Bent Crystal Monochromator Research Articles

The optics of focusing bent-crystal monochromators on X-ray powder diffractometers with application to lattice parameter determination and microstructure analysis.

The use of an Incident Beam Monochromator (IBM) in an X-ray powder diffractometer modifies both the shape of the spectrum from the X-ray source, and the relation between the apparent diffracted angle and the actual wavelength of the X-ray. For high-accuracy work, the traditional assumption of a narrow line of typically Gaussian shape does not suffice. Both the shape of the tails of peaks, and their width, can be described by a new model which couples the dispersion from the optic to the dispersion from the powder sample, and to its transport to a detector. This work presents such a model, and demonstrates that it produces excellent fits via the Fundamental Parameter Approach, and requires few free parameters to achieve this. Further, the parameters used are directly relatable to physical characteristics of the diffractometer optics. This agreement is critical for the evaluation of high-precision lattice parameters and crystal microstructural parameters by powder diffraction.

Dispersive double bent crystal monochromators Si(111) + Si(311) and Si(111) + Si(400) with a strongly asymmetric diffraction geometry of the analyzer for powder diffractometry

In this paper, some results of neutron diffraction properties of the dispersive double-crystal Si(111) + Si(311) and Si(111) + Si(400) monochromator settings containing two bent perfect crystals but with the second one – analyzer in the strongly asymmetric diffraction geometry, are presented. For the sake of possible applications, both double crystal settings were tested in the orientation of the second crystal for the output beam compression geometry. Powder diffraction test was carried out on a α-Fe(211) pin of the diameter of 2 mm. Contrary to the Si(111) + Si(400) setting, an excellent resolution represented by full width at half maximum in the (Δd/d)-scale was obtained for the Si(111) + Si(311) double crystal monochromator in the parallel as well as in the antiparallel diffraction geometry.

Characterization and on-line adjustment of the sagittal-bent Laue crystal profile.

The sagittal-bent Laue monochromator can provide an ideal way to focus high-energy X-ray beams. However, the anticlastic curvature induced by sagittal bending has a great influence on the crystal performance. Thus, characterizing the bent-crystal shape is very important for predicting the performance of the bent-crystal monochromator. In this paper the crystal profile is measured by off-line optical metrology and on-line X-ray experiments. The off-line results showed that the bent-crystal surface could be well fitted to a saddle surface apart from a redundant cubic term which was related to the different couples applied on the crystal. On-line characterization of the meridional and the sagittal radius of the bent crystal includes double-crystal topography and ray-tracing measurement. In addition, the double-crystal topography experiment could be used as a quick diagnostic method for the bending condition adjustment. The sagittal radius of the bent crystal was characterized through a ray-tracing experiment by using a particularly designed tungsten mask. Moreover, rocking curves under different bending conditions were measured as well. The results were highly consistent with analytical results derived from the elastic theory. Furthermore, radii along different vertical positions under various bending conditions were measured and showed a quadratic relationship between the vertical positions and the meridional radii.

Study of the focusing properties of Laue bent crystal by ray-tracing

The laue bent crystal is the most effective optical element of focusing, collimating and monochromating of high flux density hard X-rays (above 30 keV) from the insertion device radiation, because of reducing the high heat load effectively. So the study of the focusing optical properties is important for developing high performance and stability laue bent crystal monochromator. We give a systematic research of the focusing properties of laue bent crystal by X-ray trace program self-developed, analyze the effects of the mode of incident beam and crystal parameter on focusing spot size, focal length, diffracted beam divergence, etc., and obtain the following results: the higher the diffracted energy, the smaller the spot size is and the spot size tends to be a constant; the smaller the bend radius of crystal, the smaller the spot size is, and the spot size reaches a minimum when the bend radius is a threshold, then as the bend radius decreases, the spot size becomes larger because of aberration; the spot size is linearly directly proportional to the thickness of crystal. For the divergence of diffracted beam, it becomes smaller as the diffracted energy is high, and tends to be a constant; it is linearly directly proportional to the curvature of bent crystal. Also we obtain a reasonable range of bent crystal parameters from the study by ray-tracing.

