Mitigation of 222Rn induced background in the XENON1T dark matter experiment

Abstract

222Rn is a major source of background in many rare-event experiments such as the XENON1T dark matter search. The noble gas radon is created by radioactive decay inside all detector materials and emanates into the sensitive liquid xenon target disabling any detector shielding. Subsequent beta-decays of radon progenies are the dominant source of background in the XENON1T experiment. In order to mitigate radon induced background the detector's construction materials have been selected according to dedicated 222Rn emanation measurements. In the first part of this thesis, we summarize the results of the XENON1T radon screening campaign and present the measurement of the integral radon emanation rate of the fully assembled detector. The development of a radon removal system which continuously purifies the liquid xenon target from the emanated radon is the topic of the second part of this thesis. In order to demonstrate the suitability of cryogenic distillation as a technique to separate radon from xenon, we developed an experimental setup to measure the depletion of radon in xenon boil-off gas after a single distillation step. In the last part of the thesis, we demonstrate the operation of a radon removal system for the XENON100 experiment. For this first test employing a running dark matter detector, we integrated a multiple stage, cryogenic distillation column in the XENON100 gas purification loop. From the evolution of the radon concentration in XENON100, we investigate the distillation column's radon removal capability and discuss the design and application of a radon removal system for XENON1T and the upcoming XENONnT experiment.