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 have completed a comprehensive investigation of ‘asymmetric’ operation, meaning the use of one or more diffracting crystal plane orientations that are manifestly distinct from that of the SBCA wafer surface [1] – consider, for example, using the Si (642) planes from an SBCA fabricated from a Si (211) wafer. 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 a new laboratory-based instrument that is designed to explore asymmetric operation and also with synchrotron results (APS 25-ID) for asymmetric operation of high energy resolution fluorescence detection (HERFD) and nonresonant X-ray Raman scattering (XRS). For HERFD, we propose that hkl hopping can significantly improve the user experience and simplify beamline operations. For XRS, we propose that asymmetric operation with a sufficiently small beam size can greatly increase the clearance between sample and detector while retaining high energy resolution, even with 0.5-m radius SBCA. 
In addition to the above results, two manuscripts in preparation will be briefly discussed. These address ray tracing for asymmetric operation as a guide for experimental practice [2] and a software package for optimal selection of asymmetric reflections for a given experiment, constrained by available SBCA wafer orientations.[3] 
[1] A.J. Gironda, J.E. Abramson, Y. Chen, M. Solovyev, G.E. Sterbinsky, G.T. Seidler, “Asymmetric Rowland circle geometries for spherically bent crystal analyzers in laboratory and synchrotron applications,” J. Analytical Atomic Spectroscopy 39, 1375 (2024). https://doi.org/10.1039/D3JA00437F 
[2] Y. Chen, G.T. Seidler, “A Ray Tracing Survey of Asymmetric Operation of the X-ray Rowland Circle Using Spherically Bent Crystal Analyzers” in prep. (2024) 
[3] J.E. Abramson, G.T. Seidler, “Selection of Asymmetric Reflections of Spherically Bent Crystal Analyzers for High Resolution X-ray Spectroscopy”, in prep. (2024)
 

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