The Suppression of Destructive Sparks in Parallel Plate Proportional Counters

We find that high energy background events produce localised sparks in parallel plate counters when operated in the proportional mode. These sparks increase dead-time and lead to degradation ranging from electrode damage to spurious pulsing and continuous breakdown. The problem is particularly serious in low energy photon detectors for X-ray astronomy which are required to have lifetimes of several years in the high radiation environment of space. For the parallel plate imaging detector developed for the European X-ray Observatory Satellite (EXOSAT) we investigate quantitatively the spark thresholds, spark rates and degradation processes. We discuss the spark mechanism, pointing out differences from the situation in spark chambers and counters. We show that the time profile of the sparks allows us to devise a spark suppression system which reduces the degradation rate by a factor of ~200.