A Study of Breakdown Limits in Microstrip Gas Counters with Preamplification Structures

We have studied the charge and breakdown limits of Microstrip Gas Counters (MSGCs) with 2 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 that this is due to the spread 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.