Spherically bent mica analyzers as universal dispersing elements for X-ray spectroscopy

Spherically-bent crystal analyzers (SBCAs) see considerable use in very high-resolution hard X-ray wavelength dispersive X-ray fluorescence spectroscopy, often called X-ray emission spectroscopy (XES). While Si and Ge are the most frequently used diffractive components of SBCAs, we consider here the somewhat classical choice of muscovite mica as the dispersing element. We find that the various harmonics of a highest-quality mica-based SBCA show ~5-~40% of the integral reflectivity per unit solid angle of a typical Si or Ge SBCA in the hard X-ray range, and that the mica SBCA have comparable energy resolution to the traditional SBCAs. Interestingly, the choice of mica comes with a practical benefit: the primary (0,0,2) reflection has sufficiently strong harmonics that are fairly tightly spaced in energy so that they span the complete energy range from ~4 to ~11 keV when used at convenient Bragg angles in a Rowland circle spectrometer. Hence, a single mica SBCA can be used for every K-shell emission line of three dimensional transition metals and every L-shell emission line of the lanthanide elements simply by selecting the correct mica (0,0,2) harmonic with a final energy-dispersive solid state detector. The loss in efficiency is counteracted by an operational efficiency, i.e., the "universal" application of a single analyzer over a very large range of elements. This performance suggests future application of mica SBCAs in both laboratory-based XES and synchrotron-based photon-in, photon-out spectroscopies in the hard X-ray range.