Gamma-ray Diffraction Research Articles

Optimization of highly oriented pyrolytic graphite applied to neutron crystal optics

The neutron diffraction properties of highly oriented pyrolytic graphite (HOPG) are reviewed using experimental results that have been obtained by diffraction of high-energy gamma rays, X-rays and neutrons. The interpretation of the empirical data based on diffraction theory leads to generic diagrams that display the performance of HOPG as a function of crystal thickness, mosaic spread and neutron wavelength. The analysis of the relation between the defect structure and diffraction properties demonstrates the usefulness of a detailed X-ray diffraction study to maximize the efficiency of composite neutron monochromators and analyzers. The optimization procedure is illustrated by the configuration of a double-focusing monochromator.

The role of defects for the lock-in transition and the formation of polar nanodomains in A2BX4-compounds

ABSTRACTHigh-resolution gamma-ray diffraction in combination with in situ impedance spectroscopy has been used to characterize the lock-in transition in high-quality (K1-xRbx)2ZnCl4 mixed crystals with different doping levels x (0 < x < 7.5%). It is shown that the phase transition becomes extremely sluggish if the defect concentration is increased. While the incommensurate phase is destroyed quite rapidly, the adaption of the stable and well-ordered commensurate structure takes several days. In between there is a strongly disordered phase characterized by residual discommensurations which are pinned by the impurity defects. By careful preparation of the samples and consideration of the kinetic behavior, the lock-in transition and the interesting regime just above Tc where polar nanodomains exist can be shifted towards room temperature.

Dzyaloshinskii–Moriya interaction: How to measure its sign in weak ferromagnets?

Three experimental techniques sensitive to the sign of the Dzyaloshinskii--Moriya interaction are discussed: neutron diffraction, Moessbauer gamma-ray diffraction, and resonant x-ray scattering. Classical examples of hematite (alpha-Fe2O3) and MnCO3 crystals are considered in detail

Switching behaviour of modulated ferroelectrics I: kinetics of the field-induced lock-in transition of Rb2ZnCl4

The structural changes of ferroelectric Rb(2)ZnCl(4) have been studied in real time under the influence of pulsed electric fields by high-resolution gamma-ray diffraction close to the lock-in transition at 198.3 K. A well-defined field-induced switching between the incommensurate and the commensurate phase is observed only in virgin crystals on a time-scale of milliseconds. Crystals aged under high electric fields, however, exhibit an intermediate diffuse phase that is attributed to pinning effects of discommensurations, i.e. the walls of nano-domains in the incommensurate phase. Surprisingly, the incommensurate phase itself is stabilized and the transition into the ferroelectric phase is shifted to lower temperatures as soon as the frequency of the cycled electric field is increased. This unusual finding is discussed in terms of stress fields associated with the motion of discommensurations.

An “ESA-Affordable” Laue-lens

With ESA’s INTEGRAL mission gamma-ray astronomy has advanced to the point where major scientific advances must be expected from detailed studies of the many new point sources. The interest in developing focusing telescopes operating in the soft gamma-ray regime up to 1 MeV is therefore mounting rapidly. Telescopes based on Laue diffraction of gamma-rays from crystals appear as one promising route, although the practical difficulties of realizing a large scale Laue lens are certainly not small. In this paper I have attempted to develop an optimized lens design considering the size and mass constraints of a specific medium size launch vehicle. The introduction of the lens mass as a primary design driver has some surprising effects for the choice of material for the crystals and new tradeoff considerations are introduced.

Copper Mosaic Crystals for Laue Lenses

Large single crystals of copper with an uniform and very narrow mosaic spread between 25 seconds and 1 minute of arc are now available at I.L.L. This result is of great interest in the construction of a Laue lens for astrophysical applications for which such quality copper single crystals may be used. The X-ray diffraction properties of copper single crystals produced at I.L.L. were studied for x-ray energies ranging from 100 keV to 400 keV. Several monocrystalline plates with different thicknesses and mosaic distributions were then prepared from the as-grown crystals in order to measure their diffraction efficiency as a function of energy. As expected, the value of the peak reflectivity depends on the crystal thickness. Reflectivity measurements show the excellent properties of copper crystals for gamma-ray diffraction. A peak reflectivity of 24% was measured at 220 keV from a copper single crystal of 3.75 mm thickness having a mosaic spread of 1.5 minutes of arc. Some technical aspects on the preparation of copper single crystal plates are also discussed.

Electron-density distribution in cubic SrTiO3: a comparative γ-ray diffraction study

The electron density and atomic displacements in the perovskite SrTiO(3) have been studied using extensive and accurate gamma-ray diffraction data (lambda = 0.0392 A) at room temperature. The six strongest low-order structure factors have been determined under extinction-free conditions. Gram-Charlier series expansion of the thermal parameters have revealed no evidence for anharmonicity. The population of the 3d subshell on Ti is found to be close to zero, in agreement with the observed magnetic behaviour. The electronic properties at the bond critical points indicate ionic Ti-O and Sr-O interactions of different strengths, which is corroborated by the net charges of the atomic basins [q(Sr) = 1.18 |e|, q(Ti) = 3.10 |e|, q(O) = -1.42 |e|]. A critical comparison is made with earlier experimental results from laboratory X-ray, synchrotron X-ray, electron and neutron diffraction studies. Agreement and discrepancies are identified and resolved.

