Qualification study of LiF flight crystals for the objective crystal spectrometer on the SPECTRUM-X-GAMMA satellite

The Objective Crystal Spectrometer (OXS) on the SPECTRUM-X-GAMMA satellite will carry these types of natural crystals LiF(220), Ge(111) and RAP(001). They will be used to study, among others, the H- and the He-like emission from the cosmically important elements Fe, S, Ar and O. More than 300 LiF-crystals of dimension approximately 23 × 63 mm2 are required to cover one side of a large (approximately 1000 × 600 mm2) panel which is to be mounted in front of one of the high throughput X-ray telescopes. A qualification study, performed at the Danish Space Research Institute (DSRI), examined a large sample of LiF(220) crystals at Cu-Kα2 (8.0278 keV). Data from 124 flight crystals yields an average FWHM of rocking curves of 2.3 arcmin with a standard deviation of 0.4 arcmin. For more than 80% of the crystals, angular deviation of the (220) planes from the actual crystal surface is less than 1.5 arcmin. These data will be used to select the best crystals for the flight panel and will determine precisely the orientation of the crystals mounted on the OXS. Eight crystals were glued onto a small test panel of the OXS and for only one crystal was there measured a significant deviation of the crystal properties, including alignment relative to the others.