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}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}Rn activity concentration of 10,mathrm{,}upmu mathrm{Bq}/mathrm{kg} in 3.2,mathrm{t} of xenon. The knowledge of the distribution of the ^{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}Rn activity concentration in XENON1T. The final ^{222}Rn activity concentration of (4.5pm 0.1),mathrm{,}upmu mathrm{Bq}/mathrm{kg} in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment.
Highlights
IntroductionXENON1T operated for two years, starting from December 2016. to other astroparticle physics experiment looking for rare events, it required an extremely low background level
Many cosmological observations suggest that a large fraction of the total matter density of the Universe is made up of an unknown form of dark matter [1]
The background rate of current xenon dark matter detectors is dominated by 222Rn-induced events and it is expected that
Summary
XENON1T operated for two years, starting from December 2016. to other astroparticle physics experiment looking for rare events, it required an extremely low background level. Throughout the different generations of the XENON experiments, external background sources have been suppressed, e.g. by an improved external shield and xenon self-shielding and by the mitigation of radioactivity from materials Their level was marginal in XENON1T and intrinsic background sources became dominant. Other radon isotopes may lead to background events Their contribution was strongly suppressed due to their small abundance in the detector and much shorter half-lives, that did not allow for their dispersion within the target volume. The measured 226Ra bulk activity can in general not be used to predict how much 222Rn emanates from the material, because surface impurities may become dominant. This made dedicated 222Rn emanation measurements necessary, which are described in this article.
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