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The XENON1T experiment at the Laboratori Nazionali del Gran Sasso (LNGS) is the first WIMP dark matter detector operating with a liquid xenon target mass above the ton-scale. Out of its 3.2 t liquid xenon inventory, 2.0 t constitute the active target of the dual-phase time projection chamber. The scintillation and ionization signals from particle interactions are detected with low-background photomultipliers. This article describes the XENON1T instrument and its subsystems as well as strategies to achieve an unprecedented low background level. First results on the detector response and the performance of the subsystems are also presented.

The XENON1T experiment aims for the direct detection of dark matter in a detector filled with 3.3 tons of liquid xenon. In order to achieve the desired sensitivity, the background induced by radioactive decays inside the detector has to be sufficiently low. One major contributor is the β -emitter 85 Kr which is present in the xenon. For XENON1T a concentration of natural krypton in xenon nat Kr / Xe < 200 ppq   (parts per quadrillion,  1 ppq = 10 - 15 mol / mol ) is required. In this work, the design, construction and test of a novel cryogenic distillation column using the common McCabe–Thiele approach is described. The system demonstrated a krypton reduction factor of 6.4 · 10 5 with thermodynamic stability at process speeds above 3 kg/h. The resulting concentration of nat Kr / Xe < 26 ppq is the lowest ever achieved, almost one order of magnitude below the requirements for XENON1T and even sufficient for future dark matter experiments using liquid xenon, such as XENONnT and DARWIN.

The XENON1T dark matter experiment aims to detect weakly interacting massive particles (WIMPs) through low-energy interactions with xenon atoms. To detect such a rare event necessitates the use of radiopure materials to minimize the number of background events within the expected WIMP signal region. In this paper we report the results of an extensive material radioassay campaign for the XENON1T experiment. Using gamma-ray spectroscopy and mass spectrometry techniques, systematic measurements of trace radioactive impurities in over one hundred samples within a wide range of materials were performed. The measured activities allowed for stringent selection and placement of materials during the detector construction phase and provided the input for XENON1T detection sensitivity estimates through Monte Carlo simulations.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant 222Rn background originating from radon emanation. After inserting an auxiliary 222Rn emanation source in the gas loop, we determined a radon reduction factor of R>27 (95% C.L.) for the distillation column by monitoring the 222Rn activity concentration inside the XENON100 detector.

Abstract We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant²²²222 Rn background originating from radon emanation. After inserting an auxiliary²²²222 Rn emanation source in the gas loop, we determined a radon reduction factor ofR > 27R > 27 (95% C.L.) for the distillation column by monitoring the²²²222 Rn activity concentration inside the XENON100 detector.

We report on a search for Weakly Interacting Massive Particles (WIMPs) using 278.8 days of data collected with the XENON1T experiment at LNGS. XENON1T utilizes a liquid xenon time projection chamber with a fiducial mass of \\((1.30 \\pm 0.01)\\) t, resulting in a 1.0 t\\(\\times\\)yr exposure. The energy region of interest, [1.4, 10.6] \\(\\mathrm{keV_{ee}}\\) ([4.9, 40.9] \\(\\mathrm{keV_{nr}}\\)), exhibits an ultra-low electron recoil background rate of \\((82\\substack{+5 \\\ -3}\\textrm{ (sys)}\\pm3\\textrm{ (stat)})\\) events/\\((\\mathrm{t}\\times\\mathrm{yr}\\times\\mathrm{keV_{ee}})\\). No significant excess over background is found and a profile likelihood analysis parameterized in spatial and energy dimensions excludes new parameter space for the WIMP-nucleon spin-independent elastic scatter cross-section for WIMP masses above 6 GeV/c\\({}^2\\), with a minimum of \\(4.1\\times10^{-47}\\) cm\\(^2\\) at 30 GeV/c\\({}^2\\) and 90% confidence level.

Two-neutrino double electron capture (\\(2\\nu\\)ECEC) is a second-order Weak process with predicted half-lives that surpass the age of the Universe by many orders of magnitude. Until now, indications for \\(2\\nu\\)ECEC decays have only been seen for two isotopes, \\(^{78}\\)Kr and \\(^{130}\\)Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance. The \\(2\\nu\\)ECEC half-life provides an important input for nuclear structure models and its measurement represents a first step in the search for the neutrinoless double electron capture processes (\\(0\\nu\\)ECEC). A detection of the latter would have implications for the nature of the neutrino and give access to the absolute neutrino mass. Here we report on the first direct observation of \\(2\\nu\\)ECEC in \\(^{124}\\)Xe with the XENON1T Dark Matter detector. The significance of the signal is \\(4.4\\sigma\\) and the corresponding half-life \\(T_{1/2}^{2\\nu\\text{ECEC}} = (1.8\\pm 0.5_\\text{stat}\\pm 0.1_\\text{sys})\\times 10^{22}\\;\\text{y}\\) is the longest ever measured directly. This study demonstrates that the low background and large target mass of xenon-based Dark Matter detectors make them well suited to measuring other rare processes as well, and it highlights the broad physics reach for even larger next-generation experiments.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column is integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant \\(^{222}\\)Rn background originating from radon emanation. After inserting an auxiliary \\(^{222}\\)Rn emanation source in the gas loop, we determined a radon reduction factor of R > 27 (95% C.L.) for the distillation column by monitoring the \\(^{222}\\)Rn activity concentration inside the XENON100 detector.