keV Synchrotron Radiation Research Articles

TaSi2-Si composites as wide-bandpass optical elements for synchrotron radiation (abstract)

The wide matrix rocking curves of the in situ eutectic composite TaSi2-Si make it attractive as a wide-bandpass monochromator for synchrotron radiation. Wafers with Si[111], Si[110], or Si[100] orientation were studied to determine the origin of the wide rocking curves. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix was examined using synchrotron Laue patterns and the TaSi2 [100], TaSi2 [003], and TaSi2 [102] reflections. Double and triple axis diffractometry were used to show that the large widths were due to strain and mosaic and not long-range bending; copper radiation (for some double axis results) and 120 and 160 keV synchrotron radiation were used. At 8 keV, rocking curve widths were about twenty times broader than those from perfect Si, and peak reflectivities approached 20%. Rocking curves from Si[333] and Si[444] (120 and 160 keV, respectively) had identical profiles and reflectivities of about 25%. The triple axis results show compressive strains in the Si matrix along Si[111] (i.e., parallel to the rods) and dilational strains orthogonal to the rods. These results confirm the promise of TaSi2-Si as a wide-bandpass optical element for synchrotron radiation.

A sagittal focusing and scanning monochromator for 8–80 keV synchrotron radiation at DORIS III

A sagittal focusing and scanning monochromator for 8–80 keV synchrotron radiation at DORIS III

Time-resolved studies of fast scintillating crystals under VUV and X-ray synchrotron radiation excitation

Abstract Time-resolved luminescence studies of several new multi-component crossluminescence crystals as well as La1-x Ce x F3 (x = 0.4 ÷ 0.9) crystals have been carried out under vacuum UV (10 ÷ 30 eV) and soft X-ray (hv ∼ 1 keV) synchrotron radiation excitation. The band structure parameters of the studied crossluminescence crystals have been estimated by using the spectral data obtained. The non-monotonic dependence of the light yield on x has been observed for La1-x Ce x F3, the optimum value of x being near 0.7. The result is discussed on the basis of different models of Ce3+ emission quenching.

Origin of the critical scattering on two length scales in SrTiO3: A high-energy synchrotron-radiation diffraction study.

High-q-resolution diffraction experiments performed with 100 keV synchrotron radiation on the identical ${\mathrm{SrTiO}}_{3}$ crystal used in neutron and x-ray-scattering experiments before, unambiguously show that the bulk critical scattering above ${\mathit{T}}_{\mathit{c}}$=99.5 K does not exhibit a sharp component, which had been found in recent x-ray-diffraction experiments. The observed temperature dependence of the inverse correlation length is in excellent agreement with the neutron data. However, scattering on two length scales was observed in a near-surface volume element of the same crystal.

X-ray detection with submicron impact accuracy

Utilizing a commercial optical CCD camera with the smallest pixel size presently available (6.8 × 6.8 μm 2) we have measured impact accuracies of 0.9 μm (rms) for 15 keV synchrotron radiation in direct photon counting mode. Of crucial importance was diffusion spreading of the deposited charge such that by centroid reconstruction interpolation between discrete pixel coordinates was possible.

The resolution function of a triple-crystal diffractometer for high-energy synchrotron radiation in nondispersive Laue geometry

The k-space resolution function of a triple-crystal diffractometer is calculated for an arrangement of three perfect silicon single crystals Bragg diffracting in nondispersive Laue geometry. A comparison is made with the results of measurements using synchrotron radiation in the energy range from 80 to 150 keV. In this case, absorption is very weak and according to dynamical theory the width of the diffraction pattern of thick perfect single crystals is proportional to the wavelength λ, whereas its Lorentzian tails are proportional to λ2. Together with the fact that the Bragg angles are only of the order of 2°, this leads to a concentration of the starlike k-space resolution function into a narrow band parallel to the reciprocal-lattice vector G. For diffraction of 80 keV synchrotron radiation at the silicon 111 reflection, the full width at half-maximum (FWHM) of the intensity distribution in the scattering plane is 1.1 × 10−5 Å−1 perpendicular to G and 2.2 × 10−4 Å−1 parallel to G. The observed differences in the contributions from monochromator and analyzer crystal to the resolution function are explained by the finite width of the electronic window of the detector counting chain and the non-Bragg scattering contribution from the crystals. If annealed Czochralski-grown silicon single crystals with a mosaicity of ~3′′ are used as monochromator and analyzer, the resolution is reduced by one order of magnitude, but for studies of imperfect samples or of diffuse scattering large gains in intensity can be accomplished this way.

