X-ray Beam (x-ray + beam)

Distribution by Scientific Domains


Selected Abstracts


Radiological protection for diagnostic examination of pregnant women

CONGENITAL ANOMALIES, Issue 1 2002
Tomoko Kusama
ABSTRACT, Application of diagnostic X-ray examination to pregnant women is complicated since risks to both mother and embryo/fetus must be considered. Embryos and fetuses are more sensitive to radiation than adults or children. The threshold doses for fetal death, malformations and mental retardation which are deterministic effects, are reported to be 100,200 mGy or higher. The relative risk for childhood cancer due to radiation at an absorbed dose of 10 mGy during embryonic/fetal development has been estimated at 1.4. However, the absorbed dose of the embryo/fetus during X-ray diagnostic examination in which the X-ray beam does not irradiate the embryo/fetus directly such as maternal skull and chest X-ray is extremely low, less than 0.01 mGy. Thus these diagnostic procedures are not a problem from the perspective of radiological protection of the embryo/fetus. However, for pelvic CT scan and barium enema in which the uterus is directly within the X-ray beam, the absorbed doses to the embryo/fetus are about 20,80 mGy and 10,20 mGy, respectively. Therefore, medical staff must pay careful attention to the embryo/fetus in application of these examinations. Pregnant women who were not aware of pregnancy at the time of their diagnostic exposure have great anxiety about radiation from such X-ray examinations. However, fetal doses below 100 mGy should not be considered a reason for terminating a pregnancy. [source]


Comparative radiopacity of tetracalcium phosphate and other root-end filling materials

INTERNATIONAL ENDODONTIC JOURNAL, Issue 4 2000
C. D. Laghios
Abstract Aim This study compared the radiopacity of tetracalcium phosphate (TTCP) and 11 root-end filling materials relative to human dentine. Methodology Specimens of 2 mm thickness and a graduated aluminium stepwedge were placed on dental X-ray films and exposed to an X-ray beam. The optical densities of the specimens and aluminium steps were measured. The optical densities of the specimens were correlated to the equivalent thickness of aluminium with a regression analysis equation. The equation was used to calculate the equivalent aluminium thickness of each of the specimens. Results Nine of the materials were found to be of acceptable radiopacity (at least 2 mm Al more radiopaque than dentine). TCCP and two of the glass-ionomer compounds were found to have insufficient radiopacity to be radiographically distinguishable from human dentine. Conclusions All the materials were found to be distinguishable radiographically from dentine, except for Vitrebond, TTCP and Ketac-Fil. Amalgam was the most radiopaque material and Ketac-Fil was the least radiopaque material tested. [source]


Instrument-independent specification of the diffraction geometry and polarization state of the incident X-ray beam

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2009
Marc Schiltz
This work augments the proposal of Schwarzenbach & Flack [J. Appl. Cryst. (1989), 22, 601,605], who have advocated the use of a diffractometer-independent definition of the azimuthal angle , to specify the diffraction geometry of a Bragg reflection. It is here proposed that one additional angle ,, which is also based on a diffractometer-independent definition, is needed to encode the direction of linear polarization for those experiments where this quantity is of importance. This definition is then extended to the cases of partially and/or elliptically polarized X-ray beams, and the use of three normalized Stokes parameters, P1, P2 and P3, together with ,, is advocated in order to characterize exhaustively the polarization state of the incident beam. The conventions proposed here present a general, unambiguous and economical means of encoding the information about the diffraction geometry, without the need to record any further information about the instrument, crystal orientation matrix and goniometer angles. Data-processing software using these definitions to analyse polarization-dependent phenomena becomes instrument-independent and completely general. These methods have been implemented in the macromolecular phasing program SHARP for exploiting the polarization anisotropy of anomalous scattering in protein crystals. [source]


X-ray absorption by macromolecular crystals: the effects of wavelength and crystal composition on absorbed dose

