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1. $$^{222}$$Rn  emanation measurements for the XENON1T experiment

   https://doi.org/10.1140/epjc/s10052-020-08777-z  

   XENON Collaboration; Aprile, E.; Aalbers, J.; Agostini, F.; Alfonsi, M.; Althueser, L.; Amaro, F. D.; Antochi, V. C.; Angelino, E.; Angevaare, J. R.; et al (April 2021, The European Physical Journal C)

   Abstract The selection of low-radioactive construction materials is of utmost importance for the success of low-energy rare event search experiments. Besides radioactive contaminants in the bulk, the emanation of radioactive radon atoms from material surfaces attains increasing relevance in the effort to further reduce the background of such experiments. In this work, we present the$$^{222}$$ ${}^{222}$Rn emanation measurements performed for the XENON1T dark matter experiment. Together with the bulk impurity screening campaign, the results enabled us to select the radio-purest construction materials, targeting a$$^{222}$$ ${}^{222}$Rn activity concentration of$$10\,\mathrm{\,}\upmu \mathrm{Bq}/\mathrm{kg}$$ $10\mu \mathrm{Bq}/\mathrm{kg}$in$$3.2\,\mathrm{t}$$ $3.2t$of xenon. The knowledge of the distribution of the$$^{222}$$ ${}^{222}$Rn sources allowed us to selectively eliminate problematic components in the course of the experiment. The predictions from the emanation measurements were compared to data of the$$^{222}$$ ${}^{222}$Rn activity concentration in XENON1T. The final$$^{222}$$ ${}^{222}$Rn activity concentration of$$(4.5\pm 0.1)\,\mathrm{\,}\upmu \mathrm{Bq}/\mathrm{kg}$$ $(4.5\pm 0.1)\mu \mathrm{Bq}/\mathrm{kg}$in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment. 

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2. Radon-222 and Radium-226 seawater and ice data from Bering Sea and Chukchi Sea 2021

   https://doi.org/10.18739/A23X83N3N  

   Stephens, Mark (January 2023, Arctic Data Center)

   Mercury (Hg) concentrations and speciation within surface waters of the Arctic Ocean are controlled by a complex set of processes including photochemical and microbial transformations, redox reactions, and air-sea exchange of gaseous and particulate Hg species. In this study, our aim was to estimate the magnitude of volatile Hg fluxes across the air-sea interface, and examine the influence of ice cover on this process. While gas exchange in the open ocean has been modeled as a function of wind speed, the parameterization is problematic in the presence of sea ice, which can physically block gas exchange, as well as reduce fetch and dampen waves. By using measurements of Radon-222 (Rn-222) gas and it parent isotope, Radium-226 (Ra-226), to accurately measure gas exchange velocities (k), the relative impacts of chemical and biological processes on mercury distributions within the surface waters can then be deduced. This dataset contains Radon-222 and Radium-226 activity concentrations from R/V Sikuliaq cruise SKQ202108S in the Bering Sea, through the Bering Strait, and in shelf waters of the Chukchi Sea during May – June 2021. Samples include seawater (16 water column profiles), as well as ice cores and brine from four ice stations. At the time of the cruise, sampling locations in the Bering Sea were ice free and gas transfer velocities (k) estimated from Rn-222 deficits (with respect to Ra-226 concentrations) were in general agreement with published parameterizations of k as a function of wind speed. The springtime retreating ice edge was located at 69-70 degrees north latitude in the Chukchi Sea, and sampling locations there were located along the ice edge, in areas of open water, and at sites within the pack ice up to ~10 kilometers (km) from the ice edge. Gas transfer velocities in the marginal ice zone also reflected recent wind histories, with k values generally at the high end of or exceeding those predicted from the wind speed parameterizations. 