Avalanche photodiodes as large dynamic range detectors for synchrotron radiation

We investigated silicon-based avalanche photodiodes (APDs) as X-ray detectors in terms of their linearity, maximum counting rates, and dynamic range with 8.4 keV synchrotron radiation. Measurements resulted in counting rates that extend from the APD's noise level of 10 −2 Hz to saturation counting rates in excess of 10 8 Hz. In addition, by monitoring the APD's noise level and photon counting efficiency between synchrotron bursts, we demonstrate nine orders of magnitude dynamic range.

High energy synchrotron radiation. A new probe for condensed matter research

The absorption of 150 keV synchrotron radiation in matter is weak and, as normally done with neutrons, bulk properties are studied in large samples. However, the k-space resolution obtained with a Triple Crystal Diffractometer (TCD) for high energy synchrotron radiation is about one order of magnitude better than in high resolution neutron diffraction. The technique has been applied to measure the structure factor S(Q) of amorphous solids up to momentum transfers of the order of 32 A −1 , to study the intermediate range Ortho-II ordering in large, high quality YBa 2 Cu 3 O 6.5 single crystals and for investigations of the defect scattering from annealed Czochralski grown silicon crystals

Developments in tapered monocapillary and polycapillary glass X-ray concentrators

Tapered glass capillaries can be used to condense both wide-band and monochromatic hard (> 5 keV) X-ray beams. These long, hollow, needle-like optics have been used in a variety of experiments to demonstrate their utility in areas such as protein crystallography, hard X-ray imaging, and Laue diffraction. Monocapillary versions of these X-ray concentrators have yielded intensity gains of up to 1000 and X-ray beam sizes as small as 50 nm. Recently, in an effort to make larger condensed beam sizes (> 10 μm), polycapillary tubing has been tapered and a 68 μm diameter X-ray beam with an intensity gain of 4.6 was observed using 6 keV synchrotron radiation. Results from developments of both types of tapered capillaries and from ray-tracing design efforts are presented.

High-resolution, large-angular-acceptance monochromator for hard X rays

We have constructed the first working model of a novel diffraction geometry for the high-resolution monochromatization of hard (5–30 keV) synchrotron radiation. The monochromator has an energy bandpass of 6.7 meV FWHM and an angular acceptance of 22 μrad (4.5 arcseconds) for 14.4 keV X rays - an energy resolution ΔE E of 4.6 × 10 −7. With a second set of crystals, an energy bandpass of 23 meV FWHM and an angular acceptance of 7 μrad was achieved for 23.8 keV photons. The performance of the monochromator has been verified, in a nearly ideal measurement, using nuclear-fluorescent X rays with an energy bandwidth on the order of 10 neV.

The two-dimensional electron momentum density of carbon by angular correlation measurements of Compton scattering

We report on measurements of the angular correlation between inelastically scattered γ-rays and the recoiling electrons in 5 μg/cm 2 thin carbon foils. The coincidence count rate is proportional to the two-dimensional electron momentum density of the target. The experiment was done with 100 keV synchrotron radiation from the DORIS III storage ring at DESY, Hamburg.

Compton scattering of 141 keV X-rays from 1s-electrons of silver: A synchrotron radiation experiment

High-energy 141 keV synchrotron radiation has been used to study the Compton scattering from the 1s-electrons of silver using different scattering angles and sample thicknesses. The scattering contribution of the outer-shell electrons was subtracted using a theoretical calculation and the 1s-contribution was compared with the theoretical prediction in the energy range close to the K-absorption edge. A good agreement between the experiment and the theory based on the impulse approximation was found.

Majority- and minority-spin-band directional Compton profiles in iron

Directional magnetic Compton profiles in BCC iron have been measured with circularly polarised 60 keV synchrotron radiation. Further measurements on iron, which are of improved statistical quality, enable majority and minority directional difference profiles to be separated and a more specific test of band theory to be made. The results are generally in good agreement with APW calculations, except in low-momentum regions, where there are significant discrepancies.

