High accuracy calibration of the HXDS HPGe detector at the PTB radiometry laboratory at BESSY

The Smithsonian Astrophysical Observatory is performing the calibration of the AXAF High Resolution Mirror Assembly (HRMA) at the Marshall Space Flight Center. In order to determine the absolute incoming photon flux at the calibration facility a High Purity Germanium (HPGe) detector and an identical spare detector have been chooen as flux monitor of the HRMA X-ray Detection System (HXDS). The detectors must thus be calibrated in the photon energy range between 700 eV and 10 keV. The calibration and characterization of the detectors was performed at the radiometry laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY. Two methods have been applied for the determination of the detection efficiency. Firstly, the undispersed calculable synchrotron radiation emitted by a bending magnet of the primary source standard BESSY has been used. Secondly, the detection efficiency has been determined using monochromatized radiation and Si photodiodes as transfer detector standards. The photodiodes have been calibrated with a relative uncertainty of 0.5% in the photon energy range from 50 eV to 1500 eV against a cryogenic electrical substitution radiometer which is a primary detector standard. It will be shown that the determination of the detection efficiency in the whole desired spectral range is possible with a relative uncertainty between 1% and 2%, even in the photon energy range where the detection efficiency is affected by the x-ray absorption finestructures of the Al light blocking filter of the detector.