FEEDBACK AND BREAKDOWNS IN MICROSTRIP GAS COUNTERS

The phenomena of feedback and breakdowns in microstrip gas counters ha ve been studied. Tests have been performed on a wide variety of micros trips of different geometries and substrates in various gas mixtures a nd at several pressures. In addition, for direct comparison, similar m icrostrip gas counters without substrates were also investigated. In t he case of ''conventional'' microstrip detectors on substrates, it was found that as maximum gains were approached, self-quenched streamers occurred, but that these were present only over a very narrow voltage range after which they rapidly transited to gliding-discharge-type bre akdown sparks. For microstrip detectors without substrates, the situat ion was quite different. At high gains (> similar to 10(4)) they trans ited from proportional to self-quenched streamer-mode and would operat e stably in this mode over a very broad voltage range. When breakdown occurred in these detectors it was through a previously unreported mec hanism and at gains, without exception, 5-10 times higher than in thei r counterparts with substrates. The conclusion of these studies is tha t the substrate represents the weak link in the chain and should ideal ly be avoided or at least removed from the high-field regions. In case s where this is not possible, then the multiplication region should be concentrated to the smallest volume around the anodes and maximally s uppressed elsewhere. Recipes for achieving this are provided, along wi th new geometries which permit gas gains in excess of 10(5) to be achi eved.