A slitless double crystal monochromator for EXAFS and XANES measurements

Abstract

Recently a double crystal monochromator has been commissioned on the EXAFS station 8.1 at the Daresbury Synchrotron Radiation Source (SRS). This monochromator is designed to operate in an UHV compatible chamber in the energy range 3–14 keV. It utilises three seperate pairs of crystals to cover the total range and to provide the optimum energy resolution 1–3 eV required for EXAFS and XANES measurements. The crystal pair for the high energy range (8–14 keV) is special in that the second crystal is curved with a radius of 55 m in the plane of diffraction and also with a radius of 2.5 m perpendicular to the plane of diffraction. The crystal pairs for the other ranges are bent only in the plane of diffraction to take account of the vertical divergence of the X-rays from the SRS. In the low energy ranges, the focussing is provided by an ellipsoidal mirror. The second crystal is translated to keep the position of the exit beam fixed and is also adjusted to suppress the higher harmonics. The first crystal is water-cooled to avoid excessive thermal loading. Some results are presented for a number of X-ray edges (K-edge, Ti, V, Cu and L III-edge Pt and Au) to illustrate the performance of the monochromator.