The XENON1T water Cherenkov muon veto system and commissioning of the XENON1T Dark Matter experiment
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Abstract
There is strong evidence that roughly a quarter of the energy density content of the universe consists of dark matter. The XENON1T experiment is the most recent stage of the XENON Dark Matter Project, aiming for the direct detection of dark matter in the form of Weakly Interacting Massive Particles (WIMPs). Its sensitivity for the spin-independent WIMP-nucleon elastic scattering cross-section is σ = 7.7 · 10−47 cm2 for a WIMP mass of mχ = 35 GeV/c2. The projected sensitivity for a full 2 t·y (ton-years) exposure is σ ≈ 2 · 10−47 cm2. The XENON1T detector consists of a liquid xenon time projection chamber (TPC) and is sensitive to nuclear recoils of WIMPs scattering off the xenon atoms. A water Cherenkov muon veto surrounds the XENON1T TPC to shield external backgrounds and to tag muon-induced neutrons by detection of a transiting muon or the secondary shower induced by a muon interacting in the surrounding rock. The muon veto is instrumented with 84 8" photomultiplier tubes (PMTs) with high quantum efficiency (QE) in the Cherenkov regime. The walls of the water tank are clad with the highly reflective DF2000MA foil by 3M.
This thesis presents studies on the muon veto and the TPC subsystem. First, a
detailed description of the muon veto system, as well as series tests of its PMTs and development of individual and global calibration systems are presented. Further, a study of the reflective properties of the DF2000MA foil is described, together with a measurement of its wavelength shifting (WLS) properties. The impact of reflectance and WLS on the detection efficiency of the muon veto was studied using Monte Carlo simulations carried out with the Geant4 toolkit. The data analysis of the XENON1T muon veto system was initiated with this thesis providing a software to read the processed muon veto data. The possibility to identify muon tracks in the muon veto was verified as a first use case.
Contributions to the commissioning of the XENON1T TPC are presented further.
Within the framework of this thesis, capacitive liquid level meters have been developed to measure the liquid xenon level inside (four short level meters, SLM) and outside (two long level meters, LLM) of the TPC. They can provide a precision of tens of μm for SLMs and ≈ 3 mm for LLMs and are read out with a self-made PCB using UTI chips. Additionally, a study of the S2 signal width is presented, compared to different level meter readings and extrapolated to statements regarding wire grid warping.
Summarized, this thesis followed the XENON1T experiment from early design
phase (2012), via installation (2015-2016) to the end of commissioning phase (2016) with analyses of first commissioning data. The XENON1T experiment is taking science data since end of 2016.