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. Copyright  2003 John Wiley & Sons, Ltd.

/pdf/quantitative-determination-of-major-systematics-in-3y6971e2a7.pdf

Quantitative Determination of Major Systematics in Synchrotron X-ray Experiments : Seeing Through Harmonic Components

The x-ray mass attenuation coefficients of zinc are measured in a high-accuracy experiment between 7.2 and 15.2 keV with an absolute accuracy of 0.044% and 0.197%. This is the most accurate determination of any attenuation coefficient on a bending-magnet beamline and reduces the absolute uncertainty by a factor of 3 compared to earlier work by advances in integrated column density determination and the full-foil mapping technique described herein. We define a relative accuracy of $0.006%$, which is not the same as either the precision or the absolute accuracy. Relative accuracy is the appropriate parameter for standard implementation of analysis of near-edge spectra. Values of the imaginary components $f\ensuremath{''}$ of the x-ray form factor of zinc are derived. Observed differences between the measured mass attenuation coefficients and various theoretical calculations reach a maximum of about 5% at the absorption edge and up to 2% further than 1 keV away from the edge. The measurements invite improvements in the theoretical calculations of mass attenuation coefficients of zinc.

X-ray mass attenuation coefficients and imaginary components of the atomic form factor of zinc over the energy range of 7.2-15.2 keV

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.

https://www.ph.unimelb.edu.au/~chantler/opticshome/xrayopt/JLGloverJPB43_085001.pdf

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

X-ray absorption fine structure (XAFS) of ferrocene (Fc) and Decamethylferrocene (DmFc) have been determined on an absolute scale using transmission measurements of multiple solutions of differing concentrations (15 mM, 3 mM, pure solvent) at operating temperatures of 10–20 K. Mass attenuation coefficients and photoelectric absorption cross sections are measured and tabulated for both molecules for an extended energy range in excess of 1.5 keV from the Fe K-shell absorption edge. At these temperatures, the minimization of of dynamic disorder has enabled a critical determination of the oscillatory absorption structures created by multiple-scattering paths of the excited photoelectron. These oscillatory structures are highly sensitive to the local conformation environment of the iron absorber in organometallic structures. Crystallographic and scattering studies have reported both structures characterized by staggered cyclopentadienyl rings, in contrast with low temperature crystallography and recent density...

Accurate X-ray Absorption Spectra of Dilute Systems: Absolute Measurements and Structural Analysis of Ferrocene and Decamethylferrocene

The linearity of response of X-ray detectors is tested. Examples of linearity tests demonstrate the remarkable range of linear response of flowing-gas ion chambers in the synchrotron environment. The diagnostic is also highly sensitive to the presence in the X-ray beam of harmonic X-rays diffracted by a higher-order reflection of the monochromator. The remarkable range of linearity of ion chambers has enabled the accurate measurement of the absolute X-ray attenuation of a number of elements. It should now be possible to measure the absolute intensity of Bragg reflections, provided such measurements are carried out with extended-face single crystals. The advantages of the extended-face crystal technique for Bragg intensity measurements are summarized and a number of approaches to absolute Bragg intensity measurement are discussed. © 2011 International Union of Crystallography Printed in Singapore - all rights reserved.

https://www.ph.unimelb.edu.au/~chantler/opticshome/xrayopt/ZBarneaDetectorLinearityJApplCryst44.pdf

Measuring the linearity of X-ray detectors: consequences for absolute attenuation, scattering and absolute Bragg intensities

The appreciable differences between the absorption coefficients of a foil for the characteristic radiation and its harmonics can be utilized to determine the wavelength composition of X-rays diffracted by a monochromator or sample. A simple experimental procedure is proposed and used to determine the intensity of the λ/2 harmonic diffracted by the 400 reflection of gallium phosphide at the 200 position of the molybdenum Kα peak. At a tube voltage of 45 kV the λ/2 contribution was found to be 9%; it decreased to 2.5% when pulse-height discrimination blocked 10% of the total counts. Our procedure provides a direct check of the effectiveness of balanced filters and electronic discrimination against harmonic radiation.

Z. Barnea

Papers

Detection of the contribution of harmonics to the diffracted X-ray intensity