Determination of spectral reflectivity of spherically bent mica crystals applied for diagnostics of relativistic laser plasmas

Abstract

Mica crystals are widely applied in x-ray spectroscopy diagnostics since their ability to effectively reflect the radiation in different orders covering a wide range of photon energy, including sub-keV range hardly accessible with other crystals. Particularly, spherically bent mica crystals are commonly used in high energy density plasma imaging spectrometry diagnostics. However, the detailed reflectivity properties of bent mica crystals are not known well. Here we propose and verify the way to calibrate mica crystal spectral reflectivity in the experiment with relativistic laser plasma. The approach is based on the comparison of dense laser plasma x-ray spectra measured by focusing spectrometers with spatial resolution equipped, with examined mica and pre-calibrated alpha-quartz bent crystals. As a result, the normalized reflectivity of spherically bent mica crystal operated in 2nd order of reflection was experimentally evaluated for the first time versus wavelength in the range of 6.7-8.7 Å. The obtained spectral calibration curve for bent mica crystal demonstrates remarkable difference to that one calculated for flat mica crystal and given in Henke tables and has to be applied further for a correct interpretation of the measured x-ray spectra.