Long-Term Irradiation Studies of Large-Area Micromegas Detectors for the ATLAS NSW Upgrade

In the course of the planned upgrade of the Large Hadron Collider at CERN to higher interaction rates, detectors and electronics of the experiments located there will also be exchanged and replaced by more powerful components. One of these experiments is the ATLAS experiment. This underwent, among other things, an upgrade of the inner forward muon spectrometer in the first upgrade period (Phase 1). The Small Wheel located there was replaced by the New Small Wheel (NSW). The New Small Wheel consists of two detector technologies: the small-strip Thin Gap Chambers (sTGCs) and the MICRO-MEsh GAseous Structure (Micromegas) detectors. The central topics of this work are studies on large-area (2-3m$^2$) Micromegas detectors. These showed unstable behavior, which could lead to efficiency losses or long-term problems concerning operations in the years to come. The use of the intended operating gas, a mixture of Argon and Carbon dioxide in a ratio of 93:7 vol%, could not satisfactorily meet the requirements. A change to a ternary gas mixture of Argon, Carbon dioxide, and Isobutane in a 93:5:2 vol% ratio was considered. This admixture of a hydrocarbon gas leads to intensive investigations in terms of longevity, efficiency, and resolution of the Micromegas detectors which are the topic of this thesis. Possible aging damages as a consequence of the admixture of isobutane are discussed. Spare Micromegas detectors, identical to the series detectors installed in the NSW, are examined in this work for the parameters discussed previously. Two facilities were selected for testing the detectors. The Gamma Irradiation Facility (GIF++) at CERN provides a 14 TBq cesium source (662 keV photons). Several Micromegas detectors are irradiated with this source and acquire charges equivalent to the expected future amount in the ATLAS experiment. Furthermore, this facility offers regular beamtimes with a muon beam. Periodic tests are conducted to investigate the efficiency and precision of irradiated detectors. These tests are performed with the final readout electronics of the NSW. Different absorption filters are used in the GIF++ to simulate varying background intensities. This is done to test the expected irradiation rate's impact on detector performance and readout electronics. A calibration of the irradiation rates in the GIF++ is performed in this work to compare them with the rate in the ATLAS experiment. The basis of this calibration is data taken with the NSW at the ATLAS experiment. In addition, the best results are achieved for large-area Micromegas detectors concerning inclined particle incidents. A second long-term setup is situated in Garching near Munich. Another spare Micromegas underwent irradiation from an Americium-Beryllium neutron source for nearly three years. In contrast to pure gamma radiation exposure in the GIF++, the aim here is to expose the detector additionally to highly ionizing hadronic radiation. Long-term studies compare the performance of the legacy (Ar:CO$_2$) and the ternary gas mixture. Benchmark efficiencies of the detector are compared with intermediate measurements and a final measurement after completion of the irradiation. Cosmic muons are used for this measurement. Additional investigations with the neutron source as background and a calibration of the neutron source are performed. Based on the results of this work, the Micromegas detectors at ATLAS are now operated with the ternary gas mixture.