Speaker
Description
The SNO+ collaboration has developed a likelihood-based method to identify and tag previously inaccessible backgrounds arising from the $^{238}\mathrm{U}$ and $^{232}\mathrm{Th}$ decay chains. The method combines the time and spatial separations between candidate events with energy PDFs to evaluate correlations within radioactive decay sequences. In contrast to the sub-millisecond half-lives associated with traditional BiPo coincidence tagging, this technique enables, for the first time in SNO+, the identification of the triple-$\alpha$ sequence $^{224}\mathrm{Ra} \to {}^{220}\mathrm{Rn} \to {}^{216}\mathrm{Po}$ in the $^{232}\mathrm{Th}$ chain, as well as tagging the $^{212}\mathrm{Bi}$ $\alpha$ decay preceding the $^{208}\mathrm{Tl}$ background.
The tagging of $^{212}\mathrm{Bi}$ prior to the $^{208}\mathrm{Tl}$ decay provides a powerful tool for reducing one of the dominant backgrounds in measurements of low-energy $^{8}\mathrm{B}$ solar neutrinos. The correlations observed across different stages of the $^{232}\mathrm{Th}$ decay chain also provide an in-situ measurement of the $^{212}\mathrm{Pb}$ half-life and offer evidence for bulk scintillator motion and drift. In addition, delayed coincidence tagging techniques have been extended to isotopes above the $^{214}\mathrm{BiPo}$ coincidence in the $^{238}\mathrm{U}$ chain. SNO+ has also characterised the triple-$\alpha$ sequence $^{223}\mathrm{Ra} \to {}^{219}\mathrm{Rn} \to {}^{215}\mathrm{Po}$ in the $^{235}\mathrm{U}$ chain, providing a complementary demonstration of $\alpha$ tagging across all three naturally occurring radioactive series.
These results demonstrate the potential of extended delayed-coincidence techniques for decay-chain reconstruction, improved background characterisation, and radiopurity studies in large liquid scintillator detectors.