Studies on Gas Mixture and Gas Recirculation Effects on GEM Detectors Operation at the CERN GIF++ Facility

The future Physics Program established for the Large Hadron Collider (LHC), with the upgrade of the accelerator, sets important challenges for all detector systems. Several upgrades will affect Muon Systems, including the ones on the gas mixture in use, since their stability and correct composition are at the basis of safe long-term operation. The aim is to lower operational costs and gas emission, while maintaining high performance levels. Gas systems are therefore operated in recirculation mode. The purpose of this thesis is a detailed study of the performances of Gas Electron Multiplier detectors (GEM), commonly operated with Ar/CO$_2$ gas mixtures, but also the addition of CF$_4$, to obtain an improved time resolution. The use of CF$_4$, that is considered a greenhouse gas, force the operation in recirculated gas systems, that makes fundamental to guarantee optimal detector operation in such condition. A Triple-GEM detector characterization was realized in different gas mixture conditions. Studies on GEM performance when operated in a gas recirculation system and under high irradiation rate were realized at the CERN Gamma Irradiation Facility (GIF++), that provides irradiation with gamma emission (662 keV photons) from a $^{137}$Cs source (activity 14 TBq). Two Triple-GEM detectors are installed inside the GIF++ facility with R&D purposes. The aim of the measurement campaign at GIF++ is to progress with the validation of their operation in a system as close as possible to the ones in LHC experiments.