Monochromators for High-Energy Synchrotron Radiation

Several monochromators, which are based on the use of cylindrically bent perfect Si crystals, have been constructed at the High Energy X-ray Scattering beamlines of the ESRF. The monochromators provide different focusing conditions, and the energy band-passes are optimized for the needs of different experiments. Formulas are given for calculation of the focal distances, reflectivity curves, and energy distributions. The lay-out of the beamlines follows the Troika concept, where the radiation fan is either split in 3 beams, or the central beam is utilized successively by semi-transparent monochromators to serve three experimental stations simultaneously. The radiation sources are a 7-period permanent magnet asymmetric wiggler and a superconducting wavelength shifter. The critical energies are 45 keV and 96 keV, respectively. The lowest operation energy is 30 keV, and transmission type monochromators have been used up to 1 MeV photon energies. Typical X-ray flux at the sample is 10¹² ph/s at energies below 150 keV. The monochromator crystals are partly immersed in In-Ga eutectic, which provides thermal contact between the crystal and the water-cooled support structure. Bending mechanisms include flexible springs, so that the actuator motions are several orders of magnitude larger than the crystal motions. This eliminates the effects of the thermal drifts of the support structures.