Laue Geometry Research Articles

On parametric x-ray radiation

The spectral-angular and angular distributions of parametric x-ray radiation generated in the vicinity of the maximum at small angles to the direction of charged-particle motion are studied under the conditions of Laue and Bragg diffraction geometries. Detector parameters at which the dynamical maxima of parametric x-ray radiation can be observed against the background of transition radiation are indicated. The spectral-angular and angular distributions of parametric x-ray radiation are analyzed for the case of backward geometry. The results of the present theoretical calculations are found to be in good agreement with experimental data.

Time-resolved x-ray diffraction measurements on CdS shocked along the c axis

Time-resolved high-energy x-ray diffraction measurements were used to examine laser shock-induced changes in cadmium sulfide single crystals compressed along the c axis near 1.5 and 3.5 GPa. Macroscopic compression under different loading conditions was monitored through the evolution of the (004) Bragg reflection in Laue geometry. In the low-stress experiments, the elastic response of the sample was evidenced by reversible changes of the intensity profiles, while a plastic behavior was observed in the high-stress experiments. A residual (004) reflection is observed in the rocking curve obtained from the crystal shocked to 3.5 GPa, in addition to two peaks at higher angle.

Diffraction of ultrashort XFEL pulses by perfect crystals (abstract)

Planned fourth generation sources, i.e., XFELs, motivate the study of dynamical diffraction effects in the interesting regime where pulse durations (femtoseconds) become comparable to or shorter than extinction lengths (microns). The transient, time-dependent diffracted intensities arising from incident delta-function impulses are calculated for Bragg and Laue geometries, examining both reflected and transmitted beams. Furthermore, the alteration of an XFEL pulse’s spiky temporal structure as it passes through single- and double-crystal monochromators is discussed for both low- and high-order reflections.

Characterization of a diamond crystal x-ray phase retarder

An x-ray phase retarder plate based on a diamond single crystal diffracting in the asymmetric Laue geometry has been characterized at the X25 wiggler beamline at the National Synchrotron Light Source. The forward diffracted (transmitted) beam, using the (111) Bragg planes in a 0.5 mm thick wafer with a (001) surface normal, was employed. A polarization analyzer based on a GaAs(111) crystal oriented to diffract the (222) and a different reflection simultaneously was used to determine the Stokes–Poincaré polarization parameters of the beam transmitted by the diamond phase plate, at several settings of the diamond about its (111) rocking curve. At 7.1 keV, the phase plate performed as expected and it was proven possible to produce, with the plate, an almost completely left- or right-handed circularly polarized x-ray beam from a linearly polarized incident beam.

Initial results on logarithmic spiral bent Laue analyzer for fluorescence x-ray absorption spectroscopy (abstract)

We have previously described [Z. Zhong, D. Chapman, B. A. Bunker, G. B. Bunker, R. Fischetti, and C. U. Segre, J. Synchrotron Radiat. 6, 212 (1999); C. Karanfil et al., AIP Conf. Proc. 521, 178 (2000)] a novel concept of using highly bent single crystals in Laue geometry for wide bandwidth monochromatization of fluorescence x rays emerging from a point source, such as the sample in fluorescence x-ray absorption spectroscopy (XAFS) experiments. In this article we describe the first measurements (summer 2000) of the fluorescence XAFS of millimolar solutions of several elements. The potential of this type of analyzer is clearly demonstrated.

X-Ray Synchrotron Topography Investigation of Porous Silicon Formed by Patterning in Localized Areas

X-ray synchrotron topography was used to study the structural features of porous silicon (PS) layers formed by (boron)-type silicon anodization within localized areas. Selective anodization was obtained by using a masking film with good chemical inertness to the anodic dissolution or a surface layer of different doping type and level resulting from phosphorus implantation. The overall structural picture of the PS/substrate systems obtained by the two different masking procedures was built up by combining different topographic methods. The samples were oriented both in Bragg (reflection) and Laue (transmission) geometry to diffract white or monochromatic radiation with extended (projection topography) or limited (section topography) beam widths. By comparing the effects of the two masking types on the anodization process, the implantation method is unquestionably preferable because of the lower stress at the window edges, the better lateral porosity homogeneity inside the windows, and the flatter interface between porous silicon and substrate. © 2001 The Electrochemical Society. All rights reserved.

