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Abstract

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.