A Ray Tracing Survey of Asymmetric Operation of the X-ray Rowland Circle Using Spherically Bent Crystal Analyzers

TL;DR

This paper uses ray tracing simulations to evaluate how asymmetric operation of spherically bent crystal analyzers improves energy resolution and reduces broadening effects in X-ray spectroscopy, guiding future experimental setups.

Contribution

It provides a detailed analysis of asymmetric SBCA configurations, demonstrating their advantages over symmetric operation in energy resolution and outlining strategies to minimize efficiency loss.

Findings

Asymmetric SBCA generally outperforms symmetric in energy resolution.

Proper detector placement can mitigate defocusing effects.

Asymmetric operation enables broader energy range access.

Abstract

The spherically bent crystal analyzer (SBCA) is a widely-adopted hard x-ray optic, renowned for its good energy resolution and large collection solid angle. It is frequently employed in synchrotron-based techniques like Resonant Inelastic X-ray Scattering (RIXS) and X-ray Emission Spectroscopy, as well as in the rebirth of laboratory-based X-ray Absorption Fine Strucure (XAFS) and XES, and its predominant use has been in 'symmetric' operation on the Rowland circle. The recent study of Gironda et al. (J. Anal. At. Spectrom., 2024, 39, 1375) emphasizes the benefits of 'asymmetric' SBCA operation, demonstrating the use of multiple crystal reflections from a single SBCA to broaden its accessible energy range. Furthermore, Gironda et al. demonstrate that asymmetric operation frequently mitigates energy broadening intrinsic to Johann optics and propose that under a specific Rowland circle configuration, designated here the Johann Normal Alignment (JNA), such broadening is eradicated altogether. We report extensive ray tracing simulations to scrutinize the impact of asymmetric configurations on energy broadening and detector plane defocusing. We find that the performance of asymmetric SBCA operation generally exceeds its symmetric counterpart in energy resolution when no analyzer masking is used and, with strategic detector placement, the decrease in detection efficiency due to defocusing can be minimized. Spectroscopic imaging is adversely affected by the detector plane blurring, but rejection of scattering from special environment windows in x-ray Raman imaging is still feasible. These results help inform future, more common implementation of asymmetric reflections with SBCA.