Electron densities in prototypic antiferromagnets from gamma-ray diffraction

Electron densities in prototypic antiferromagnets from gamma-ray diffraction

Electron densities in relation to magnetism: gamma-ray diffraction from transition-metal monoxides and difluorides.

In this article, results are described from an area of solid-state physics that heretofore has remained unexplored by Bragg charge-scattering experiments. It is shown that gamma-ray electron densities can provide detailed information on magnetic ground-state properties at a subatomic resolution. The use of 316 keV gamma radiation has allowed low-order structure factors at better than 0.1% accuracy to be determined and, at the same time, extended data sets to be collected up to high momentum transfer. Prototypic antiferromagnets such as the late 3d transition-metal monoxides and difluorides have been systematically investigated, in most cases both in the paramagnetic and in the fully ordered phase. At transitions with a lowering of symmetry, multidomain formation was suppressed by application of external mechanical stress or magnetic fields. The results are considered in the context of information provided by recent magnetic neutron and X-ray studies.

Reconsideration of the wavelength dependence of extinction: a γ-ray case

The wavelength dependence of extinction has been re-examined by means of new gamma-ray diffraction data collected from NiF(2) at 0.0205, 0.0265 and 0.0392 A. The standard model of extinction has been confirmed and substantiated by experimentally accessible parameters such as the intrinsic mosaic width. The predicted kinematic limit values are in accordance with the results from charge-density analyses. The model proposed by Mathieson & Stevenson [Acta Cryst. (2002), A58, 185-189] leads to a purely phenomenological polynomial fit function. The extrapolated values are shown to be inconsistent with structure analyses based on extended gamma-ray data sets and cannot be recommended for use.

Orbital ordering in LaMnO3: estimates of structure factors and comparison of measurement methods.

This paper shows that the phenomenon of orbital ordering should be detectable by energy-filtered quantitative convergent-beam electron diffraction (QCBED). The structure factors of LaMnO(3) crystals are calculated using a non-spherical atomic scattering model of the Mn(3+) ion. Several low-order electron structure factors showed pronounced change with orbital ordering, in which the e(g)(1) electron orders in the 3d(3z(2) - r(2)) orbital leaving the 3d(x(2) - y(2)) unoccupied. In contrast, the X-ray structure factors showed very small change. Orbital order is important in transition-metal oxides, including colossal magnetoresistive manganite oxides. The calculations show that by using QCBED it is possible to measure the subtle changes in electron structure factors due to orbital ordering of the e(g)(1) electron of the Mn(3+) ion in an LaMnO(3) crystal. A comparison of methods for structure-factor measurement is given, including Bragg X-ray and gamma-ray diffraction, X-ray Pendellösung and critical-voltage methods. New measurements by QCBED of structure factors in rutile are compared with the Bragg X-ray values. These show that QCEBD can provide an accurate extinction-free measurement of low-order structure factors, which is extremely difficult or perhaps impossible when using other methods applied to real crystals.

Correction for thermal diffuse scattering in single-crystal time-of-flight neutron diffraction: first applications

The thermal diffuse scattering (TDS) correction developed by Popa &amp; Willis [Acta Cryst.(1997), A53, 537–545] for single-crystal pulsed neutron diffraction is examinedviaa series of experimental data sets collected at the spallation neutron source IPNS. The effect on the derived temperature parameter values is assessed and compared with reference structures obtained from gamma-ray diffraction. The importance of carrying out TDS corrections is emphasized. Suggestions are made regarding the method of evaluation of the TDS in situations where the elastic constants are unknown.

SANS measurements of deuteride (hydride) formation in single crystal Pd

Abstract In situ small-angle neutron scattering (SANS) measurements of deuteride precipitation in well-annealed, undeformed single crystal Pd have been performed at room temperature to observe the morphology of the precipitate as the deuterium fraction was increased over a range of 0.005 to 0.097 [D]/[Pd]. Supporting measurements were made of the room temperature deuterium solubility isotherm in the single crystal material and of the progressive lattice mosaic broadening by gamma-ray diffraction analysis. The use of single crystal material eliminated the effect of grain boundaries on the precipitation process and allowed the orientation of the deuteride precipitates relative to the host Pd lattice to be established. The SANS data for deuterium loading beyond the solid solution phase are dominated by a d Σ /d Ω ∼ Q −2 scattering response well fit by a model cross section of small plate-like precipitates with a thickness of 25–30 A. This response showed little indication of a preferred habit plane and the plate thickness was independent of the total deuterium loading. The volume fraction of these small plates grew uniformly with increased deuterium loading, but was surprisingly small, reaching only 8×10 −5 at 0.060 [D]/[Pd]. This means that almost all of the deuterium resides in some other precipitate structure. The SANS data in the lowest Q region showed a second, anisotropic scattering component that can be interpreted as the Porod tail from plate-like particles of grossly larger dimensions. These plates favor a (001) α habit plane, normal to the elastically soft [001] α direction. Post-measurement visual inspection of the sample surfaces showed the presence of an oriented, quasi-periodic one-dimensional array of plates with thicknesses of approximately 0.1 to 0.2 mm and with an orientation in agreement with the anisotropy of the SANS data. We conclude that most of the deuteride phase precipitates in these large structures.