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3. Observation of radon mitigation in MicroBooNE by a liquid argon filtration system

   https://doi.org/10.1088/1748-0221/17/11/P11022  

   Abratenko, P.; Anthony, J.; Arellano, L.; Asaadi, J.; Ashkenazi, A.; Balasubramanian, S.; Baller, B.; Barnes, C.; Barr, G.; Barrow, J.; et al (November 2022, Journal of Instrumentation)

   Abstract The MicroBooNE liquid argon time projection chamber (LArTPC) maintains a high level of liquid argon purity through the use of a filtration system that removes electronegative contaminants in continuously-circulated liquid, recondensed boil off, and externally supplied argon gas. We use the MicroBooNE LArTPC to reconstruct MeV-scale radiological decays. Using this technique we measure the liquid argon filtration system's efficacy at removing radon. This is studied by placing a 500 kBq 222 Rn source upstream of the filters and searching for a time-dependent increase in the number of radiological decays in the LArTPC. In the context of two models for radon mitigation via a liquid argon filtration system, a slowing mechanism and a trapping mechanism, MicroBooNE data supports a radon reduction factor of greater than 97% or 99.999%, respectively. Furthermore, a radiological survey of the filters found that the copper-based filter material was the primary medium that removed the 222 Rn. This is the first observation of radon mitigation in liquid argon with a large-scale copper-based filter and could offer a radon mitigation solution for future large LArTPCs. 

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4. The XENONnT dark matter experiment

   https://doi.org/10.1140/epjc/s10052-024-12982-5  

   XENON_Collaboration; Aprile, E.; Aalbers, J.; Abe, K.; Ahmed_Maouloud, S.; Althueser, L.; Andrieu, B.; Angelino, E.; Angevaare, J_R; Antochi, V_C; et al (August 2024, The European Physical Journal C)

   Abstract The multi-staged XENON program at INFN Laboratori Nazionali del Gran Sasso aims to detect dark matter with two-phase liquid xenon time projection chambers of increasing size and sensitivity. The XENONnT experiment is the latest detector in the program, planned to be an upgrade of its predecessor XENON1T. It features an active target of 5.9 tonnes of cryogenic liquid xenon (8.5 tonnes total mass in cryostat). The experiment is expected to extend the sensitivity to WIMP dark matter by more than an order of magnitude compared to XENON1T, thanks to the larger active mass and the significantly reduced background, improved by novel systems such as a radon removal plant and a neutron veto. This article describes the XENONnT experiment and its sub-systems in detail and reports on the detector performance during the first science run. 

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5. Offline tagging of radon-induced backgrounds in XENON1T and applicability to other liquid xenon time projection chambers

   https://doi.org/10.1103/PhysRevD.110.012011  

   Aprile, E; Aalbers, J; Abe, K; Ahmed_Maouloud, S; Althueser, L; Andrieu, B; Angelino, E; Angevaare, J R; Antón_Martin, D; Arneodo, F; et al (July 2024, Physical Review D)

   This paper details the first application of a software tagging algorithm to reduce radon-induced backgrounds in liquid noble element time projection chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T was mapped out using ${}^{222}\mathrm{Rn}$and ${}^{218}\mathrm{Po}$events, and the rms convection speed was measured to be $0.30\pm 0.01\mathrm{cm}/\mathrm{s}$. Given this velocity field, ${}^{214}\mathrm{Pb}$background events can be tagged when they are followed by ${}^{214}\mathrm{Bi}$and ${}^{214}\mathrm{Po}$decays, or preceded by ${}^{218}\mathrm{Po}$decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for ${}^{214}\mathrm{Bi}$and ${}^{214}\mathrm{Po}$decays or ${}^{218}\mathrm{Po}$decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a ${}^{214}\mathrm{Pb}$background reduction of ${6.2}_{-0.9}^{+0.4}\%$with an exposure loss of $1.8\pm 0.2\%$, despite the timescales of convection being smaller than the relevant decay times. We show that the performance can be improved in XENONnT, and that the performance of such a software-tagging approach can be expected to be further improved in a diffusion-limited scenario. Finally, a similar method might be useful to tag the cosmogenic ${}^{137}\mathrm{Xe}$background, which is relevant to the search for neutrinoless double-beta decay. Published by the American Physical Society2024 

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