The commissioning of the GEM detector for the muon apparatus of the LHCb experiment

The 24 triple-GEM detectors for the innermost region of the first muon station (M1R1) of the LHCb experiment are ready to be installed. The major challenges have been the improvement of the time performance in terms of efficiency of the bunch crossing identification and a high radiation hardness in the harsh environment typical of LHC experiments. During the R&D phase we demonstrated, with several laboratory and beam tests, that GEMs operated with an Ar/CO₂/CF₄ (45/15/40) gas mixture can achieve a time resolution below 3 ns r.m.s, giving an efficiency better than 96% in 25 ns time window. A rate capability better than 50 MHz/cm² and a robustness against discharges (tests with alpha and low momentum particles at PSI ) have been measured. A severe global irradiation test of the final detector has been required to study the compatibility of the construction materials with the gas mixture, showing that the detector survives, without permanent damages and serious performance losses, after an integrated charge of 2.2 C/cm², corresponding to 12 years of LHCb. The construction of chambers, based on an innovative stretching technique of the GEM foils, has been realized in a class 1000 clean room. Quality control of components and the HV test of GEM foil, as well as the gas leak test and X-ray tomography of the detectors have been performed. The performances of the final GEM detector, integrated with the front-end electronics based on the CARIOCA-GEM chip, have been measured with the LHCb DAQ electronic chain in a recent beam test at the SPS operated with a 25 ns time structure, similar to the LHC running condition. The results demonstrate that the GEMs fulfill the experiment requirements with a safe working range of 90 V corresponding to a factor of 4 in gain.