Detector Lifetime

The lifetime of detectors in the harsh space environment is a major concern to the involved experimenters. The radiation environment can damage a proportional counter in two ways. Heavy ionizing particles can deposit energies 3-4 orders of magnitude higher than X-rays within the nominal operating range. These huge amount of charges must not trigger a permanent discharge or destroy the detector by spark discharge. In low earth orbit (ROSAT orbit at 580 km), heavy ionizing events have a trigger rate of about 2 x 10~4 cm~2 s^1 behind 1.5 cm of aluminum.

Gas detectors suffer a permanent aging because of the cracking of the quench gas molecules during normal operation. Hydrocarbons like CH4 tend to deposit polymerization products on anode and cathode wires, resulting in gain shifts and or permanent discharge (Malter effect) of the detector after accumulation of a not very well-defined critical charge per millimeter of anode wire. Many parameters like gas purity, gas composition, wire, housing and sealing materials, and last but not least the electrical field contribute to the radiation dose tolerated by the detector [19]. Sealed detectors tolerate a charge dose of 10-5 C mm-1 anode wire for an Ar-CH4 gas filling but only 10-7 C mm-1 for a Xe-CH4 gas before serious degradation occurs [26]. Therefore, many large area proportional counters for X-ray astronomy use CO2 as quench gas. CO2 does not polymerize. Only carbon deposits have been observed [21].

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