Krypton Assay in Xenon at the ppq level for the Commissioning of the Xenon1T Dark Matter Experiment

The Xenon1T experiment aims for the detection of dark matter with liquid xenon as target material for scattering events of so-called WIMPs (Weakly Interacting Massive Particles), which represent one of the main candidates for particle dark matter. Due to the expected very low event rate of this process, ultra-low background conditions are required to reach the sensitivity necessary for WIMP detection. Among the most serious internal background contributions is the radioactive krypton isotope 85Kr, which is intrinsically present in commercially available xenon at the ppm or ppb level, and, as a β-emitter, produces signals in the detector that can mimic the searched-for WIMP signals. How-ever, krypton traces are successfully reduced from liquid xenon by cryogenic distillation. This thesis has been carried out in the context of monitoring krypton traces, using a gas-chromatographic mass spectrometry setup (RGMS) for an off-line krypton in xenon assay. We develop a novel data analysis procedure that allows to be sensitive to krypton in xenon concentrations of 10 ppq at 90 % C.L. for the RGMS conditions present during this work. We further present assay results covering three di˙erent xenon distillation campaigns. With these we verify the excellent performance of the cryogenic distil-lation column designed for and installed at Xenon1T and project the Xenon1T detector to preserve the required purity in krypton over months of operation