hklhop: a Selection Tool for Asymmetric Reflections of Spherically Bent Crystal Analysers for High Resolution X-ray Spectroscopy

High resolution, hard x-ray spectroscopy at synchrotron x-ray light sources commonly uses spherically bent crystal analyzers (SBCAs) formed by shaping a single crystal wafer to a spherical backing. These Rowland circle optics are almost always used in a 'symmetric' (or nearly symmetric) configuration wherein the reciprocal lattice vector used for energy selectivity via diffraction is coincident with the normal vector to the curved wafer surface. However, Gironda, et al., recently proposed that asymmetric operation of SBCA, wherein the reciprocal lattice vector is no longer normal to the wafer surface, has significant operational benefits and has been an underutilized opportunity. First, those authors find improved energy resolution through decreased Johann error, or equivalently find increased solid angle at a chosen experimental tolerance for energy broadening. Second, they find productive, high-resolution use of a large number of reciprocal lattice vectors from a single SBCA, thus enabling operation over a wide energy range without need to exchange SBCA upon making large changes in desired photon energy. These observations hold the potential to improve performance, increase flexibility and decrease cost for both laboratory and synchrotron applications. Given these motivations, we report an open-source software package, hklhop, that enables exploration of the complex space of analyzer wafer choice, experimental energy range or ranges, and desired suppression of Johann error. This package can guide both the design and the day-to-day operations of Rowland spectrometers enabled for asymmetric use.