AGING STUDIES WITH MICROSTRIP GAS-CHAMBERS

Microstrip gas chambers (MSGCs) are intrinsically capable of operating at very high radiation fluxes, up to and above 10(6) particles mm-2 s -1. It was noticed, however rather early in the development of the det ectors that sustained exposure to radiation could produce irreversible damage (decrease of gain and discharges) easily assimilated with the ageing phenomena which are known to occur in proportional counters, an d caused by the polymerization of gas molecules in the avalanche proce ss. We describe the results of a systematic investigation of ageing un der sustained irradiation of MSGC plates manufactured on several subst rates and operated in clean laboratory containers as well as in mechan ical assemblies better suited for the use of the detectors in experime ntal set-ups. In the best experimental conditions, we have demonstrate d survival without degradation of the detectors up to a collected char ge of 120 mC cm-1; in less than optimal conditions, ageing is observed already at a level of a few muC cm-1. A careful selection of the oper ating gas and of the materials used in manufacturing appears mandatory to guarantee survival of the devices in a high radiation environment such as that of the Large Hadron Collider (LHC) detectors. The present study has been realized at CERN in the framework of the RD-28 Collabo ration (Development of microstrip gas chambers for high-rate applicati ons).