DEVELOPMENT OF MICRO-STRIP GAS-CHAMBERS FOR HIGH-RATE OPERATION

We describe the developments of micro-strip gas chambers able to withs tand the very high rates foreseen for operation as vertex detector in high luminosity experiments, and for applications in medical diagnosti cs. To avoid surface charging-up processes, we have used as supports e lectron-conducting glass with resistivity in the range 10(9) to 10(12) Omega cm, and bore-silicate glass with thin coatings of lead silicate with surface resistivity between 10(14) and 10(16) Omega/square. A sy stematic research has been undertaken to find the purity levels of the gas filling and of the manufacturing materials necessary for long-ter m stability of operation, the goal being 10 years of operation at LHC (or about 140 mC cm(-1) of collected charge). In particular, we have t ested high-grade polymers that can be injection-moulded into the shape required to make MSGC frames, and epoxies that satisfy the stringent outgassing requirements. A strong dependence of ageing from the charge rate used in the irradiation has been found, indicating that measurem ents realized at high current densities may be too optimistic in terms of expected lifetime of the detectors; this seems to be particularly true for MSGCs manufactured on high resistivity bore-silicate glass.