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Intrinsic 85Kr and 222Rn Backgrounds in the XENON Dark Matter Search


Lindemann,  Sebastian
Division Prof. Dr. Manfred Lindner, MPI for Nuclear Physics, Max Planck Society;

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Lindemann, S. (2013). Intrinsic 85Kr and 222Rn Backgrounds in the XENON Dark Matter Search. PhD Thesis, Ruprecht-Karls-Universität, Heidelberg.

The XENON Dark Matter search is aiming for the direct detection of Dark Matter in the form of weakly interacting massive particles (WIMPs) scattering of xenon nuclei. This process is expected to be extremely rare, if at all existing. To be detectable, competing background has to be suppressed to unprecedented level. This de_nes the framework for this thesis. First, the data analysis is sketched that resulted in todays strongest limit on the spin-independent WIMPnucleon scattering for WIMP masses above 8 GeV/c2 using an exposure of 225 live days x 34 kg collected by the Xenon100 detector. For this analysis we develop a successfully employed consistency condition rejecting non-physical background. In the main part of this thesis, we investigate the intrinsic backgrounds 85Kr and 222Rn. To be sensitive to a potential WIMP signal, ultra-low concentrations have to be achieved in the liquid xenon target. We developed a method to determine krypton traces in xenon above a detection limit of only 6 parts per quadrillion (ppq)- two orders of magnitude below previous achievements. We prove that the cryogenic fractional distillation reaches a krypton level in xenon below 1 parts per tril (ppt). This represents a crucial proof-of-principle for the needs of the upcoming Xenon1T detector. Moreover, we present a 222Rn emanation assay of the Xenon100 detector and apply our results to cast projections on the future background handling in Xenon1T. Finally, we introduce two promising realizations of radon removal systems and argue for the utmost importance of 222Rn emanation assays.