ON SOME FACTORS AFFECTING DISCHARGE CONDITIONS IN MICRO-STRIP GAS-CHAMBERS

We describe the results of an investigation on the influence of geomet rical parameters on the maximum gain that can be attained in a micro-s trip gas chamber. Single electron emission from the cathode was confir med to be a precursor of discharge, particularly for gas mixtures with a high fraction of quencher, confirming our previous observations. In contradiction however with the results of a theoretical model propose d in earlier work, we found that the maximum gain is obtained in a nar row range of cathode strip width. This suggests the presence of other mechanisms initiating a discharge, such as photon- or ion-initiated fe edback. The influence of strongly ionising irradiation on discharge co nditions has also been investigated; we have observed an increasing pr obability of discharge in detectors irradiated with a particles, relea sing around 1 MeV in the gas, as compared to 6 keV X-rays. This result s in a reduction by a factor of 2 of the maximum gain at which a MSGC can be safely operated, a concern for the use of micro-strip chambers in vertex detectors for colliders where low energy, heavily ionizing p articles and thermal neutrons are produced together with minimum ioniz ing particles.