Soft x-ray calibration of the Co/C multilayer mirrors for the Objective Crystal Spectrometer on the Spectrum Roentgen-Gamma satellite

The objective crystal spectrometer (OXS) on the forthcoming Spectrum-Roentgen-Gamma satellite is designed to carry three kinds of crystals: LiF(220), Si(111) and RAP(001), placed in front of the SODART telescope. Thirty six super polished (RMS roughness < 0.1nm) Si(111) substrates were coated with 65-80 periods of Co/C multilayers using electron beam evaporation deposition combined with ion polishing for the metal layers. These crystals are to be used in the energy band immediately below the C-K absorption edge of 0.284 keV. Because the crystals are to be assembled as one crystal on the OXS, the reflectivity performance as a function of energy and angle of incidence of all crystals has been measured using line radiation from an x-ray tube which provides 1.487 keV and 0.277 keV and using synchrotron radiation from 0.16 keV to 0.28 keV at the Synchrotron Ultraviolet Radiation electron storage ring a t the National Institute of Standards and Technology. The results from these measurements are discussed.