Characterization of synthetic quartz crystals grown from cylindrical seeds produced by ultrasonic machining

The potential of ultrasonic machining is investigated as a tool to produce seed crystals for hydrothermal growth of synthetic quartz with required surface finish and unconventional geometries. For this purpose, cylindrical seeds with 5, 10 and 18 mm diameter were obtained along the optic axis of a Z-bar using a conventional ultrasonic machining process with abrasive particles ranging from 6 to 50 μm. The topography of seed surfaces was characterized by surface profilometry and scanning electron microscopy. Later, synthetic crystals were grown in a standardized hydrothermal growth condition using NaOH solution. The characterization of as-grown crystals was performed by double-crystal X-ray topography with Laue geometry. Based on X-ray topographic images, it was observed that the density of dislocations, nucleated at the early stage of the growth period, increases with increasing roughness of the seed-crystal surface.

A design of mammography units using a quasimonochromatic x-ray source

In this article we present a mammography unit design using a parametric x-radiation (PXR) source. We show that PXR can provide a fanned quasimonochromatic x-ray beam that can be used to obtain mammography images of higher contrast and lower dose than those obtained from a conventional x-ray system. Changing the Bragg angle of the PXR crystal with respect to the electron beam changes the photon energy, improves image quality, and minimizes dose. Monte Carlo computer simulations are given that show that the PXR source with a 5% bandwidth gives a figure of merit close to that of the ideal monoenergetic source and significantly higher than that of the filtered-x-ray-tube sources. In order to simultaneously obtain adequate flux and achieve bandwidths below 5%, we utilized an electron-beam energy of 35 MeV and an average current of 300 μA to 1 mA for 3 s (depending upon breast thickness and density). Slits after the PX radiator are used to define both the spatial distribution and the spectral bandwidth of the x-ray beam, which is scanned over the breast in approximately 3 s. A graphite crystal C (002) in the Laue geometry is utilized as the PX radiator. Lower electron-beam currents might be possible as higher efficiency PX or hybrid radiators become available.

X-ray measurement of a microdistortion tensor and its application in an analysis of the dislocation structure of thick GaN layers obtained by hydrochloride gaseous-phase epitaxy

The method of two-and three-crystal x-ray diffractometry (TCD) is used for studying the dislocation structure of thick GaN layers grown by chloride gaseous-phase epitaxy (CGE) on sapphire, as well as on a thin GaN layer, which is grown by the metalloorganic synthesis (MOS) method. Five components of the microdistortion tensor 〈ɛij〉 and the sizes of the coherent scattering regions along the sample surface and along the normal to it are obtained from the measurements of diffracted intensity in the Bragg and Laue geometries. These quantities are used to analyze the type and geometry of the dislocation arrangement and to calculate the density of the main types of dislocations. The density of the vertical screw dislocations, as well as of the edge dislocations, decreases (by a factor of 1.5 to 3) during growth on a thin GaN layer. The diffraction parameters of the thick layer on the MOS-GaN substrate suggest that it has a monocrystalline structure with inclusions of microcrystalline regions.

The nuclear-resonance-scattering station at the PETRA II undulator beamline

PETRA II, a 12 GeV accelerator at DESY, Hamburg, is used to produce synchrotron radiation (SR) for experiments in the hard X-ray regime when it is not running as injector for HERA. The beam from an undulator is split by a diamond crystal in Laue geometry to feed two experimental stations, one of which is now dedicated for nuclear resonance experiments. The X-ray energy may be chosen in the range from 5 to 55 keV covering all isotopes already observed with SR and many interesting candidates for new resonances. Tuning may be performed by optimising the magnetic gap and the storage ring energy. In particular, the opportunities for timing experiments are unique due to a very flexible filling mode of the storage ring. The flux at the sample position is comparable to undulator beams at ESRF. The second beamline covers higher energies up to some 300 keV and may also be used for nuclear resonance experiments.