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2004
James W. Murray
Radiation damage restricts the useful lifetime for macromolecular crystals in the X-ray beam, even at cryotemperatures. With the development of structural genomics pipelines, it will be essential to incorporate projected crystal lifetime information into the automated data collection software routines. As a first step towards this goal, a computer program, RADDOSE, is presented which is designed for use by crystallographers in optimizing the amount of data that can be obtained from a particular cryo-cooled crystal at synchrotron beamlines. The program uses the composition of the crystal and buffer constituents, as well as the beam energy, flux and dimensions, to compute the absorption coefficients and hence the theoretical time taken to reach an absorbed dose of 2,,,Gy, the so-called `Henderson limit'. At this dose, the intensity of the diffraction pattern is predicted to be halved. A `diffraction,dose efficiency' quantity is introduced, for the convenient comparison of absorbed dose per diffracted photon for different crystals. Four example cases are considered, and the implications for anomalous data collection are discussed in the light of the results from RADDOSE. [source]


Pole figure analysis of mineral nanoparticle orientation in individual trabecula of human vertebral bone

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003
Daniel Jaschouz
The spongious interior of human vertebral bone consists of interconnected trabecula with a thickness of about 0.2 mm. The texture of the bone material, a collagen-mineral nanocomposite, was studied within single trabecula by collecting two-dimensional small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) data exactly at the same specimen positions with an X-ray beam of 0.2 mm diameter. Three-dimensional information was reconstructed by combining measurements with different specimen orientations. The XRD data, and for the first time also the SAXS data, were subjected to a pole figure analysis. The method provides a quantitative description of the crystallographic orientation distribution as well as the morphological orientation distribution of the plate shaped nanoparticles, both with respect to the orientation of the investigated trabecula. As a main result it could be shown that a fibre-texture exists in the trabecula, and that the plate shaped nanoparticles are aligned with the lamellae within the trabecula. [source]


Waveguide-enhanced scattering from thin biomolecular films

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2002
F. Pfeiffer
An X-ray diffraction experiment on multilamellar membranes incorporated into an X-ray waveguide structure is reported. In the device, the lipid bilayers are confined to one side by the silicon substrate and to the other side by an evaporated thin metal cap layer. Shining a highly brilliant X-ray beam onto the system, resonantly enhanced, precisely defined and clearly distinguishable standing-wavefield distributions (modes) are excited. The in-plane structure of the acyl chain ordering is then studied by grazing incidence diffraction under simultaneously excited modes. A significant gain in signal-to-noise ratio as well as enhanced spatial resolution can be obtained with such a setup. [source]


A furnace for in situ X-ray diffraction studies of insertion processes in electrode materials at elevated temperatures

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2001
T. Eriksson
A furnace is described for in situ X-ray diffraction studies, in transmission mode, of structural changes in electrode materials for Li-ion (polymer) batteries in the ambient to 300C temperature range. The method exploits the thin flat-cell geometry of the lithium-polymer battery concept. The flat sample is able to oscillate about a horizontal axis in its own plane in the X-ray beam, to provide better averaging during the diffraction experiment. The use of the device is demonstrated in a study of lithium intercalation in graphite (a commonly used anode material in lithium-ion batteries) during electrochemical cycling and storage at 70C. [source]


MarqX: a new program for whole-powder-pattern fitting

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2000
Y. H. Dong
MarqX is a computer program for the modelling of powder diffraction data. It can be used for an unconstrained profile fitting (pattern decomposition, PD) or constrained modelling of the whole powder pattern (Pawley method, PM), for single- as well as multiple-phase samples. The program output includes: lattice parameters or peak positions (for PM and PD, respectively), width and shape of the diffraction peak (in terms of half width at half-maximum and mixing parameter of a pseudo-Voigt function), corrected for the instrumental broadening component, intensity, peak area and profile asymmetry. In addition, errors on the goniometer zero and shift in sample position with respect to the goniometric axis can also be modelled, together with distance and relative intensity of the spectral components of the X-ray beam (e.g.K,1 and K,2). Specific output files are provided for line-profile analysis, including the Williamson,Hall plot and Warren,Averbach method. [source]