Electron momentum densities by inelastic gamma ray-recoil electron scattering: A (γ, eγ) experiment

Coincidence measurements of inelastically scattered γ-rays and the recoiling electrons for thin Al foils are reported. They demonstrate the possibility of the direct determination of the ground state electron momentum density in solids by photon-electron scattering. The experiments were performed using 144 keV synchrotron radiation from the DORIS storage ring at DESY.

Three-crystal spectrometer for 150-keV synchrotron radiation

A triple-axis spectrometer for high-energy (≳150 keV) synchrotron radiation has been constructed and characterized. The reciprocal space resolution function of this instrument has been measured using perfect silicon crystals as monochromator, sample, and analyzer. It was found to have dimensions of 1.0×10−5 Å−1 and 2.3×10−4 Å−1 in the directions perpendicular and parallel to the momentum transfer Q, respectively, comparable to those achievable using standard 8-keV triple-axis instruments. The large penetration depths of high-energy x-ray photons, combined with the high instrumental resolution, opens new possibilities in the study of truly bulk condensed matter systems.

Compton profiles of aluminium and silicon

Compton profiles of aluminium and silicon have been measured using 29.5 keV synchrotron radiation X-rays with a momentum resolution of 0.084 au. Comparison of the profiles with the corresponding angular correlation curves of positron annihilation radiation gives for the first time direct information about the effect of the positron on real interacting electron systems.

Test of annealed Czochralski grown silicon crystals as X-ray diffraction elements with 145 keV synchrotron radiation

The X-ray diffraction properties of annealed Czochralski grown silicon single crystals have been studied on a triple axis diffractometer using 145 keV synchrotron radiation from the 30 keV critical energy wiggler at CHESS. The silicon crystals, containing approximately 20 ppm oxygen atoms, were annealed at 1200°C for 8 h in order to create a homogeneous distribution of defects. A 10 mm thick crystal produced a Laue diffraction rocking curve with a 4 arcsec wide plateau at 50% reflectivity, and a full width at half maximum of approximately 8 arcsec. The shape of the rocking curve could be reproduced by calculations based on Darwin's extinction theory, using a mosaic distribution determined from high resolution γ-ray rocking curves measured from the same crystal. The momentum transfer resolution function of the triple axis spectrometer using these deformed crystals as monochromator and analyzer was also measured. It was found to be enlarged by factors of 11 and 18 in the perpendicular and parallel directions, respectively, compared with the resolution using perfect crystals. This enlarged resolution element can be advantageous in diffuse scattering experiments.

A high-vacuum triple-axis-diffractometer for soft X-ray scattering experiments

We present the design of a diffractometer for diffraction and reflection experiments with soft X-rays. The main part of this instrument is a triple-axis-diffractometer consisting of three one-circle-goniometers which are all mounted on a single 14 in. o.d. vacuum flange. The goniometers with their stepper motors are operated in high vacuum ( ⋍1 × 10 −6 Torr ). The performance of the instrument is demonstrated by diffraction experiments using 7.95 Å (1.56 keV) synchrotron radiation. Results are presented for cadmium arachidate Langmuir-Blodgett multilayer films on a quartz substrate.

Multiple photoionisation of Xe atoms between 4.1 and 8.0 keV: mean charge of Xe ions

The charge state distribution of Xe ions produced by 4.1-8.0 keV synchrotron radiation has been measured by means of a time-of-flight mass spectrometer. It is found that the mean charge of Xe ions resulting from atomic rearrangement following the creation of the inner-shell vacancies in Xe atoms significantly depends on the photon energy and is clearly affected by L-subshell ionisation thresholds. The experimental results are found to be in good agreement with the Monte Carlo calculations.

Sagittal focusing of synchrotron radiation

We describe a simple device that has successfully been used to sagittally focus about 2.5 mrad of synchrotron radiation with an energy resolution equivalent to that of a double crystal channel cut monochromator. Deep narrow slots are cut into a 6 mm thick silicon crystal wafer cut into a triangular shape. The slots leave a connecting link of 0.25 mm thickness. The triangular shape allows a simple means of providing a cylindrical surface approximated polygonally by the thick crystal segments between the narrow slots. The device was tested with 10 and 15 keV synchrotron radiation on one of the CHESS white beam lines at a magnification of 1 3 as suggested by Sparks et al. At both energies the diffracted beam power in the bent crystal was equivalent to that in the unbent state to within a few percent.