Muonic Background Analysis in GERDA Phase II (II+) and in LEGEND-200 Commissioning Data

Abstract

Muons can generate a non-negligible background in rare-event experiments, such as GERDA and LEGEND which are dedicated to the search of neutrinoless double-beta decay (0νββ) in the isotope of 76 Ge. Cosmogenic muons, with their variable kinetic energy depending on the progenitor particle, have the capability to penetrate even the most deep underground laboratories. For this reason, both GERDA and its successor, LEGEND-200, are equipped with a powerful Cherenkov veto system which was initially designed, constructed, and recently refurbished by the University of Tübingen. In this work, both direct and indirect muonic-induced background in the ent- tire GERDA Phase II and Phase II+ data was studied. Coincidence events with the germanium detectors were examined and muon-germanium coincidences were clearly identified by using a timestamp correlation method between germanium and muon events. The residual muonic background after all the analysis cuts were studied in detail, and it highlights the essential requirement for the muon veto system in the next-generation LEGEND experiment. Two approaches were used to detect the cosmogenic isotopes produced by muons through spallation. Both yield approached results consistent with zero, indicating the absence of cosmogenic events in GERDA Phase II and Phase II+ data. Existing hardware components were carefully maintained, subject to frequent calibrations and monitoring over the duration of this work, ensuring a reliable performance of the veto. In addition, the radioactive background peak attributed to 137 Cs was investigated in the GERDA data. The source of this radioactive peak was traced back to the detector holder plates and bars employed in the GERDA setup. Furthermore, Monte Carlo simulations were employed within the GERDA dataset to study the impact of removing oil from the PMTs. These simulations provided the foundation for affirming that the sensitivity of the Cherenkov veto system would remain sufficient following the repair, ensuring its effectiveness in the LEGEND-200 experiment. This thesis also presents the upgrades to the data acquisition and analysis systems to cope with the new software and hardware requirements of the LEGEND-200 experiment. Notably, the new hardware trigger threshold was chosen in a way that the muon sample is recorded with minimal random coincidences. The first set of digital signal processing parameters for analyzing LEGEND-200's new data was implemented. Additionally, further selection criteria were defined to accurately identify true muon events in the data. Through a comprehensive analysis of commissioning data, together with regular monitoring, and frequent calibrations after the refurbishment of the muon veto, the good performance of the veto system is precisely verified.