X-ray fluorescence microprobe imaging in biology and medicine

JOURNAL OF CELLULAR BIOCHEMISTRY, Issue 6 2006
Tatjana Paunesku
Abstract Characteristic X-ray fluorescence is a technique that can be used to establish elemental concentrations for a large number of different chemical elements simultaneously in different locations in cell and tissue samples. Exposing the samples to an X-ray beam is the basis of X-ray fluorescence microscopy (XFM). This technique provides the excellent trace element sensitivity; and, due to the large penetration depth of hard X-rays, an opportunity to image whole cells and quantify elements on a per cell basis. Moreover, because specimens prepared for XFM do not require sectioning, they can be investigated close to their natural, hydrated state with cryogenic approaches. Until several years ago, XFM was not widely available to bio-medical communities, and rarely offered resolution better then several microns. This has changed drastically with the development of third-generation synchrotrons. Recent examples of elemental imaging of cells and tissues show the maturation of XFM imaging technique into an elegant and informative way to gain insight into cellular processes. Future developments of XFM,building of new XFM facilities with higher resolution, higher sensitivity or higher throughput will further advance studies of native elemental makeup of cells and provide the biological community including the budding area of bionanotechnology with a tool perfectly suited to monitor the distribution of metals including nanovectors and measure the results of interactions between the nanovectors and living cells and tissues. J. Cell. Biochem. 99: 1489,1502, 2006. 2006 Wiley-Liss, Inc. [source]


Application of kinoform lens for X-ray reflectivity analysis

JOURNAL OF SYNCHROTRON RADIATION, Issue 2 2010
M. K. Tiwari
In this paper the first practical application of kinoform lenses for the X-ray reflectivity characterization of thin layered materials is demonstrated. The focused X-ray beam generated from a kinoform lens, a line of nominal size ,50,m 2,m, provides a unique possibility to measure the X-ray reflectivities of thin layered materials in sample scanning mode. Moreover, the small footprint of the X-ray beam, generated on the sample surface at grazing incidence angles, enables one to measure the absolute X-ray reflectivities. This approach has been tested by analyzing a few thin multilayer structures. The advantages achieved over the conventional X-ray reflectivity technique are discussed and demonstrated by measurements. [source]


The ID23-2 structural biology microfocus beamline at the ESRF

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2010
David Flot
The first phase of the ESRF beamline ID23 to be constructed was ID23-1, a tunable MAD-capable beamline which opened to users in early 2004. The second phase of the beamline to be constructed is ID23-2, a monochromatic microfocus beamline dedicated to macromolecular crystallography experiments. Beamline ID23-2 makes use of well characterized optical elements: a single-bounce silicon (111) monochromator and two mirrors in Kirkpatrick,Baez geometry to focus the X-ray beam. A major design goal of the ID23-2 beamline is to provide a reliable, easy-to-use and routine microfocus beam. ID23-2 started operation in November 2005, as the first beamline dedicated to microfocus macromolecular crystallography. The beamline has taken the standard automated ESRF macromolecular crystallography environment (both hardware and software), allowing users of ID23-2 to be rapidly familiar with the microfocus environment. This paper describes the beamline design, the special considerations taken into account given the microfocus beam, and summarizes the results of the first years of the beamline operation. [source]


Use of a hexapod in diffraction measurements of substrate-supported crystals of organic semiconductors

JOURNAL OF SYNCHROTRON RADIATION, Issue 6 2009
Lin Yang
Thin films of organic semiconductor prepared on substrates generally contain crystals that have one common crystal plane parallel to the substrate but random in-plane orientations. In diffraction measurements of these structures, it is often required to anchor the X-ray beam on a fixed spot on the sample, such as an optically visible crystallite or island. Here, a hexapod is used in place of a traditional multi-circle diffractometer to perform area-detector-based diffraction measurements on an actual device that contains 6,13-bis(triisopropylsilyethynyl)-pentacene (TIPS-pentacene) crystals. The hexapod allows for sample rotations about any user-defined rotation center. Two types of complex sample motions have been programmed to characterize the structure of the TIPS-pentacene crystal: an in-plane powder average has been performed at a fixed grazing-incident angle to determine the lattice parameters of the crystal; then the in-plane component of the scattering vector was continuously rotated in transmission geometry to determine the local crystal orientation. [source]


Polycapillary-optics-based micro-XANES and micro-EXAFS at a third-generation bending-magnet beamline

