High-Focusing Bragg-Crystal Polychromator Design for Energy-Dispersive X-ray Absorption Spectroscopy

The design of a highly focusing profiled Si(111) Bragg crystal polychromator for dispersive EXAFS (extended X-ray absorption fine structure) experiments is described. The contour of the crystal has been optimized to give the best focus over the full 4-13 keV energy range. The profile optimization has been improved taking into account all the degrees of freedom of the geometry and the results of X-ray tracing simulations. The profile of the crystal has been calculated to take full advantage of the new possibilities given by the undulator source and the optics of beamline ID24 at the European Synchrotron Radiation Facility. Full spot sizes have been measured to be between 20 and 40 µm in the 5-12 keV energy range. These values compare well with X-ray tracing simulations and are the smallest spots ever obtained with energy-dispersive EXAFS optics, keeping, however, a wide enough energy bandpass for most X-ray absorption spectroscopy experiments.