X-ray bandwidth: Determination by on-edge absorption and effect on various absorption experiments

Knowledge of the monochromaticity of an x-ray source is increasingly important in fundamental experiments and critical applications. The bandwidth of an x-ray beam, selected from a synchrotron radiation spectrum for example, ultimately defines the limiting resolution of the synchrotron source. The development of x-ray technology through the use of characteristic line sources of fixed line shape has impaired the development of understanding of this parameter. The bandwidth is particularly relevant with the modern trend towards the use of synchrotron sources in conjunction with monochromating devices, where the monochromaticity of the x-ray beam is not known a priori. The ability to control the bandwidth of the beam can lead to significant new experiments. We have observed the effect of the x-ray bandwidth on precise but relatively scaled measurements of the mass attenuation coefficient of molybdenum made on the absorption edge. We derive an expression describing the effect of the x-ray bandwidth on these measurements and invert this to determine the bandwidth of a highly monochromatized 20-keV synchrotron x-ray beam to be 1.57eV±0.03eV. The technique presented here determines the bandwidth, a parameter critically dependent on the x-ray optical elements in the beam, in such a manner as to require no knowledge of these elements. We demonstrate that the x-ray bandwidth has significant effects upon measured edge energies, mass attenuation coefficients, x-ray anomalous-fine structure and x-ray absorption near-edge structure (XANES). In particular, the observed x-ray bandwidth necessitates a correction of up to 1.4% in the measurement of the mass attenuation coefficient of molybdenum on the absorption edge and is shown to shift the observed absorption-edge energy location by up to 0.5eV.