JOURNAL OF SYNCHROTRON RADIATION, Issue 2 2009
Geert Silversmit
A focusing system based on a polycapillary half-lens optic has been successfully tested for transmission and fluorescence -X-ray absorption spectroscopy at a third-generation bending-magnet beamline equipped with a non-fixed-exit Si(111) monochromator. The vertical positional variations of the X-ray beam owing to the use of a non-fixed-exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K -edge is approximately 200,m on the lens input side and this was reduced to ,1,m for the focused beam. Beam sizes (FWHM) of 12,16,m, transmission efficiencies of 25,45% and intensity gain factors, compared with the non-focused beam, of about 2000 were obtained in the 7,14,keV energy range for an incoming beam of 0.5 2,mm (vertical horizontal). As a practical application, an As K -edge -XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant. [source]


Image contrast in X-ray reflection interface microscopy: comparison of data with model calculations and simulations

JOURNAL OF SYNCHROTRON RADIATION, Issue 6 2008
P. Fenter
The contrast mechanism for imaging molecular-scale features on solid surfaces is described for X-ray reflection interface microscopy (XRIM) through comparison of experimental images with model calculations and simulated measurements. Images of elementary steps show that image contrast is controlled by changes in the incident angle of the X-ray beam with respect to the sample surface. Systematic changes in the magnitude and sign of image contrast are asymmetric for angular deviations of the sample from the specular reflection condition. No changes in image contrast are observed when defocusing the condenser or objective lenses. These data are explained with model structure-factor calculations that reproduce all of the qualitative features observed in the experimental data. These results provide new insights into the image contrast mechanism, including contrast reversal as a function of incident angle, the sensitivity of image contrast to step direction (i.e. up versus down), and the ability to maximize image contrast at almost any scattering condition defined by the vertical momentum transfer, Qz. The full surface topography can then, in principle, be recovered by a series of images as a function of incident angle at fixed momentum transfer. Inclusion of relevant experimental details shows that the image contrast magnitude is controlled by the intersection of the reciprocal-space resolution function (i.e. controlled by numerical aperture of the condenser and objective lenses) and the spatially resolved interfacial structure factor of the object being imaged. Together these factors reduce the nominal contrast for a step near the specular reflection condition to a value similar to that observed experimentally. This formalism demonstrates that the XRIM images derive from limited aperture contrast, and explains how non-zero image contrast can be obtained when imaging a pure phase object corresponding to the interfacial topography. [source]


Diffraction-based automated crystal centering

JOURNAL OF SYNCHROTRON RADIATION, Issue 2 2007
Jinhu Song
A fully automated procedure for detecting and centering protein crystals in the X-ray beam of a macromolecular crystallography beamline has been developed. A cryo-loop centering routine that analyzes video images with an edge detection algorithm is first used to determine the dimensions of the loop holding the sample; then low-dose X-rays are used to record diffraction images in a grid over the edge and face plane of the loop. A three-dimensional profile of the crystal based on the number of diffraction spots in each image is constructed. The derived center of mass is then used to align the crystal to the X-ray beam. Typical samples can be accurately aligned in ,2,3,min. Because the procedure is based on the number of `good' spots as determined by the program Spotfinder, the best diffracting part of the crystal is aligned to the X-ray beam. [source]


Cryoradiolytic reduction of crystalline heme proteins: analysis by UV-Vis spectroscopy and X-ray crystallography

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2007
Thorsten Beitlich
The X-ray crystallographic analysis of redox-active systems may be complicated by photoreduction. Although radiolytic reduction by the probing X-ray beam may be exploited to generate otherwise short-lived reaction intermediates of metalloproteins, it is generally an undesired feature. Here, the X-ray-induced reduction of the three heme proteins myoglobin, cytochrome P450cam and chloroperoxidase has been followed by on-line UV-Vis absorption spectroscopy. All three systems showed a very rapid reduction of the heme iron. In chloroperoxidase the change of the ionization state from ferric to ferrous heme is associated with a movement of the heme-coordinating water molecule. The influence of the energy of the incident X-ray photons and of the presence of scavengers on the apparent reduction rate of ferric myoglobin crystals was analyzed. [source]


