The CMS Micro-strip Gas Chamber Project: Development of a high resolution tracking detector for harsh radiation environments

Thirty-two large area Micro-Strip Gas Chambers were tested in a high intensity, 350~MeV pion beam at PSI to prove that we had reached a Milestone for the Compact Muon Solenoid (CMS) experiment. The particle rate was approximately 6 kHz/mm2, distributed over the whole active area of the detectors, and this rate was maintained for a total integrated time of 493 hours. All of the chambers were operated with signal-to-noise values at or above that corresponding to 98 % hit detection efficiency at CMS; the average S/N was 31. No indications of any gain instabilities or ageing effects were observed. In the official 3-week Milestone period, three strips from a total of 16384 were damaged, a result which is twenty times lower than the minimal requirement for CMS. The spark rate of the detectors was very low and decreased with time to an average of one spark per chamber per day. The cathode voltages of 24 of the chambers were increased over a one week period to investigate the behaviour of the detectors at higher gains; the maximum S/N value was 2.4 times that at the normal working point. No significant increase in spark rate or strip loss rate was detected and the chambers operated stably. The detector efficiencies and imaging capabilities were also investigated. The MSGC design features and the assembly and test methodologies that enabled us to achieve these results are reported.