Simulation Based Software and Hardware Development for the Active Reduction of Muon Induced Background in the Liquid Scintillator Detectors JUNO and OSIRIS

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid scintillator neutrino detector currently under construction in southern China. Its main goal is the determination of the neutrino mass ordering by measuring the energy spectrum of reactor electron antineutrinos from nearby nuclear power plants. In addition, JUNO pursues a broad physics program by observing neutrinos from terrestrial and extra-terrestrial sources, including the sun, supernovae, the atmosphere and potentially dark matter. Besides the unprecedented energy resolution of 3% sqrt(Evis(MeV)), a detailed understanding and reduction of background signals in the detector is necessary. Among the main background sources are muon induced cosmogenic isotopes and radioactive contamination in the liquid scintillator. The first part of this thesis is dedicated to the rejection of cosmogenic background produced in events with multiple muons in the detector. For this event type, a dedicated algorithm is developed which aims to reconstruct the muon tracks in this event type with high precision and reliability. The performance of the algorithm is tested with simulated muon events and it is demonstrated that the method can contribute essentially to the rejection of cosmogenic background. In the second part of the thesis, the estimation of cosmogenic background and the development of a muon veto system for the JUNO pre-detector OSIRIS (Online Scintillator Internal Radioactivity Investigation System) is presented. The purpose of the sub-system is the monitoring of radioactive contamination in the liquid scintillator, before it is filled into the JUNO detector. As part of this thesis, the OSIRIS simulation framework is extended to address the production of cosmogenic isotopes in the detector, important features in the detector geometry and the components of the veto system. Based on comprehensive muon simulations, it is found that muon induced background contributes significantly to the OSIRIS sensitivity and its active rejection with a veto system is necessary. For that reason, detailed Monte-Carlo studies are conducted to determine the veto design which facilitates the maximum muon detection efficiency. Observed parameters are the number and positions of veto PMTs, the optical properties of the tank surfaces and the optical separation, the impact of dark counts and background on false trigger signals and potential incidents like PMT failure or aggravated water quality. Finally, it is shown that the anticipated veto design meets the requirements and rejects cosmogenic background efficiently.