BREAKDOWN FEATURES OF VARIOUS MICROSTRIP-TYPE GAS COUNTER DESIGNS ANDTHEIR IMPROVEMENTS

Breakdown mechanisms and spurious pulses, the precursors to some break downs, were studied experimentally for both uncoated and coated Micros trip Gas Counters (MSGC's) of different geometries, as well as for Mic roGap Counters (MGC's) and for the ''Compteur A Trou'' (CAT). It was f ound that in all cases the breakdowns occurred through surface streame rs, although the exact mechanism of streamer formation depended on the particular detector design. Based on these studies, new designs of mi crostrip detectors, in which the role of the substrate was minimized, were elaborated and tested. In some of these detectors, especially wit h large pitches (>2 mm), gains up to 2-3x10(5) were achieved together with good rate characteristics. The ultimate gain limit in all geometr ies was still set by spark-inducing streamers which appeared at some c ritical charge density in the avalanche. To avoid this, and particular ly to enhance the performance of small-pitch MSGC's, preamplification structures can be used. Utilizing a parallel plate avalanche chamber a s a front end to an MSGC resulted in an overall gain of similar to 10( 6), limited in this case only by charge saturation.