A study of breakdown limits in microstrip gas counters with preamplification structures

Abstract

We have studied the charge and breakdown limits of Microstrip Gas Counters (MSGC's) with two different preamplification structures: the Gas Electron Multiplier (GEM) and the Parallel-Plate Avalanche Counter (PPAC). It was found that in both cases the breakdown limit was increased by 1-2 orders of magnitude compared to the bare MSGC and that this was due to the spreading of the primary electron cloud during pre-amplification. This spreading reduces the charge density in the final MSGC avalanche, permitting much higher total gains before streamers form. The real practical gain limitations in these two-stage detectors arose not from sparking, but from a loss of proportionality due to space charge effects.