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

Abstract

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 ${}^{222}\text{Rn}$ and ${}^{218}\text{Po}$ events, and the root-mean-square convection speed was measured to be $0.30\pm 0.01$ cm/s. Given this velocity field, ${}^{214}\text{Pb}$ background events can be tagged when they are followed by ${}^{214}\text{Bi}$ and ${}^{214}\text{Po}$ decays, or preceded by ${}^{218}\text{Po}$ decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for ${}^{214}\text{Bi}$ and ${}^{214}\text{Po}$ decays or ${}^{218}\text{Po}$ decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a ${}^{214}\text{Pb}$ 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 ${}^{137}\text{Xe}$ background, which is relevant to the search for neutrinoless double-beta decay.

17 pages, 19 figures