On the design of ultrafast shutters for time-resolved synchrotron experiments

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2007
Milan Gembicky
A comprehensive treatment of the limitations and possibilities for single-pulse selection in synchrotron operating modes with ,150,ns bunch separation, as occurs in the standard operating mode at the Advanced Photon Source, is presented. It is shown that the strength of available materials and allowable kinetic energy build-up limit single-bunch selection for this separation to sample sizes of ,100,m, and that for minimization of kinetic energy build-up it is preferable to increase the r.p.m. within physically acceptable limits rather than increase the disc radius to obtain a desirable peripheral speed. A slight modification of the equal-bunch spacing standard fill patterns is proposed that allows use of samples as large as 500,m. The corresponding peripheral speed of the chopper wheel is ,600,m,s,1, which is within the limits of high-strength titanium alloys. For smaller samples, peripheral speeds are proportionally lower. Versatility can be achieved with interchangeable chopper wheels and the use of different orientations of the rotation axis relative to the X-ray beam, which opens the possibility of larger, rather than one-of-a-kind, production runs. [source]


A sagittally focusing double-multilayer monochromator for ultrafast X-ray imaging applications

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2007
Jinyuan Liu
The development of a sagittally focusing double-multilayer monochromator is reported, which produces a spatially extended wide-bandpass X-ray beam from an intense synchrotron bending-magnet source at the Advanced Photon Source, for ultrafast X-ray radiography and tomography applications. This monochromator consists of two W/B4C multilayers with a 25, period coated on Si single-crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X-ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X-ray radiography and tomography. [source]


The ID23-1 structural biology beamline at the ESRF

JOURNAL OF SYNCHROTRON RADIATION, Issue 3 2006
Didier Nurizzo
The demand for access to macromolecular crystallography synchrotron beam time continues to increase. To meet this demand the ESRF has constructed a dual station beamline using a canted undulator system as the X-ray source. The first phase of the beamline to be constructed is ID23-1, a tunable MAD-capable station with a mini-focus X-ray beam. The beamline makes use of well characterized optical elements: a channel-cut monochromator with a high-precision toroidal mirror to focus the X-ray beam. The beamline has been conceived with the aim of providing high levels of automation to create an industrial-like environment for protein crystallography. A new software suite has been developed to permit reliable easy operation for the beamline users and beamline staff. High levels of diagnostics are built in to allow rapid trouble-shooting. These developments are now being exported to the ESRF macromolecular crystallography beamline complex and have been made in a modular fashion to facilitate transportability to other synchrotrons. [source]


X-ray induced reduction of the crystal of high-molecular-weight cytochrome c revealed by microspectrophotometry

JOURNAL OF SYNCHROTRON RADIATION, Issue 1 2004
Midori Sato
The crystal structures of high-molecular-weight cytochrome c (HMC) from Desulfovibrio vulgaris Hildenborough in the transient and reduced states have been determined at 2.8 resolution. An absorption spectrum measured with microspectrophotometor indicated that about 86% of the hemes were reduced after 45 min irradiation of the X-ray beam. Further exposure for 90 min did not significantly change the spectrum. These results suggest that HMC in the crystalline state is easily reduced by illumination of the X-ray beam from synchrotron radiation. [source]


A compact optical design for Bragg reflections near backscattering

JOURNAL OF SYNCHROTRON RADIATION, Issue 5 2001
Alfred Q. R. Baron
A very compact in-line X-ray optical design is presented which is optimized for use with Bragg reflections close to backscattering (Bragg angles near 90). The essential idea is to use a low-order Bragg reflection to couple the X-ray beam into a small channel-cut backscattering crystal. The design is demonstrated in an almost theoretically performing high-resolution monochromator providing 2 108 photons,s,1 in a 0.52,meV bandwidth at 25.65,keV. The monochromator is used to measure inelastic nuclear scattering from phonons in 161Dy-containing samples. [source]


A position-sensitive ionization chamber for XAFS studies at synchrotron sources

JOURNAL OF SYNCHROTRON RADIATION, Issue 2 2001
Kazumichi Sato
A position-sensitive ionization chamber has been developed with backgammon-type-segmented electrodes. This novel detector possesses a linear range of 8 mm for determining the incident position of the X-ray beam incoming. The position resolution was found to be less than 10 m, probably close the sub-micrometer region. Owing to its high spatial resolution, the position-sensitive ionization chamber was able to commit that the gradual decrease observed in the X-ray beam intensity at a SPring-8 beamline was mainly due to the spatial variation of the X-ray beam in time. The present work also confirmed the applicability of the novel detector to the accurate monochromator adjustment for experiments using diamond anvil cells. [source]


