Z. Barnea

X-ray extended-range technique for precision measurement of the X-ray mass attenuation coefficient and Im (f ) for copper using synchrotron radiation

We reconsider the long-standing problem of accurate measurement of atomic form factors for fundamental and applied problems. We discuss the X-ray extended-range technique for accurate measurement of the mass attenuation coefficient and the imaginary component of the atomic form factor. Novelties of this approach include the use of a synchrotron with detector normalisation, the direct calibration of dominant systematics using multiple thicknesses, and measurement over wide energy ranges with a resulting improvement of accuracies by an order of magnitude. This new technique achieves accuracies of 0.27– 0.5% and reproducibility of 0.02% for attenuation of copper from 8.84 to 20 keV, compared to accuracies of 10% using atomic vapours. This precision challenges available theoretical calculations. Discrepancies of 10% between current theory and experiments can now be addressed.  2001 Elsevier Science B.V. All rights reserved.

Measurement of the x-ray mass attenuation coefficient of silver using the x-ray-extended range technique

We used the x-ray-extended range technique to measure the x-ray mass attenuation coefficients of silver in the 15–50 keV energy range with a level of uncertainty between 0.27% and 0.4% away from the K-edge. The imaginary part of the atomic form factor of silver was derived by subtracting the scattering component from the measured total mass attenuation coefficients. Discrepancies between the measured mass attenuation coefficients and alternative theoretical predictions are discussed.

Focusing of X-rays by Total External Reflection from a Paraboloidally Tapered Glass Capillary

The first observation of a true geometrical focus of X-rays well beyond the exit of a paraboloidally tapered glass monocapillary is reported. An intensity gain of 250 +/- 20 into a 6 x 9 mum pinhole for 8 keV X-rays and transmission efficiencies of more than 90% below 20 keV were observed.

The Australian diffractometer at the Photon Factory

Outlined are design features of a versatile high‐resolution two‐axis diffractometer that is being constructed for operation at the Photon Factory as an Australian national facility. The instrument features optional use of multiple‐imaging plates on a translating cassette to allow rapid recording of an almost complete range of data covering both the high‐angle and small‐angle scattering regime or alternatively the use of electronic detectors. The instrument will be capable of operation in various modes including the following: (i) high‐resolution powder diffraction with single‐channel counter and crystal analyzer, (ii) high‐resolution, high‐speed powder diffraction in the Debye–Scherrer mode with imaging plates as recording medium, either stationary or translating (for time‐dependent studies), (iii) small‐angle x‐ray scattering with imaging plates as recording medium, (iv) protein crystallography in screenless Weissenberg mode, and (v) two‐ or three‐axis single‐crystal diffractometry. The salient features ...

Absolute determination of the effect of scattering and fluorescence on x-ray attenuation measurements

We investigate the effect of x-ray scattering and fluorescence upon measurements of the x-ray mass attenuation coefficient. Measurements of scattering and fluorescence are obtained from a comparison of attenuation measurements using different sized apertures to admit varying amounts of the scattering and fluorescence into the detectors. The result of such a comparison is found to be in good agreement with a theoretical calculation of the fluorescent and scattered photons reaching the ion chambers and, under our experimental conditions, decreases the measured attenuation coefficients of silver by up to 0.2%.

Protein Crystal Diffraction Patterns Using a Capillary-Focused Synchrotron X-ray Beam

A paraboloidally tapered glass monocapillary was used to focus an 8 keV monochromated synchrotron bending-magnet X-ray beam into a 40 (+/-5) mum focal spot located 45 (+/-5) mm from the exit of the capillary. This focal spot had a measured intensity gain of 120 (+/-10) times the intensity present in an equivalent cross section of the unfocused beam from the monochromator. This focused beam was used to obtain oscillation diffraction patterns on image plates from a hen egg-white lysozyme protein crystal in two distinct geometries: one with the specimen crystal at the capillary exit and the other with the crystal at the beam focus. In the first geometry, focused Bragg reflections were observed at the focal plane. In the second geometry, diverging Bragg reflections of high intensity from a small crystal volume were observed. Image-plate diffraction patterns for these two geometries were compared with exposures with equivalent integrated diffracted intensities obtained using a 100 x 100 mum unfocused X-ray beam with the same crystal. The use of the focused beam resulted in a reduction in the exposure time required to produce equivalent patterns by a factor of between 70 and 100.

