Spherically bent crystal analyzers (SBCA) on the Rowland circle are the most common approach to high-energy resolution hard x-ray spectroscopy at synchrotron facilities and in the laboratory. Excluding a small number of conceptually important examples in prior work, such instruments operate with the diffracting plane practically coincident with the surface of the SBCA, i.e., they operate ‘symmetrically’ with respect to the SBCA optic’s cylindrical axis. After a careful survey of the foundations of the x-ray Rowland circle, we will report a comprehensive experimental and ray-tracing investigation of ‘asymmetric’ operation, meaning the use of one or more crystal orientations that are manifestly distinct from that of the SBCA wafer surface.[1, 2, 3] We find that a single SBCA can be used to access all energies from ~5 keV and higher by “hkl hopping” and we also find that asymmetric operation frequently permits reduction of Johann error without analyzer masking. These benefits are demonstrated with both synchrotron results and a new laboratory-based X-ray absorption fine structure (XAFS) and X-ray emission spectroscopy (XES) instrument that is designed to explore asymmetric operation. Looking to the future, asymmetric operation can play an important role in synchrotron and XFEL photon-in/photon-out spectroscopy in two ways. First, the energy range flexibility of asymmetric SBCA systems enables much simpler access and operation of high energy resolution fluorescence detection (HERFD): it is likely possible to develop an SBCA spectrometer that can easily and quickly jump to most needed energy ranges without beamline scientist intervention. Second, for x-ray Raman scattering, we propose that asymmetric operation combined with a sufficiently small beam size can greatly increase the clearance between sample and detector while retaining high energy resolution, even when using unmasked 0.5-m radius SBCA. We will report progress in the development of a next-generation LERIX instrument at the Advanced Photon Source that uses these concepts.
[1] Gironda, et al., https://doi.org/10.1039/D3JA00437F
[2] Chen, et al., https://doi.org/10.1039/D4JA00339J
[3] Abramson, et al., https://doi.org/10.1039/D4JA00340C
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Frederico Alves Lima / Gabriella Mulá Mathews