C. T. Chantler

Theoretical Form Factor, Attenuation, and Scattering Tabulation for Z=1–92 from E=1–10 eV to E=0.4–1.0 MeV

Tables for form factors and anomalous dispersion are widely used in the UV, x‐ray, and y‐ray communities, and have existed for a considerable period of time. Much of the recent theoretical basis for these was contributed by Cromer, Mann, and Liberman while much of the experimental data were synthesized by Henke et al. More recent developments in both areas have led to new and revised tables. These works have employed numerous simplifications compared to detailed relativistic S‐matrix calculations; the latter do not lend themselves to convenient tabular application for the range of Z and energy of general interest. Conversely, the former tables appear to have large regions of limited validity throughout the range of Z and energies, and in particular have important limitations with regard to extrapolation to energies outside tabulated ranges. In the present study, the primary interactions of x‐rays with isolated atoms from Z=1 (hydrogen) to Z=92 (uranium) are described and computed within a self‐consistent ...

Spatial coherence measurement of X-ray undulator radiation

We measure the spatial coherence function of a quasi-monochromatic 1.1 keV X-ray beam from an undulator at a third-generation synchrotron. We use a Youngs slit apparatus to measure the coherence function and find that the coherence measured is poorer than expected. We show that this difference may be attributed to the effects of speckle due to the beamline optics. The conditions for successful coherence transport are considered.

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.

A curved crystal spectrometer for energy calibration and spectral characterization of mammographic x‐ray sources

Clinical efficacy of diagnostic radiology for mammographic examinations is critically dependent on source characteristics, detection efficiency, image resolution and applied high voltage. In this report we focus on means for evaluation of source-dependent issues including noninvasive determination of the applied high voltage, and characterization of intrinsic spectral distributions which in turn reflect the effects of added filtration and target and window contamination. It is shown that a particular form of x-ray curved crystal spectrometry with electronic imaging can serve to determine all relevant parameters within the confines of a standard clinical exposure.

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.

Overview of the electron beam ion trap program at NIST

This paper surveys the ongoing physics experiments at the Electron Beam Ion Trap (EBIT) facility at NIST, with particular attention paid to the underlying physical principles involved. In addition, some new data on the performance of our EBIT are presented, including results related to the determination of the trap width, ion temperature, and number of highly charged ions in the trap.

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%.

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.

X-ray diffraction of bent crystals in Bragg geometry. I: Perfect-crystal modelling

X-ray reflectivity, widths, centroid shifts and profiles for curved perfect crystals are calculated from a model. The crystal is approximated by a stack of perfect-crystal lamellae or blocks with a gradually changing (mean) orientation. A computer program has been developed to calculate the above quantities in the Johann geometry for the composite crystal from the dynamic theory of diffraction. Focusing and defocusing aberrations and the use of photographic detection methods are included. Correction of omissions from earlier theory and modelling is noted, together with observed effects. Incoherent scattering can give dramatic changes in diffracted intensities and significant shifts of final parameters. Effects of depth penetration on shifts, cosine ratios and other parameters are included. Assumptions of the model and implementation are detailed. It is shown that interference effects between waves of roughly equal amplitudes require use of lamellar thicknesses greater than those corresponding to the Darwin range. Internal tests demonstrate agreement with the literature at extremes. The theory is applied to first- and fourth-order spectra in differential Lyman α wavelength measurements. Results for pentaerythritol 002 crystals are presented. Paper II of this series extends this model to non-ideally imperfect crystals and other crystals of interest and discusses experimental agreement.

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X-ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C(5)H(5))(2), are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C(5)H(5) rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT-like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS-based estimates of the bond distances have accuracies comparable with those obtained using single-crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.