High-accuracy calibration of the HXDS flow proportional counter for AXAF at the PTB laboratory at BESSY

The Smithsonian Astrophysical Observatory uses the HRMA X-ray Detection System (HXDS) to calibrate the High-Resolution Mirror Assembly (HRMA) of the Advanced X-ray Astrophysics Facility AXAF. Apart from two high-purity-germanium solid-state detectors (SSDs) with good energy resolution and very high efficiency at higher energies, the detection sq stem comprises seven now proportional counters (FPCs) and one microchannel-plate High-Speed Imager. For the lower energy range, the FPCs are more appropriate. They have been calibrated at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt, using the electron storage ring BESSY. For the determination of the absolute quantum efficiency two methods have been applied. First, the detector response was measured in the lower energy range 0.1 keV to 1.7 keV at several discrete energies using monochromatized radiation. The absolute photon flux has been determined by Si n-on-p photodiodes, calibrated against a cryogenic electrical-substitution radiometer used as primary detector standard. The second method uses the undispersed synchrotron radiation emitted by a bending magnet of the primary source standard BESSY, which can be calculated very accurately. Combining both measurements the determination of the detection efficiency over the entire desired spectral range (0.1 keV to 10 keV) was possible with a typical relative uncertainty around 1% to 2% in the central energy range (0.2 keV to 1.7 keV).