Development of the Hard-X-ray Angle Resolved X-ray Photoemission Spectrometer for Laboratory Use

This article reports development of a practical laboratory hard X-ray photoelectron spectroscopy (HXPS) system by combining a focused monochromatic Cr K(alpha) X-ray source, a wide angle acceptance objective lens and a high kinetic energy electron analyzer. The Cr K(alpha) source consists of a Cr target, a 15 kV focused electron gun, and a compact bent crystal monochromator. The X-ray spot size is variable from 10 microm (1.25 W) to 200 microm (50 W). A wide acceptance angle objective lens is installed in front of a hemispherical analyzer. The total resolution of 0.53 eV was obtained by Au Fermi-edge measurements. Angle acceptance of +/-35 degrees with angle resolution of 0.5 degrees was achieved by measuring Au 3d(5/2) peak through a hemicylinder multi-slit on an Au thin strip, in angle resolution mode. As the examples, silicon based multilayer thin films were used for showing the possibilities of deeper (larger) detection depth with the designed system.

Development of Hard X-ray Photoelectron Spectroscopy for Laboratory Use

A hard X-ray photoelectron spectroscopy (HXPS) instrument has been developed for laboratory use by combining a monochromatic Cr Kα focused X-ray source, a wide acceptance angle objective lens and a high energy electron analyzer. The Cr Kα X-rays excited by a micro focused electron beam at 20 kV beam voltage are diffracted by an ellipsoidal bent crystal monochromator and focused on the sample. A wide acceptance angle objective lens was integrated as the first section of an input lens of a hemispherical analyzer. A 9.9 μm diameter minimum X-ray spot size was achieved and the energy resolution of 0.56 eV was obtained by Au Fermi edge measurements. The maximum acceptance angle and angular resolution were 70o and 0.54o, respectively. The typical acquisition time is about 30 min for a wide energy survey spectrum and about 1 h for Si 1s angle resolved spectra obtained without tilting the sample. The application of this instrument for the analyses of overlayers and multilayer thin films is also discussed.

Experiment investigation of La 1− xSr xMnO 3 by high-resolution X-Ray emission and spin-polarized X-ray absorption spectroscopy

Big changes in resistivity along with the changing of local structure in some oxide systems, such as high-temperature superconductors and colossal magnetoresistance system, strongly suggest the need of a systematic investigation of their local electronic and atomic structures. In this work we present the high-resolution X-ray emission spectra and the spin-polarized X-ray absorption near-edge spectroscopy (SPXANES) data at the Mn K-edge in the La 1− x Sr x MnO 3. This experiment is based on a high-resolution large-acceptance crystal analyzer based on Si (1 1 1) and optimized for X-ray fluorescence spectroscopy. With a spherical bent crystal monochromator, a Mn Kβ emission spectra with high resolution was obtained with a short collection time and SPXANES spectra of La 1− x Sr x MnO 3 at room temperature were also measured at high temperature.

A high quality bent crystal monochromator based on asymmetric Laue geometry

A high quality bent crystal monochromator based on asymmetric Laue geometry

Detectors for energy-dispersive EXAFS (EDE) experiments

EXAFS, X-ray absorption spectroscopy fine structure, has been an important tool within synchrotron radiation science for some years. It has been used extensively in step scanning mode to provide information on the local environment within both static and slowly changing samples. Energy-dispersive EXAFS (EDE) allows similar information to be collected in a significantly shorter time frame, enabling the study of truly dynamically changing samples. The technique uses a bent crystal monochromator to focus an energy-dispersed X-ray beam onto a sample which then impinges upon an array detector positioned behind the sample. This allows the absorption over a range of energies to be collected in a very short period. Until recently only few systems have existed for this technique, typically employing commercially available detectors originally constructed for other applications. These presented limitations in speed, energy resolution, linearity and dynamic range and could only follow changes in the millisecond range. Using UK ESPRC grant funds, a new detector has been developed (XSTRIP) which shows great improvements over the above limitations and has demonstrated the potential to open new areas of science to researchers. This paper will present the technique, some of the historical limitations, the performance increase afforded by the XSTRIP system and an overview of the future direction of detectors for this most exciting and important technique.

Asymmetric diffraction geometry of the bent crystal monochromator - A way to improve the properties of strain diffractometers

The possibility to advantageously exploit the asymmetric diffraction geometry of bent perfect crystals was studied with the aim of further increasing the detector signal and/or resolution of the strain diffractometer in Studsvik. It was proved that, in the case of strain diffractometers, the asymmetric diffraction geometry of the bent perfect crystal monochromator with output-beam extension can be considerably superior to the asymmetric geometry with output-beam compression usually used in powder diffractometry.

Optimization of Kalpha bursts for photon energies between 1.7 and 7 keV produced by femtosecond-laser-produced plasmas of different scale length.

The conversion efficiency of a 90 fs high-power laser pulse focused onto a solid target into x-ray Kalpha line emission was measured. By using three different elements as target material (Si, Ti, and Co), interesting candidates for fast x-ray diffraction applications were selected. The Kalpha output was measured with toroidally bent crystal monochromators combined with a GaAsP Schottky diode. Optimization was performed for different laser intensities as well as for different density scale lengths of a preformed plasma. These different scale lengths were realized by prepulses of different intensities and delay times with respect to the main pulse. Whereas the Kalpha yield varied by a factor of 1.8 for different laser intensities, the variation of the density scale length could provide a gain factor up to 4.6 for the Kalpha output.

Neutron powder diffractometer beam line with a toroidally bent asymmetric crystal monochromator: Monte Carlo simulation studies

Monte Carlo simulation studies are carried out for beam lines for neutron powder diffraction. Programs are developed for studying the propagation of neutrons starting from reactor end of beam tube and progressing through various optical elements—collimators, slits, monochromator crystal, sample and finally detector. The simulation results are seen to be in fair agreement for an existing beam line using a plane crystal monochromator on Dhruva reactor, BARC, Mumbai. The programs are then used to design a beam line using a toroidally bent monochromator crystal and open collimator geometry. The simulation results show that a perfect bent crystal monochromator based diffractometer provides better resolution and higher intensity as compared to that based on a plane crystal monochromator. A complete optical design of the beam line is worked out taking into consideration vertical and horizontal radii of curvature for the asymmetrically cut monochromator crystal, sample size and detector resolution function.

Coronary angiography using laser plasma sources: X-ray source efficiency and optimization of a bent crystal monochromator

A monochromator has been developed for coronary angiography, comprising a single bent crystal of silicon in Laue transmission geometry. K spectra of laser irradiated solid tin and tantalum (Z=50 and 73, respectively) targets were measured. The high resolution crystal spectrometer resolve the Sn and Ta Kα doublets, allowing in a proof-of-principle experiment the absolute Kα photon numbers emitted by the source to be determined. The Ta Kα yield is measured as a function of the laser pulse energy, allowing an assessment to be made of the suitability of such sources for medical applications.

High-energy X-ray diffraction beamline: BL04B2 at SPring-8

High-energy X-ray diffraction beamline, one of the bending-magnet beamline at SPring-8, was constructed, for the diffraction and scattering experiments at high energy ( E>37.8 keV). A single-bounce, bent-crystal monochromator that deflects the beam horizontally is adopted for the use of high-energy, modest-resolution, and focused X-rays. Four kinds of experimental device were equipped in tandem in the experimental hutch. This beamline has the advantage for structural studies in amorphous and liquid, and high-pressure diffraction experiments which need high penetration ability.

Improvements on water-cooled and doubly bent crystal monochromator for Compton scattering experiments

Improvements on the water-cooled and doubly bent crystal monochromator, which has been installed at the KEK, PF-AR NE1 beamline for Compton scattering experiments, are described. An as grown Si crystal is replaced by an annealed Si crystal (950° C for 24 h in air) in order to get a much higher flux. As a result, the obtained flux for 60 keV monochromatic X-rays has become four times higher. The focused beam size has not changed. The energy resolution is 90 eV, which is 1.5 times worse than the previous value, but it is acceptable for high resolution Compton scattering experiments. Two Si crystals whose directions of the surface normal are (1 1 1) and (1 0 0) were successfully mounted. With this, the beamline now covers the energy ranges of the monochromatized X-rays: 40–70 keV by Si (1 1 1) and 90–160 keV by Si (4 0 0).

Instrumental line shapes in neutron powder diffraction with focusing monochromators

The optics of neutron powder diffractometers with bent-crystal monochromators is considered with respect to the computation of instrumental line shapes, accounting for second-order aberrations. A procedure of Monte Carlo simulation of instrumental line shapes has been developed. An analytical approximation of these shapes is described. Computational results for the case of the MURR diffractometer with position-sensitive detection and a focusing silicon monochromator are presented.

