Analysis of the Po-210 Alpha-Decay Peak to Support the Search for Neutrinoless Double Beta Decay

Neutrinos are among the most mysterious particles in physics, and one of the most important open questions is whether they are their own antiparticles. The discovery of a process called neutrinoless double beta decay would answer this question and provide direct insight into the origin of neutrino mass. The SNO+ experiment is designed to search for this process by observing rare signals in a large volume of liquid scintillator deep underground. For such a search to be successful, the detector’s energy calibration must be extremely precise, since even small instabilities can obscure or mimic the rare signals of interest. One key calibration point comes from the natural alpha decay of polonium-210 (Po-210), which produces a peak in the energy spectrum. By carefully studying this peak under different conditions, we can test the stability of the detector response and improve background modelling. In my work, I analyzed data from the scintillator phase of SNO+, focusing on how consistently the Po-210 peak could be reconstructed. Candidate neutrino interactions known as inverse beta decays were used as reference points, and distance cuts of 2.5 m around events were applied to probe spatial uniformity across the detector. The resulting energy distributions were then fit with Gaussian functions to extract the peak position, width, and uncertainties under varying selections. I found that the Po-210 peak was consistently reconstructed at 0.455 +/- 0.005 MeV across different regions and data-taking periods, confirming the stability of the analysis method. At later times, however, the peak shifted toward higher energy (~0.75 MeV), suggesting potential changes in the detector environment following the bis-MSB fill period. This work demonstrates a reliable framework for monitoring natural radioactivity in SNO+. Analysis of the Po-210 peak directly strengthens the sensitivity of the experiment to neutrinoless double beta decay, helping SNO+ probe fundamental questions in particle physics.