Characterization of gaseous detectors at the CERN Gamma Irradiation Facility: GEM performance in presence of high background radiation

Muon detection is an efficient tool to recognize interesting physics events over the high background rate expected at the Large Hadron Collider (LHC) at CERN. The muon systems of the LHC experiments are based on gaseous ionization detectors. In view of the High-Luminosity LHC (HL-LHC) upgrade program, the increasing of background radiation could affect the gaseous detector performance, especially decreasing the efficiency and shortening the lifetime through ageing processes. The effects of charge multiplication, materials and gas composition on the ageing of gaseous detectors have been studied for decades, but the future upgrade of LHC requires additional studies on this topic. At the CERN Gamma Irradiation Facility (GIF++), a radioactive source of cesium-137 with an activity of 14 TBq is used to reproduce reasonably well the expected background radiation at HL-LHC. A muon beam has been made available to study detector performance. The characterization of the beam trigger will be discussed in the present work. GIF++ allows to carry out accelerated ageing tests on gaseous detectors. The R&D studies, discussed in this thesis, are focused on the performance of a triple gas electron multiplier (triple-GEM detector) under gas recirculation in presence of high background radiation at GIF++. A purifier module is used inside the gas system to remove pollutants from the gas mixture. Several studies of stability and reliability can be carried out with different gas mixtures and recirculation fractions. Indeed in gaseous detectors, the ageing effects involve a progressive degradation of the performance. Several processes in gas mixture can lead to ageing phenomena, such as dissociation, polymerization and etching. Unwanted pollutants, due to the outgassing of materials, can contaminate the gas mixture. Furthermore the combined action of electric field, charge multiplication and high background radiation might produce contaminations, which can decrease the lifetime of gaseous detectors by many orders of magnitude. In a first step, the triple-GEM detector and two single wire proportional chambers, used for the gas mixture monitoring in the recirculation system, have been characterized in laboratory. Afterwards they have been installed at GIF++ and the triple-GEM has been irradiated with the cesium-137 source. A data acquisition system has been developed for monitoring the performance of triple-GEM detector and the environmental parameters of the system, which might affect the detector response. The effects of some common pollutants have been simulated with GARFIELD++ software to evaluate the performance of triple-GEM detector with different gas mixtures, in particular Ar/CO2 and Ar/CO2/CF4 in different compositions