Sagittal focusing inducing energy structure in medium to high energy resolution x-ray monochromators

Abstract

Sagittal focusing is a widely used geometry to horizontally focus X-rays in synchrotron beamlines. Usually, two types of devices are used: toroidal mirrors combined with flat X-ray monochromators or flat mirrors combined with sagittal focusing monochromators. The former option seems to be better since the stresses caused by the bending of the second crystal in a double crystal monochromator (DCM) are avoided. In this work we explore the effect of sagittal focusing in medium to high energy resolution X-ray monochromators. Our studies are based on a theoretical explanation of experimental data acquired in two different experiments performed at the XRD2 beamline at Laboratorio Nacional de Luz Sincrotron (LNLS / Brazilian Synchrotron) and X18A at the National Synchrotron Light Source (NSLS / BNL / USA). The first experiment was carried out with a flat meridional bendable mirror followed by a DCM with the second crystal being sagitally bent. The second experiment was carried out with a flat Si 111 DCM followed by a toroidal, but meridional bendable mirror. Both setups are completed by a Si 551 4-bounce monochromator to scan the energy spectrum around 13.7 keV and 13.9 keV, respectively. The results show that the sagittal curvature induces an energy structure. Theoretical studies made by analyzing the reflecting angles in the mirror surface joined with the dynamical theory of X-ray diffraction in the 4-bounce X-ray monochromators confirm these results, which are also explored by the DuMond diagram.