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Radon Induced Background in the XENON1T Dark Matter Search Experiment and Studies on Xenon Purity in the HeXe System
The XENON1T experiment is aiming for the direct detection of dark matter in the form of weakly interacting massive particles (WIMPs) scattering off xenon nuclei. Detecting these rarely-interacting particles requires an unprecedented low background level. In XENON1T, the noble gas 222Rn induces the dominant background, either as electronic recoil (ER) background admixed in the liquid xenon (LXe) volume or as surface background on the Time- Projection-Chamber’s (TPC) wall. In the first part of this thesis, we constrain the amount of 222Rn induced ER background during the complete run-time of the experiment. We furthermore survey the considerable 222Rn reduction during the XENON1T operation, resulting in a final activity concentration of (4.5 ± 0.1) μBq/kg, the lowest one ever achieved in a LXe dark matter experiment. Conclusions drawn are relevant not only for the interpretation of the background level in XENON1T, but also for the next-generation experiment XENONnT. The 222Rn induced surface background can be removed from the PTFE (Polytetrafluoroethylene) wall of the TPC by dedicated surface cleaning methods based on 32% HNO3. In the second part of this work, we constructed a small-scale LXe TPC, the HeXe set-up, to test for the first time if a PTFE surface cleaning based on 32 % HNO3 degrades the xenon purity and the performance of a TPC. No degradation of the xenon purity is observed within the system’s sensitivity, indicating that 32 % HNO3 is a promising candidate for PTFE surface cleaning in LXe TPCs. Our result lays the groundwork for future research towards confirming that the treatment is applicable in large-scale LXe detectors.
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