Feasibility study of the use of synchrotron radiation in the calibration of AXAF: Initial reflectivity results

The Advanced X-ray Astrophysics Facility (AXAF) has a calibration goal of 1% accuracy. We have begun test experiments at the National Synchrotron Light Source (NSLS), to demonstrate the attainable accuracy for reflectance measurements on coated flat mirrors in the 50 eV to 12 keV energy range. The goal of the experiments is to develop hardware and procedures for a continuum source reflectance calibration of witness coupons to the grazing incidence mirrors of AXAF. These test experiments will include measurements on commercially produced gold, nickel, and iridium coatings deposited by e-beam deposition and/or sputtering. Initial measurements were made on beamline X8C of NSLS, over the 5 to 10 keV range for sputtered gold and nickel, and e-beam-deposited gold. Beam size and incident flux were limited by horizontal and vertical slits, allowing a minimum grazing angle of 5 arc minutes. A sealed proportional counter was used as the detector for the reflected beam. Instantaneous flux monitors included a double window air ionization chamber and a sealed proportional counter monitoring X-rays Compton scattered from the air, both upstream from the slits. Normalizations for reflectance, obtained from energy scans of the direct beam by the detector with no sample present, are repeatable to within 5% RMS. Reflectance versus angle of incidence curves have been collected at 5 keV, as well as energy scans at several grazing angles over 5 to 10 keV. Optical constants will be estimated using the reflectance versus angle of incidence method. We discuss results and modifications of the configuration which will be implemented to improve the accuracy in future runs.