of defocusing distance provides information on the lateral coherence of the beam as well as on the phase modulation in the object.

PHOTOELECTRON DIFFRACTION: SPACE, TIME, AND SPIN DEPENDENCE OF SURFACE STRUCTURES

The current status of photoelectron-diffraction studies of surface structures is briefly reviewed, and several recent developments and proposals for future areas of application are then discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low-energy electron-diffraction and scanning-tunneling microscopy, to epitaxial growth is considered. Several new avenues that are being opened up by third-generation synchrotron-radiation sources are also considered. These include photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements, and circular dichroism in photoelectron angular distributions. The addition of spin to the photoelectron-diffraction measurement is also considered, and can be achieved either through core-level multiplet splittings or by circular-polarized excitation of spin–orbit-split levels. This last development should make it possible to study short-range magnetic order, perhaps even in a holographic fashion.

X-ray diffraction from single fibres of spider silk

Wide-angle X-ray diffraction patterns of single spider-silk fibres of ≤ 5 μm diameter can be obtained at a third-generation synchrotron-radiation source in a few seconds per pattern. This is sufficient to observe the strongest equatorial and first layer line reflections. For higher-resolution structural work, data have to be recorded with cryocooling techniques in order to maintain the crystallinity of the sample for several minutes.

DIFFRACTION AND HOLOGRAPHY WITH PHOTOELECTRONS AND FLUORESCENT X-RAYS

We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emission of fluorescent x-rays. The current status of photoelectron diffraction studies of surfaces, interfaces, and other nanostructures is first briefly reviewed, and then several recent developments and proposals for future areas of application are discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low energy electron diffraction and scanning tunneling microscopy, to the epitaxial growth of oxides and metals is considered. Several new avenues that are being opened up by third-generation synchrotron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements of surface reactions with chemical-state resolution, and circular dichroism in photoelectron angular distributions from both non-magnetic and magnetic systems. The addition of spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the measurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or circular-polarized excitation of spin-orbit-split levels. The direct imaging of short-range atomic structure by both photoelectron holography and two distinct types of x-ray holography involving fluorescent emission is also discussed. Both photoelectron and x-ray holography have demonstrated the ability to directly determine at least approximate atomic structures in three dimensions. Photoelectron holography with spin resolution may make it possible also to study short-range magnetic order in a holographic fashion. Although much more recent in its first experimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, including both surface and bulk regions.