Application of the statistical dynamical theory to gamma-ray diffraction experiments on low-dislocation quartz single crystals.

Highly monochromatic (Deltalambda/lambda = 10(-6)) short-wavelength (lambda = 0.03 Å) gamma radiation has been used in the study of the diffraction processes in real single crystals. Dislocation-free silicon crystals and quartz crystals with small concentrations of chaotically distributed dislocations were investigated. The experimental results were processed with use of statistical dynamical theories of diffraction, both fundamental and advanced. It is shown that the advanced version is more reliable in both cases. However, the values of the average lattice phase E, describing the long-range distortions of the lattice, can differ markedly from the value of 1 for quartz crystals, and parameter L = -ln(E) has a square dependence on the extinction length.

Gamma-ray diffraction in dislocation-containing quartz single crystals

Gamma-ray diffraction in dislocation-containing quartz single crystals

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Vacancy-related defects introduced into n-Si during annealing or aluminium diffusion at high temperature (1000–1250°C) have been studied. Different ambients (argon, nitrogen, vacuum and chlorine-containing atmosphere) were used to create a vacancy supersaturation during heat treatments. Three deep-level centers whose formation is governed by the presence of vacancies have been identified. They were characterized by the following temperature dependences of the thermal emission rate:e3 = 7.92 × 107T2 × exp(− 0.455/kT),e5 = 2.64 × 106T2 × exp( − 0.266/kT),e7 = 7.26 × 106T2 × exp (− 0.192/kT). The influence of different factors, such as heat-treatment conditions, concentration of oxygen and doping level in initial crystals, on center formation was studied. An asymmetric diffuseγ-ray scattering was observed near the surface of a crystal irradiated by thermal neutrons and annealed in a chlorine-containing atmosphere. This scattering is related to the formation of structural defects of the vacancy type. In the same region of the crystal, the concentration of the E7 center was one order of magnitude higher than that of other deep-level centers. Comparison of theγ-ray diffraction and deeplevel transient spectroscopy (DLTS) data suggests that the formation of the center occurs under the conditions of Si supersaturation with vacancies.

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Mössbauer diffraction measurements on polycrystalline bcc iron

In the early days of Mossbauer spectroscopy, it was demonstrated that coherent scattering of photons emitted by a Mossbauer source can be observed. In spite of the fact that scattering experiments could give information not accessible by the absorption method, they are not widely used. The reasons for this are, on the one hand, experimental difficulties, and on the other hand, the form of the samples, which should be large single crystals in most cases. In this work, we present a Mossbauer diffraction measurement on polycrystalline bcc iron where these problems have been overcome. We demonstrate that using the kinematical theory of gamma-ray diffraction, useful information on the relative orientation of the crystallographic axis to the hyperfine field directions can be subtracted.

Gamma-ray diffraction study of surface regions in Si single crystals and substrates of CaF 2/Si MBE heterostructures and (Ti-Ni)/Si mono- and bilayer systems

For the first time γ-ray diffractometry investigations of narrow near-surface regions of samples have been carried out. The original technique has been developed and successfully applied for the separation of the micro-defect and macro-distortion influences upon γ-radiation diffraction processes in real imperfect crystals. The results indicate that the γ-ray diffraction is sensitive to the influence of even thin layers on a thick substrate

Gamma-ray diffraction in the study of silicon

Abstract Diffraction of high-monochromaticity (δλ/λ⩽10 −6 ) , short-wavelength (λ=0.003 nm ) gamma radiations was used to study the defect structure changes arising during different treatments of silicon single crystals and p−n structures. Absolute values of the integrated reflectivity for a few orders of reflection as well as rocking curves were derived by ω-step scanning in transmission geometry. The theoretical foundations relevant to the interpretation of the experimental data and characterization of the samples are given. The main advantages of γ-ray diffraction, in particular high sensitivity to lattice distortions, have been demonstrated in a study of the defect structure changes taking place during crystal growth, neutron transmutation doping and device structure formation. The investigations showed high efficiency of the heat treatment in chlorine-containing atmosphere as a means of decreasing the structural defect concentration in single crystals and p−n structures. The role of non-equilibrium intrinsic point defects in defect formation during thermochemical treatments is discussed.