Measurement of External Radioactive Backgrounds in the SNO+ Water Phase

The SNO+ experiment has taken data with water and will search for neutrinoless double beta decay using 780 tonnes of tellurium loaded liquid scintillator. Because the rate of external radioactive backgrounds are not expected to change significantly between phases of SNO+, they can be measured in the water phase and used for sensitivity projections in future phases. These backgrounds are critical to the search for neutrinoless double beta decay as they are the dominant factor in the choice of fiducial volume. Two independent analyses were performed to measure the rate of the most significant external radioactive backgrounds, ²⁰⁸Tl and ²¹⁴Bi. Both analyses find these rates to be consistent with expectations.