X-RAY OPTICS OF A DYNAMICAL SAGITTAL-FOCUSING MONOCHROMATOR ON THE GILDA BEAMLINE AT THE ESRF

The performance of a dynamical sagittal-focusing monochromator for har d X-rays is described. It consists of a flat first crystal and a diamo nd-shaped ribbed second crystal which is clamped by its central rib an d dynamically bent by applying a force on its two apices. The system h as proved to perform very well on the GILDA beamline at the ESRF. The horizontal acceptance varies with energy and with focusing geometry as predicted theoretically; :he total available horizontal fan of radiat ion (3.6 mrad) is in fact collected in the 1:3 geometry. The system is routinely run in a dynamical focusing mode for XAFS spectroscopy in t he energy range 5-30 keV with Si(311) crystals, with a constant spot s ize FWHM similar to 1 mm on the sample and without degradation of ener gy resolution or reproducibility. Using simple geometrical considerati ons we calculate the variations of the horizontal profile of the refle cted beam during rocking-curve scans in different focusing geometries and find them in agreement with observed ones. Not only is this is a p ractical aid in alignment but it illustrates the X-ray optics of sagit tal focusing in an elegant way.