No abstract

This work presents the x-ray extended range technique for measuring x-ray mass attenuation coefficients. This technique includes the use of multiple foil attenuators at each energy investigated, allowing independent tests of detector linearity and of the harmonic contributions to the monochromated synchrotron beam. Measurements over a wide energy range allow the uncertainty of local foil thickness to be minimized by the calibration of thin sample measurements to those of thick samples. The use of an extended criterion for sample thickness selection allows direct determination of dominant systematics, with an improvement of accuracies compared to previous measurements by up to factors of 20. Resulting accuracies for attenuation coefficients of copper ͑8.84 to 20 keV͒ are 0.27-0.5 %, with reproducibility of 0.02%. We also extract the imaginary component of the form factor from the data with the same accuracy. Results are compared to theoretical calculations near and away from the absorption edge. The accuracy challenges available theoretical calculations, and observed discrepancies of 10% between current theory and experiments can now be addressed.Compilations of experimental data of PE over the last decade show large variations of up to 30%, although many authors have claimed 1% precision or better using various experimental techniques ͓16,17͔. These variations are due to unresolved systematics relating to sample thickness determination and purity, detector linearity, harmonic contamination of the x-ray beam, scattering, energy calibration, and beam divergence. The most reliable results quoted in the literature relate to the work of Creagh and Hubbell ͓17͔, Gerward ͓18͔, and Mika et al. and Chantler and Barnea ͓19͔. We have recently adapted the techniques of these authors and developed them to be appropriate for synchrotron research ͓16,20͔.The availability of modern synchrotron radiation brought near-edge absorption of x-rays within the reach of many fields of research. Previously, conventional x-ray diffraction PHYSICAL REVIEW A, VOLUME 64, 062506

No abstract

We use the x-ray extended-range technique ͑XERT͒ ͓Chantler et al., Phys. Rev. A 64, 062506 ͑2001͔͒ to measure the mass attenuation coefficients of molybdenum in the x-ray energy range of 13.5-41.5 keV to 0.02-0.15 % accuracy. Measurements made over an extended range of the measurement parameter space are critically examined to identify, quantify, and correct where necessary a number of experimental systematic errors. These results represent the most extensive experimental data set for molybdenum and include absolute mass attenuation coefficients in the regions of the x-ray absorption fine structure ͑XAFS͒ and x-ray-absorption near-edge structure ͑XANES͒. The imaginary component of the atomic form-factor f 2 is derived from the photoelectric absorption after subtracting calculated Rayleigh and Compton scattering cross sections from the total attenuation. Comparison of the result with tabulations of calculated photoelectric absorption coefficients indicates that differences of 1-15 % persist between the calculated and observed values.

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%.

We used the x-ray extended-range technique to measure the x-ray mass attenuation coefficients of silicon with an accuracy between 0.27% and 0.5% in the 5 keVϪ20 keV energy range. Subtraction of the x-ray scattering contribution enabled us to derive the corresponding x-ray photoelectric absorption coefficients and determine the absolute value of the imaginary part of the atomic form factor of silicon. Discrepancies between the experimental values of the mass attenuation coefficients and theoretically calculated values are discussed. New approaches to the theoretical calculation will be required to match the precision and accuracy of the experimental results.

We use the x-ray extended-range technique ͑XERT͒ ͓C. T. Chantler et al., Phys. Rev. A 64, 062506 ͑2001͔͒ to measure the mass attenuation coefficients of tin in the x-ray energy range of 29-60 keV to 0.04-3 % accuracy, and typically in the range 0.1-0.2 %. Measurements made over an extended range of the measurement parameter space are critically examined to identify, quantify, and correct a number of potential experimental systematic errors. These results represent the most extensive experimental data set for tin and include absolute mass attenuation coefficients in the regions of x-ray absorption fine structure, extended x-ray absorption fine structure, and x-ray absorption near-edge structure. The imaginary component of the atomic form factor f 2 is derived from the photoelectric absorption after subtracting calculated Rayleigh and Compton scattering cross sections from the total attenuation. Comparison of the result with tabulations of calculated photoelectric absorption coefficients indicates that differences of 1 -2 % persist between calculated and observed values.

In line with an ongoing programme to determine accurately x-ray attenuation coefficients, we have developed a method for the quantitative determination of the effect on experimental results of monochromator harmonic components in a synchrotron beam. The technique can be adapted to suit a wide variety of experiments, and is of particular interest because it determines the effect of the harmonic components directly. This avoids the necessity for modelling and is therefore robust. Results of a direct determination of the effect of harmonic components illustrate the power of the technique. We extended the technique to quantify the effects of dark current-induced errors.