Design and in-orbit-performance of the position sensitive proportional counter onboard the X-ray astronomy satellite ROSAT

A Position Sensitive Proportional Counter (PSPC) was the prime focal plane instrument onboard the X-ray astronomy satellite ROSAT, a conventional multiwire counter for X-ray imaging in the energy range from 0.1 to 2.4 keV. The detector gas was a mixture of 65% Ar, 20% Xe and 15% CH4 at a pressure of about 1.5 bar, replenished at a flow rate of 2.5 cm3 min−1. At 1 keV the detector had an energy resolution of 41% FWHM, a position resolution of 230 μm FWHM, and a quantum efficiency of 65%. The background rejection capability in the space environment was 99.85%. The gain degradation in orbit was 1% per month for the nominal PSPC A and 1% per year for the redundant PSPC B. Within the first 4 years of the mission, the PSPC was used for 80% of the observing time. Thereafter, the gas supply was nearly exhausted and the PSPC was used only for special observations. After eight years a final observation was carried out with the PSPC using extremely reduced gas flow at the border of the radiation belts with a very high particle background. During this observation the detector probably suffered a discharge and within several days an increasing area with 50% gain loss developed. We report on the detector design, the in-orbit-performance of the PSPC and some results of the ROSAT mission.