On some factors affecting discharge conditions in micro-strip gas chambers

We describe the results of an investigation on the influence of geometricla parameters on the maximum gain that can be attained in a micro-strip gas chamber. Single electron emission from the cathode was confirmed 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 proposed in earlier work, we found that the maximum gain is obtained in a narrow range of cathode strip width. This suggests the presence of other mechanisms initiating a discharge, such as photon- or ion-initiated feedback. The influence of strongly ionising irradiation on discharge conditions has also been investigated; we have observed an increasing probability of discharge in detectors irradiated with α particles, releasing around 1 MeV in the gas, as compared to 6 keV X-rays. This results 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 particles and thermal neutrons are produced together with minimum ionizing particles.