The Operational Experience of the Triple-GEM Detectors of the LHCb Muon System: Summary of 2 Years of Data Taking

The LHCb muon system consists of more than a thousand gas detectors, mostly MWPC, located in five different stations. The Muon detector is used to define the muon trigger and to identify muons at the high-level triger and at the recontruction stage. The first station of the muon detector, located in front of the calorimetric system, is made of 274 chambers. The 12 most irradiated chambers, the ones closer to the beam pipe, are double triple-GEM detectors with pad readout. These detectors have an active area of 200x240 mm$^2$ and are routinely operated at rates close to 300 kHz/cm$^2$. With the gas mixture used (Ar/CO$_2$/CF$_4$ at 45/15/40) these detectors have the requested efficiency (>96% in a 20 ns time window for the logical OR of the two sensitive gaps) when operated at gains of about 2300. In this presentation we will report on the performance of these 24 triple-GEM detectors after more than 2 years of operation in the harsh LHCb conditions. We will also show some problems occurred during data taking, in particular on the failure of a few GEM foils following repeated discharge phenomena, and the solutions implemented to reduce the occurrence of such problems.