Surface streamer breakdown mechanisms in microstrip gas counters

We have studied breakdown mechanisms in MSGCs. For comparison, ''microstrip detectors'' without dielectric substrates were also tested. We found that detectors without substrates can always operate at gas gains 5-10 times higher than those with substrates, and that these higher gains are limited by self-quenched streamers. In the case of microstrip detectors with substrates, streamers also occur, but have a very narrow (in voltage) self-quench region and then transit rapidly to a ''gliding'' discharge. We have tested new geometries of MSGC which allow high gains (>105) to be reached. (orig.)