Background Analysis for the SNO+ Experiment

Abstract

SNO+ aims to conduct a world leading search for neutrino-less double beta decay of 130Te with a 5 year half-life sensitivity of 1.9×1026 years using 3.9 tonnes of natural Tellurium isotropically loaded in 780 tonnes of liquid scintillator. The total background budget within 0.5σ to 1.5σ of the 0νββ energy is 13.4 events per year, dominated by 8B solar neutrinos. We discuss SNO+ analysis strategies to measure the residual Uranium and Thorium chain backgrounds through the timing coincidence of short half-life components 214Bi – 214Po and 212Bi – 212Po. We show that these so-called Bi-Po events can be rejected from the 0νββ region of interest (ROI) with 99.995% efficiency and minimal (<2%) signal sacrifice. Pure samples of Bi-Pos can also be created to accurately measure the rates of Uranium and Thorium decays in-situ. In a fraction of decays, the Polonium alpha decay will occur within the same trigger window as the beta, resulting in a higher energy ‘pile-up’ event. Separate classifications based on the hit timing structure efficiency reject these events with minimal (<1%) signal loss. A final class of background events results from the pile up of higher frequency low energy backgrounds with events such as 2νββ, which would not otherwise contribute to the ROI. We present a final set of of event classifiers developed specifically to reject these events and show that their contribution to the double beta analysis can be reduced to negligible levels.