OPERATION OF A DYNAMICALLY BENT SAGITTALLY FOCUSING DOUBLE CRYSTAL MONOCHROMATOR FOR XAFS STUDIES

The X-11A beamline at the NSLS has been upgraded to include a dynamic sagitally focusing second crystal in the monochromator, currently operating in the range 5.5-11 keV. The monochromator maintains a constant focus at the sample by adjustment of the bent crystal radius during the XAFS scan. The monochromator accepts 3.3 mrad of horizontal radiation and a spot size of approximately 1.5 mm2 is achieved at the sample. The sagittally focusing mechanism in the monochromator is described and the results are presented of intensity profiles, spatial stability, and energy resolution between the energy ranges 6 and 10 keV, where the system performs most favorably. Intensity gains over unfocused operation are substantial and XAFS studies of dilute fluorescence samples reveal the expected S/N improvements, without any introduction of additional noise from the bending process. The mechanism operates with close to 100% efficiency between the energy ranges 6 and 8 keV. Bending errors hinder the performance at higher energies and also preclude effective harmonic rejection, by piezocrystal detuning, over the entire energy range.