Modulated anomalous X-ray scattering

To determine the partial structure associated with a particular element in a multicomponent system, contrast variation is proposed based on the use of modulated anomalous X-ray scattering (MAXS). The photon energy of the X-ray beam incident on the sample is modulated over a range of energies below an absorption edge of the selected element. Because of anomalous dispersion, measurement of the scattered-intensity gradient with respect to the energy gives the required information. MAXS can be used in both small-angle and wide-angle diffraction and is applicable, in principle, to crystalline, amorphous and liquid materials. Energy modulation obtained by oscillating the Bragg angle of the monochromator by a small amount, followed by phase-sensitive detection, leads to a significant reduction of both systematic and statistical errors. Results of a simulation study of MAXS from an aqueous solution of NiCl₂ under extreme conditions of dilution and scattering contrast are presented.