Effects of gas mixture quality on GEM detectors operation

Gas Electron Multiplier (GEM) detectors have been successfully operated in the LHCb experiment and they will be installed in the CMS and ALICE experiments during LHC Long Shutdown 2. As for others LHC gaseous detector systems, gas mixture is individually provided by dedicated Gas Systems. Several studies have been performed in laboratory to characterize GEM performance from the point of view of their use in LHC Gas Systems, where many variables can influence detectors operation. A Triple-GEM prototype was tested with the aim of probing the possible effects of specific changes that could occur in the Gas System, such as variations in gas mixture composition, input gas flow and presence of impurities. A complete overview of the obtained results will be presented. The test confirms the importance of having a stable gas mixture composition, as it influences GEMs working point, with significant variations in the amplification gain. It was also seen how the input gas flow can affect GEMs performance, since it conditions the accumulation of humidity and air. Moreover, it was found that the presence of pollutants such as O2, N2 and H2O, commonly present during operation in the experiments, influences GEMs performance in terms of signal efficiency and amplification gain. While detector gain is weakly affected by the presence of N2, the presence of even small concentrations of O2 causes a significant performance decrease in terms of amplification gain.