Optimization and operation of a BBB X-ray interferometer

A special arrangement of Bonse–Hart X-ray interferometers, which uses the Bragg geometry of X-ray diffraction for the beam splitter, mirrors and the analyser (so-called BBB interferometers), has been designed and characterized both computationally and experimentally in different versions. The design includes wavelength-dependent optimization of transmission through the lamella, which is used both for beam splitting and for recombination. For characterization of the function of the instrument, interference patterns were evaluated from BBB interferograms, as well as from LLL (employing the Laue geometry for coherent beam splitting, reflecting and analysing) topograms and from interferograms resulting from a new combined geometrical layout. The influence of gravitation on the horizontally running diffraction planes was visible as a system of fringes in the interferograms, resulting from bending of the lamella under its own weight.

Diffraction of 100 to 200 keV X-rays from an Si1-xGex gradient crystal: comparison with results from dynamical theory.

In a large Czochralski-grown Si(1-x)Ge(x) (0.02 </= x </= 0.07) gradient crystal, diffraction patterns have been measured in symmetrical Laue geometry using synchrotron radiation in the energy range 100-200 keV. The experimental data are in very good agreement with the results from geometrical optics theory for distorted crystals, if the creation of new wavefields for strain gradients larger than a critical value is taken into account. In this sense, the crystal behaves like an ideal gradient crystal. If the normal absorption is disregarded, for reflection 111 and 100 keV energy, the full width at half-maximum values and the peak reflectivities of the diffraction patterns range from 14.6" and 97%, respectively, to 70.9" and 74%, respectively, for a variation in the Ge concentration from 3.5 to 5.3 at.%.

Accurate and highly complete synchrotron protein crystal Laue diffraction data using the ESRF CCD and the Daresbury Laue software

Developments in electronic area detectors such as CCDs and image plates have transformed the capability of the synchrotron Laue protein crystallography technique compared with film. The rapid readout of CCDs makes practical the use of rather fine angular interval settings of the crystal between each Laue exposure and a large overall angle coverage. The use of the ESRF CCD (image intensifier type) presented here in the Laue data collection on ESRF ID09 (the `Laue beamline') from a single crystal of the 34 kDa wild-type hydroxymethylbilane synthase (HMBS), space group P21212 a = 88.06, b = 75.73, c = 50.35 A, yielded 47 Laue exposures in 2.5° angle intervals from a single crystal. The data processed by the Daresbury Laue software is highly complete (∞–2dmin = 77.5%; 2dmin–dmin= 91.7%) to 2.3 A with high redundancy (11.2). Comparison with calculated structure factors and careful analysis of the Laue geometry shows that between ∞ and 5dmin better completeness still should be possible, which can ideally be realized from CCD detector dynamic range hardware improvements and/or software algorithms to integrate saturated spot profiles. Prospects for Laue diffraction data collection using yet faster detectors such as the `pixel detector' to study irreversible catalytic structural processes in a crystal, the most challenging of all time-resolved experiments, are bright.

Instrumentation of the ESRF medical imaging facility

At the European Synchrotron Radiation Facility (ESRF) a beamport has been instrumented for medical research programs. Two facilities have been constructed for alternative operation. The first one is devoted to medical imaging and is focused on intravenous coronary angiography and computed tomography (CT). The second facility is dedicated to pre-clinical microbeam radiotherapy (MRT). This paper describes the instrumentation for the imaging facility. Two monochromators have been designed, both are based on bent silicon crystals in the Laue geometry. A versatile scanning device has been built for pre-alignment and scanning of the patient through the X-ray beam in radiography or CT modes. An intrinsic germanium detector is used together with large dynamic range electronics (16 bits) to acquire the data. The beamline is now at the end of its commissioning phase; intravenous coronary angiography is intended to start in 1999 with patients and the CT pre-clinical program is underway on small animals. The first in vivo images obtained on animals in angiography and CT modes are presented to illustrate the performances of these devices.

Pattern of x-ray scattering by thermal phonons in Si

Intensity distribution of x-ray scattering by thermal phonons in Si was recorded using synchrotron undulator radiation. A high-energy beam sent through a Si(111) wafer in a transmission Laue geometry yielded a threefold symmetric pattern for the scattering cross section with rich details governed by phonon dispersion, population, and polarization.

