Laboratory and Synchrotron Applications of Asymmetric Operation of the Rowland Circle

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 orientations 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 reduction of Johann error without analyzer masking. These benefits are demonstrated with a new laboratory-based X-ray absorption fine structure (XAFS) and X-ray emission spectroscopy (XES) instrument that is designed to explore asymmetric operation. We also report preliminary synchrotron demonstrations of asymmetric operation for high energy resolution fluorescence detection (HERFD) and non-resonant X-ray Raman scattering (XRS). For XRS, 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.

[1] A. Gironda, J. Abramson, Y. Chen, G.T. Seidler, “The Asymmetric Implementation of Spherical Analyzers on the Rowland Circle for Applications in the Laboratory and at Synchrotron Facilities,” to be submitted (2023).