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 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 system comprises seven flow 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 Bundesandstalt, 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 source standard BESSY, which can be calculated very accurately. Combining both measurements the determination of the detection efficiency over the entire desired spectral range was possible with a typical relative uncertainty around 1 percent to 2 percent in the central energy range.