A New Synchrotron-Based Diffraction Technique for Perturbation Crystallography

A new diffraction technique based on a wide-energy-bandpass X-ray beam in connection with lock-in detection for diffraction studies of crystals subjected to external perturbations has been developed. The broad-energy-bandpass beam is obtained by combining a wiggler or bending-magnet source with a curved Si monochromator in Laue geometry. This new technique has been applied in diffraction experiments on single crystals in external electric fields. Both the speed and accuracy of the measurements are increased by more than two orders of magnitude compared with conventional methods.

Microfocusing of Hard X-rays with Cylindrically Bent Crystal Monochromators

High-energy X-ray focusing with bent-crystal monochromators is known to be hampered by so-called depth or crystal-thickness aberrations. A theoretical model of focus broadening based on the geometrical theory of X-ray diffraction in slightly deformed crystals is presented and compared with experimental data. First, it is shown that depth broadening can be avoided in the Laue geometry by an appropriate choice of asymmetry angle. Based on this finding, a monochromator for high-pressure diffraction experiments has been designed and a source-size-limited focal spot below 10 microns is observed. As a consequence of the box-shaped rocking curve of bent Laue crystals, the focus is free of long-ranging tails. Diffraction patterns of standard powder samples were recorded on imaging plates and a theoretical description of the energy-dispersion-related peak broadening is given. Finally, diffraction patterns of N2 at 180 kbar demonstrate the excellent data quality achievable with this monochromator.

Detection of charge scattering associated with stripe order inLa1.775Sr0.225NiO4by hard-x-ray diffraction

In the past few years neutron-scattering experiments have shown very intriguing stripe correlations of spins and holes in hole-doped ${\mathrm{La}}_{2}{\mathrm{NiO}}_{4}$ and ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$. As yet, no x-ray-diffraction experiment has confirmed the neutron results and the topic is still controversial. In this paper we report the observation of stripe correlations of holes by x-ray diffraction. The experiments were performed in Laue geometry at the hard-x-ray beamline BW5 at HASYLAB on a crystal of ${\mathrm{La}}_{1.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_{4}$ which was previously studied by neutron diffraction. Temperature dependences of the intensities and wave vectors of the charge-density-modulation peaks were characterized and found to be in good agreement with the neutron-scattering results. Interestingly, weak, temperature-dependent scattering was also observed at positions consistent with nonresonant x-ray magnetic scattering from the spin correlations; however, a definitive determination of its origin remains lacking.

X-ray scattering study of charge scattering associated with stripe order in La2NiO4

In this paper we report the first observation of stripe correlations of holes in hole-doped La2NiO4 by X-ray diffraction. The experiment was performed in Laue geometry at the hard X-ray beamline BW5 at HASYLAB on a crystal of La1.77Sr0.22NiO4 previously studied by neutron diffraction. Temperature dependences of the intensities and wave vectors of the charge-density-modulation peaks were characterized and found to be in good agreement with the neutron scattering results. Interestingly, weak, temperature-dependent scattering was also observed at positions consistent with nonresonant X-ray magnetic scattering from the spin correlations; however, a definitive determination of its origin remains lacking.

A Universal Treatment of X-ray and Neutron Diffraction in Crystals. I. Theory

The reflection power ratio PH/P0, transmission power ratio PT/P0 and absorption power ratio PA/P0 for a mosaic crystal plate under asymmetric Bragg and Laue geometry are obtained by solving power transfer equations employing three dimensionless parameters: an asymmetry factor b, the ratio ξ of the absorption to the scattering cross section, and the reduced thickness Ak of the crystal. Expressions are given for the optimum monochromator thickness for asymmetric Laue geometry, and the angular dependence of PH/P0, PT/P0 for Bragg geometry in the crystals having different ξ0 and Ak0. As a demonstration of the feasibility of this method, the integrated reflection power ratio experiment by Mathieson [Acta Cryst. (1975), A31, 769–774] is reanalyzed and good agreement is obtained. The standard formulae for integrated reflection by a mosaic crystal given in International Tables for X-ray Crystallography [(1972). Birmingham: Kynoch Press] based on the kinematic approximation are shown to be only the limiting case of the exact solution for ξ0 >> 1.