The SNO+ liquid scintillator response to low-energy electrons and its effect on the experiment’s sensitivity to a future neutrinoless double beta decay signal

Abstract

The SNO+ experiment is set to join the international competition of experiments searching for neutrinoless double beta decay. By loading 780 t of liquid scintillator with 0.5% natural tellurium, and with its location 2 km underground at SNOLAB, SNO+ aims to have sensitivity to determining the Majorana nature of the neutrino, a question currently at the forefront of particle physics, approaching the inverted hierarchy of the neutrino masses. To reach this sensitivity, it is crucial that SNO+ understands the response of the liquid scintillator, as systematic uncertainties on the energy scale and resolution, in particular any non-Gaussian shape of the energy resolution, may diminish the experiment’s sensitivity in a significant way. A 60Co calibration source that tags calibration events within the liquid scintillator will enable SNO+ to precisely study the shape of the energy resolution near the endpoint of the 130Te double beta decay. Monte Carlo simulations of the calibration source predict it will measure a 3.24% energy resolution at an energy of 2.51 MeV. Because 60Co emits two gamma-rays upon decaying, whereas the expected signal of neutrinoless double beta decay is the sum of two electrons, it is also crucial for SNO+ to understand how the response of the liquid scintillator depends on particle type and energy. This dissertation provides the first measurement of the SNO+ tellurium-loaded liquid scintillator response to low-energy electrons. Ionization quenching of low-energy electrons in the tellurium-loaded liquid scintillator is small, with Birks’ constant measured to be kB = (4.1 ± 2.9) × 10−6 cm/keV.