Abstract:
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. We report here a comprehensive experimental and ray-tracing investigation of ‘asymmetric’ operation, meaning the use of one or more crystal planes that are manifestly distinct from that of the SBCA wafer surface. [1] 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 significant rejection of Johann error without analyzer masking. These benefits are demonstrated with laboratory XAFS and XES results and with initial synchrotron for both high energy resolution fluorescence detection (HERFD) and nonresonant X-ray Raman scattering (XRS). Follow-up work has led to a software package to assist with hkl-hopping reflection selection and to better explain asymmetric operation through raytracing immediate relevant for implementing asymmetric use cases. [2, 3] We conclude by discussing the design and (imminent) testing of a new HERFD spectrometer at 25-ID that incorporates hkl-hopping to give the highest flexibility to users for HERFD and XES microprobe studies.

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