Abstract: The characteristics of a conventional cylindrical geometry proportional counter filled with high pressure xenon gas up to 10 atm were fundamentally investigated for use as a detector in hard x-ray astronomy. With a 2% methane gas mixture, the energy resolutions at 10 atm were 9.8% and 7.3% for 22 keV and 60 keV x rays, respectively. From calculations of the Townsend ionization coefficient, it is shown that proportional counters at high pressure operate at weaker reduced electric field than low- pressure counters. The characteristics of a parallel grid proportional counter at low pressure showed similar pressure dependence. It is suggested that this is the fundamental reason for the degradation of resolution observed with increasing pressure. !5
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