Diagnostics of bent x-ray diagnostic crystals

Recently the bending magnet beam line 1-BM at the Advanced Photon Source was reconfigured to be a test bed for bent crystals. The beamline's a water-cooled Si(111) × Si(111) double crystal monochromator is best from 6 keV to 28 keV x-rays. A channel cut crystal can improve monochromaticity further, and enlarge the beam so that it can illuminate the entire bent crystal. This paper illustrates the beam line's capabilities, with the results obtained on spherically bent quartz crystals as used for plasma spectroscopy or imaging. It is 60 mm by 40 mmand 0.1 mm thick, and bent to a 672 mm radius of curvature by optical (direct) contact to a substrate. The crystal surface can be are parallel to quartz'11-20 or 10-11 crystal plane. Bragg diffraction of a monochromatic X-ray beam across the entire crystal, a topograph, produces a spatially resolved spectrum that is a straight line for an ideal crystal; a wavy line comes from local strain, such as imperfect adhesion between the bent crystal and the substrate. These can cause artefacts in the spectrum in a spectroscopy application, or imperfect images when imaging. A slightly different angle makes an adjacent location diffract, and stepping the angle probes the whole crystal surface. Lower order Bragg diffraction with softer X-rays respond to the strain pattern closer to the crystal surface than higher order Bragg diffraction with harder X-rays, but qualitatively the topographs are similar for soft and hard X-rays and indicate the same problem locations. Hence, a bent crystal's quality can be gauged from the X-rays that are most conveniently available from a synchrotron, not necessarily the X-rays that the crystal will see. The paper will discuss the connections between the detailed topographs obtainable with the synchrotron and the performance of the crystal in its intended application, to plasma spectroscopy and X-ray imaging, and how to get the quality of bent crystals assessed with the standard protocols that the beam line has now established.