R&D on Triple-GEM detectors for the optimal and safe operation in the CMS experiment

In the framework of the High-Luminosity Large Hadron Collider project (HL-LHC), the LHC experiments will require upgrades of their detectors to cope with the new accelerator performance. The upgrade of the CMS Muon Spectrometer foresees the installation of three new muon stations based on the Gas Electron Multiplier (GEM) technology, referred to as GE1/1, GE2/1 and ME0. While the installation and commissioning of the GE1/1 detectors is currently underway, the GE2/1 and ME0 detectors are expected to be installed between 2023 and 2026. This thesis work took place in such framework. The main goal was to assure a safe operation of all the GEM based stations from the gas mixture point of view, in particular by developing a reliable system to detect any systematic variation of the gas mixture composition. On the other hand the focus was on ensuring the correct operation of GEM detectors when irradiated with harsh and intense particle fluxes, in particular testing the detector rate capability and aging properties. The first chapter gives a general overview on the LHC machine and the CMS experiment with some details about its GEM based upgrades. The second will introduce some basic concepts about the particle interactions with the matter and the physical processes behind gaseous detectors operations, along with an overview of the main gaseous detectors technologies. The third chapter will focus on the work done to develop a reliable system for gases concentrations detection in the gas mixture supplied to CMS GEM detectors (nominally Ar/CO2 (70/30)). The R&D tests performed to reach the necessary knowledge level to use a GEM detector for determining the relative ratio of the two gases will be described. The fourth chapter will present the results that convinced the CMS Group to change the GEM foil design to exploit the maximum performance of the technology in the high hit rate environment. The results, along with the new design, will be described up to the current R&D stage. The fifth chapter will discuss the advanced aging tests performed to test the radiation hardness and the aging mechanisms taking place in GEM detector when exposed to heavy irradiation and gas contamination. Many experiments were performed also to test different variables that play a role in the aging of both Wire Chambers and GEM detectors.