Offline tagging of radon-induced backgrounds in XENON1T and applicability to other liquid xenon detectors

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 ^222Rn and ^218Po events, and the root-mean-square convection speed was measured to be 0.30 ± 0.01 cm/s. Given this velocity field, ^214Pb background events can be tagged when they are followed by ^214Bi and ^214Po decays, or preceded by ^218Po decays. This was achieved by propagating a point cloud as directed by the velocity field, and searching for ^214Bi and ^214Po decays or ^218Po decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a ^214Pb background reduction of 6.2^+0.4_-0.9% with an exposure loss of 1.8± 0.2 %. The tagging algorithm was also used to produce a population of tagged events with a large enhancement in the ^214Pb fraction. 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 ^137Xe background, which is relevant to the search for neutrinoless double-beta decay.