222 Rn emanation measurements for the XENON1T experiment

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 experi- ments. In this work, we present the 222 Rn emanation mea- surements performed for the XENON1T dark matter exper- iment. Together with the bulk impurity screening campaign, the results enabled us to select the radio-purest construc- tion materials, targeting a 222 Rn activity concentration of 10 μBq/kg in 3.2 t of xenon. The knowledge of the distri- bution of the 222 Rn sources allowed us to selectively elim- inate problematic components in the course of the exper- iment. 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 (4.5 ± 0.1) μBq/kg in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment.