X-ray Optics of a Dynamical Sagittal-Focusing Monochromator on the GILDA Beamline at the ESRF

TLDR

The study describes the performance of a dynamical sagittal‑focusing monochromator for hard X‑rays. The monochromator uses a flat first crystal and a diamond‑shaped ribbed second crystal that is clamped and dynamically bent by forces on its apices, and its horizontal beam profile variations during rocking‑curve scans are calculated geometrically and agree with observations. The system performs excellently on the GILDA beamline, collecting a 3.6 mrad horizontal fan in 1:3 geometry, maintaining a ~1 mm spot size across 5–30 keV for XAFS spectroscopy with Si(311) crystals, and its predicted horizontal acceptance variations match observations, demonstrating reliable energy resolution, reproducibility, and useful alignment guidance.

Abstract

The performance of a dynamical sagittal-focusing monochromator for hard X-rays is described. It consists of a flat first crystal and a diamond-shaped ribbed second crystal which is clamped by its central rib and dynamically bent by applying a force on its two apices. The system has 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; the total available horizontal fan of radiation (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 the energy range 5-30 keV with Si(311) crystals, with a constant spot size FWHM ~1 mm on the sample and without degradation of energy resolution or reproducibility. Using simple geometrical considerations we calculate the variations of the horizontal profile of the reflected beam during rocking-curve scans in different focusing geometries and find them in agreement with observed ones. Not only is this is a practical aid in alignment but it illustrates the X-ray optics of sagittal focusing in an elegant way.