Effect of surface roughness on the spatial coherence of X-ray beams from third-generation synchrotron radiation sources

JOURNAL OF SYNCHROTRON RADIATION, Issue 4 2000
Yun Wang
The effect of the surface roughness of optical elements, such as Be windows and reflection mirrors, in synchrotron radiation beamlines on the spatial coherence of the X-ray beam is investigated systematically by means of digital simulation, in which a new model for X-ray reflection from a rough surface is proposed. A universal factor is employed to evaluate the spatial coherence quantitatively, based on which critical values for surface roughness are reached. The results from simulation are consistent with those from experiments. [source]


Design of a vacuum-compatible high-precision monochromatic beam-position monitor for use with synchrotron radiation from 5 to 25,keV

JOURNAL OF SYNCHROTRON RADIATION, Issue 2 2000
R. W. Alkire
The Structural Biology Center beamline, 19ID, has been designed to take full advantage of the highly intense undulator radiation and very low source emittance available at the Advanced Photon Source. In order to keep the X-ray beam focused onto the pre-sample slits, a novel position-sensitive PIN diode array has been developed. The array consists of four PIN diodes positioned upstream of a 0.5,m-thick metal foil placed in the X-ray beam. Using conventional difference-over-the-sum techniques, two-dimensional position information is obtained from the metal foil fluorescence. Because the full X-ray beam passes through the metal foil, the true beam center-of-mass is measured. The device is compact, inexpensive to construct, operates in a vacuum and has a working range of 8,mm 10,mm that can be expanded with design modifications. Measured position sensitivity is 1,2,m. Although optimized for use in the 5,25,keV energy range, the upper limit can be extended by changing metals or adjusting foil thickness. [source]


2D mapping of the response of CVD diamond X-ray detectors: defects and device dynamics

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2004
M. J. Guerrero
Abstract To investigate the influence of intrinsic defects in polycrystalline CVD diamond, we have used a micro-focused X-ray beam to induce local photo-currents in solid state ionisation chambers. The device behaviour was studied as a function of the defect level populations. This microscopic study of the X-ray sensitivity was then performed using varying initial states of the devices as well as varying device temperatures. These measurements, coupled with the study of the temporal evolution of the photocurrent as a function of the temperature, seemed to demonstrate the existence of highly localised regions that may detrimentally affect the overall device response. This may demonstrate that the imperfections CVD diamond devices exhibit may be caused by extremely localised point defects. ( 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Structural and optical characterization of (11-22) semipolar GaN on m -plane sapphire without low temperature buffer layer

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010
Sung-Nam Lee
Abstract We reported the high quality semipolar (11-22) GaN grown on m-sapphire by using the novel two-step growth method without low temperature GaN or AlN buffer layer. It is found that macroscopic surface morphology of semipolar GaN epilayer was very smooth, while microscopic surface structure was arrowhead-like surface structure toward the direction of [1-21-1]. Anisotropic crystal properties of semipolar GaN/m-sapphire were also observed by two incident directions of X-ray beam. Therefore, we suggested that the anisotropic crystal properties and arrow-head like surface structure would be caused by heteroepitaxial crystallograhpic difference between semipolar GaN and m-sapphire. Additionally, photoluminescence (PL) measurements showed the strong bandedge emission of n-type semipolar GaN without yellow luminescence ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Synchrotron microscopy and spectroscopy for analysis of crystal defects in silicon

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2009
Winfried Seifert
Abstract The paper discusses the synchrotron-based microprobe techniques XBIC (X-ray beam induced current), ,-XRF (X-ray fluorescence microscopy) and ,-XAS (X-ray absorption microspectroscopy) and their application for studying electrical activity of defects and precipitation of transition metals in Si materials. Investigations were performed on samples of block-cast multicrystalline Si and on model samples cut from a bonded monocrystalline wafer. To analyze the precipitation sites, Ni, Cu and Fe were introduced intentionally into the samples. The detected precipitates were found to consist of silicides. Evidence for metal precipitates was also found in virtually uncontaminated as-grown block-cast Si. Besides Ni precipitates detected at a recombination active grain boundary, particles containing one or several metals (Cu, Fe, Ti, V) were observed. Unexpectedly, these particles seem to exhibit low only recombination activity. Further studies are necessary to identify their nature. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Diffraction with a coherent X-ray beam: dynamics and imaging

