Compensation of chromatic dispersion in rocking curve imaging for high-resolution X-ray diffraction profile measurements

Ammonium dihydrogen phosphate (ADP) crystals of large dimensions (80 × 80 × 20 mm), to be used as X-ray beam expanders in the BEaTriX facility at INAF-OABrera, have been characterized on the ESRF BM05 beamline using the rocking curve imaging technique to test the planarity of the lattice planes. The difference in Bragg angles between the Si(111) monochromator and ADP(008) strongly affects the angular position of the diffraction peak as a function of sample position due to wavelength dispersion, which must be corrected in order to measure lattice curvature accurately. It is found that the diffracted image size exhibits a 4.5% elongation parallel to the scattering plane, an effect which has not been previously accounted for and which may lead to an incorrect wavelength dispersion correction. This contribution is critical for accurate measurement of resolution and lattice plane curvatures.