Operation of MSGC+GEM detectors in a high rate environment

The assembly and test of 18 MSGC+GEM modules each equipped with four wedge-shaped substrates is described. Measurements of gain and current were performed to determine the fields for maximum transparency of the GEM. These detectors were operated for five weeks under a high rate environment at the Paul Scherrer Institute (Switzerland) to study their stability and uniformity. The large scale MSGC+GEM system was operated for 376 h at the nominal working point of signal to noise around 37 with a negligible sparking rate. A safety margin of a factor of two to three in signal to noise guarantees a safe operation under LHC like conditions.