A bent Laue–Laue monochromator for a synchrotron-based computed tomography system

We designed and tested a two-crystal bent Laue–Laue monochromator for wide, fan-shaped synchrotron X-ray beams for the program multiple energy computed tomography (MECT) at the National Synchrotron Light Source (NSLS). MECT employs monochromatic X-ray beams from the NSLS's X17B superconducting wiggler beamline for computed tomography (CT) with an improved image quality. MECT uses a fixed horizontal fan-shaped beam with the subject's apparatus rotating around a vertical axis. The new monochromator uses two Czochralski-grown Si 〈1 1 1〉 crystals, 0.7 and 1.4 mm thick, respectively, and with thick ribs on their upper and lower ends. The crystals are bent cylindrically, with the axis of the cylinder parallel to the fan beam, using 4-rod benders with two fixed rods and two movable ones. The bent-crystal feature of the monochromator resolved the difficulties we had had with the flat Laue–Laue design previously used in MECT, which included (a) inadequate beam intensity, (b) excessive fluctuations in beam intensity, and (c) instability of the shape of the beam's horizontal profile. Compared with that earlier monochromator, the bent Laue–Laue device tested at 42 and 108 keV showed about a 10-fold larger beam flux, about 5 times better beam stability, 10-fold less harmonic contamination, and a smaller energy bandwidth at certain bending radii. The present work gave us better understanding of the basis for the beam-smiling effect in bent-crystal monochromators, and allowed us to refine the theoretical method of estimating the beam-harmonic contamination in bent-crystal monochromators.

Scanning X-ray spectrometer for high-resolution Compton profile measurements at ESRF

A scanning-type crystal spectrometer for high-resolution Compton profile measurements has been constructed at the High Energy Inelastic Scattering Beamline (ID15B) of the ESRF. Radiation from a seven-period asymmetrical permanent-magnet wiggler or from a superconducting wavelength shifter is focused horizontally onto the sample by a bent-crystal monochromator. Typical energies are 30, 50 and 60 keV, the flux on the sample is 1012 photons s−1, and the relative energy bandwidth is 3 × 10−4. The spectrometer operates in the Rowland circle geometry, where the sample is fixed and the cylindrically bent analyser crystal and the detector move on the focusing circle by synchronized translations and rotations. The main detector is a large-diameter NaI scintillation counter, the incident beam is monitored by an Si diode, and scattering from the sample is detected using a Ge detector. The recorded spectrum is corrected for the energy-dependent response of the spectrometer, background and multiple scattering, and converted to the momentum scale. The resolution of the spectrometer is calculated from the geometrical factors and the reflectivity curve of the analyser crystal, and the result is checked against the widths of the elastically scattered line and fluorescent lines. So far, 0.1 a.u. resolution in electron momentum has been achieved. The typical average count rate over the Compton profile is about 1000 counts s−1 from a weakly absorbing sample.

A new water-cooled and doubly bent crystal monochromator for Compton scattering experiments.

The design and performance of a new water-cooled and doubly bent crystal monochromator for Compton scattering experiments are described. Its mechanical system is similar to that of a usual bent cylindrical X-ray mirror. A monolithic Si (111) crystal is mounted on a water-cooled and cylindrically polished copper crystal holder with liquid Ga-In alloy between them in order to obtain good thermal and mechanical contact. The sagittal bending radius of the holder, R(s), is 508 mm. The holder is mounted on a mechanical bender making a meridian radius, R(m), of 596 m. The performance of the monochromator, which has been installed at the Photon Factory Accumulator Ring beamline NE1, is as follows. The focused beam size, flux and energy resolution for 60 keV X-rays are 0.5 mm in height and 2.0 mm in width, 5 x 10(12) photons s(-1) and about 60 eV, respectively. The new monochromator gives one order higher brightness at the sample position and also better energy resolution than the previous monochromator. The overall momentum resolution for high-resolution Compton scattering experiments becomes 0.08 atomic units, and the background can be reduced by a third, because it is possible to insert a fine slit after the analyser crystal without losing any Compton scattering signal.

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.