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2007
Frdric Livet
Methods for carrying out coherent X-ray scattering experiments are reviewed. The brilliance of the available synchrotron sources, the characteristics of the existing optics, the various ways of obtaining a beam of controlled coherence properties and the detectors used are summarized. Applications in the study of the dynamics of speckle patterns are described. In the case of soft condensed matter, the movement of inclusions like fillers in polymers or colloidal particles can be observed and these can reflect polymer or liquid-crystal fluctuations. In hard condensed-matter problems, like phase transitions, charge-density waves or phasons in quasicrystals, the study of speckle fluctuations provides new time-resolved methods. In the domain of lensless imaging, the coherent beam gives the modulus of the sample Fourier transform. If oversampling conditions are fulfilled, the phase can be obtained and the image in the direct space can be reconstructed. The forthcoming improvements of all these techniques are discussed. [source]


Systematic intensity errors caused by spectral truncation: origin and remedy

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2001
A. T. H. Lenstra
The wavelength dispersion of graphite(002)-monochromated X-ray beams has been determined for a Cu, a Mo and an Rh tube. The observed values for ,,, were 0.03, 0.14 and 0.16, respectively. The severe reduction in monochromaticity as a function of wavelength is determined by the absorption coefficient , of the monochromator. ,(monochromator) varies with ,3. For an Si monochromator with its much larger absorption coefficient, ,,, values of 0.03 were found, regardless of the X-ray tube. This value matches a beam divergence defined by the size of the focus and of the crystal. This holds as long as the monochromator acts as a mirror, i.e.,(monochromator) is large. In addition to monochromaticity, homogeneity of the X-ray beam is also an important factor. For this aspect the mosaicity of the monochromator is vital. In cases like Si, in which mosaicity is practically absent, the reflected X-ray beam shows an intensity distribution equal to the mass projection of the filament on the anode. Smearing by mosaicity generates a homogeneous beam. This makes a graphite monochromator attractive in spite of its poor performance as a monochromator for , < 1,. This choice means that scan-angle-induced spectral truncation errors are here to stay. These systematic intensity errors can be taken into account after measurement by a software correction based on the real beam spectrum and the applied measuring mode. A spectral modeling routine is proposed, which is applied on the graphite-monochromated Mo K, beam. Both elements in that spectrum, i.e. characteristic ,1 and ,2 emission lines and the Bremsstrahlung, were analyzed using the 6318 reflection of Al2O3 (s = 1.2,,1). The spectral information obtained was used to calculate the truncation errors for intensities measured in an ,2, scan mode. The results underline the correctness of previous work on the structure of NiSO46H2O [Rousseau, Maes & Lenstra (2000). Acta Cryst. A56, 300,307]. [source]


Unusual hydrate stabilization in the two-dimensional layered structure of quinacrinium bis(2-carboxy-4,5-dichlorobenzoate) tetrahydrate, a proton-transfer compound of the drug quinacrine

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009
Graham Smith
The crystal structure of the hydrated proton-transfer compound of the drug quinacrine [rac- N,-(6-chloro-2-methoxyacridin-9-yl)- N,N -diethylpentane-1,4-diamine] with 4,5-dichlorophthalic acid, C23H32ClN3O2+2C8H3Cl2O4,4H2O, has been determined at 200,K. The four labile water molecules of solvation in the structure form discrete ...O,H...O,H... hydrogen-bonded chains parallel to the quinacrine side chain, the two N,H groups of which act as hydrogen-bond donors for two of the water acceptor molecules. The other water molecules, as well as the acridinium H atom, also form hydrogen bonds with the two anion species and extend the structure into two-dimensional sheets. Between these sheets there are also weak cation,anion and anion,anion ,,, aromatic ring interactions. This structure represents the third example of a simple quinacrine derivative for which structural data are available but differs from the other two in that it is unstable in the X-ray beam due to efflorescence, probably associated with the destruction of the unusual four-membered water chain structures. [source]