Precision X-ray optics for fundamental interactions in atomic physics, resolving discrepancies in the X-ray regime

Reliable knowledge of the complex X-ray form factor (Re(f) and f″) is required for many fields including crystallography, medical diagnosis and XAFS studies. However, there are discrepancies between theory and theory, experiment and experiment and theory and experiment of 10% and more, over central X-ray energies. Discrepancies exist for most elements, despite claimed experimental accuracies of 1%. This paper summarises the current variation between experimental and theoretical results, and outlines key issues for obtaining experimental accuracies of 1% in critical wavelength ranges for selected elements to address these issues. This paper critically surveys available experimental data for attenuation coefficients and suggests a procedure for obtaining significantly higher accuracy measurements in the future.

Measurement of the x-ray mass-attenuation coefficients of gold, derived quantities between 14 keV and 21 keV and determination of the bond lengths of gold

The x-ray mass-attenuation coefficients of gold are measured at 91 energies between 14 keV and 21 keV using synchrotron radiation. The measurements are accurate to between 0.08% and 0.1%. The photoelectric mass-absorption coefficients and the imaginary component of the form factors of gold are also determined. The results include the LI edge and are the most accurate and extensive gold dataset available in this energy range. An analysis of the LI edge XAFS showed excellent agreement between the measured and simulated XAFS and yielded highly accurate values of the bond lengths of gold. When our results are compared with earlier measurements and with predictions of major theoretical tabulations, significant discrepancies are noted. The comparison raises questions about the nature of discrepancies between experimental and theoretical values of mass-attenuation coefficients.

Full-foil x-ray mapping of integrated column density applied to the absolute determination of mass attenuation coefficients

Recent measurements of mass attenuation coefficients have identified the determination of the thickness of the absorbing specimen as the major limitation to the accuracy of the measurement. We present a technique for determining the mass attenuation coefficient with high accuracy. The technique uses the integral of the density along a column extending through the thickness of the absorber, which we term the integrated column density. Attenuation measurements mapped across the entire absorber are used to determine a relative map of the integrated column density. These relative measurements are then placed on an absolute scale by comparison with the average integrated column density and are used to determine the mass attenuation coefficient. This approach correctly treats variations in the integrated column density across the foil. We illustrate the technique with an absolute measurement of the x-ray mass attenuation coefficient of molybdenum using a synchrotron beam of energy 41.568 keV ? 0.005 keV. We obtain cm2 g?1 ? 0.0032 cm2 g?1, accurate to 0.028%?over one order of magnitude more accurate than any previous work. The full-foil technique used to determine the mass attenuation coefficient is used to determine an integrated column density profile of a sample to a precision of around 0.05% of the thickness of the absorber. We demonstrate the sensitivity of the technique by observing a periodic thickness variation of order 0.1 ?m occurring over a 5 mm length scale on a nominally 50 ?m thick molybdenum foil.

Monitoring fluctuations at a synchrotron beamline using matched ion chambers: 2. isolation of component noise sources, and application to attenuation measurements showing increased precision by two orders of magnitude

The significance of statistical fluctuations in a synchrotron beam is often neglected, with a consequent loss of precision or accuracy of up to two orders of magnitude. We illustrate this for the specific example of an x-ray attenuation measurement. Since all x-ray measurements involve either scattering or absorption (or both), the net potential gain in precision is similar for all such experiments, including crystallographic and XAFS determinations. We demonstrate the net gain with data obtained with two matched ion chambers on a monochromatized bending magnet beam at the Photon Factory, Tsukuba, Japan. Isolating and measuring component contributions to the overall fluctuations allows a robust determination of the limiting experimental precision. This approach also determines the absolute incident flux without measuring the absolute photon count. The type of statistical analysis described is not only a post facto diagnostic tool but, by being incorporated into the experiment on-line, can provide a real-time optimizing intervention in the measurement process. Copyright ” 2000 John Wiley & Sons, Ltd.