Background Characterisation for the Water and Scintillator Phases of SNOPLUS

Introduction
SNO+ is a multinational collaboration working on the detection of the radioactive decay process known as the neutrinoless double beta decay. The experiment is based at the Creighton nickel mine in Sudbury, Ontario and builds upon previous work carried out at the location on the SNO detector (a comprehensive overview of the SNO+ experiment is available at [1]).
Experimental Motivations
The principal aim, of observing the neutrinoless double beta decay, is to test for physics beyond the Standard Model. If this decay process is observed, it will demonstrate that neutrinos are in fact Majorana fermions i.e. that they are their own anti-particles. This would therefore result in lepton number conservation being broken and would have wider reaching implications for mechanisms of leptogenesis.
Furthermore, if neutrinos are Majorana particles, this would enable one to identify the neutrino mass hierarchy and calculate the absolute mass of the 3 neutrino types. SNO+ may not completely constrain the Majorana neutrino mass due to the detector's sensitivity. However, it will have the ability to set an upper limit of between 55 - 133 meV on this effective mass.
SNO+ will also be able to conduct other physical studies during its operation. The detector will be sensitive to high energy solar neutrinos, supernovae neutrinos and invisible nucleon decays for the duration of the experiment. Moreover, SNO+ will be able to detect low-energy solar, reactor and geoneutrinos during specific phases.
Experimental Context
The SNO+ detector will have 3 distinct phases during its lifetime. Initially, SNO+ functioned as a water Cherenkov detector and was filled with approximately 900 tonnes of Ultra Pure Water (UPW). During the second phase, the detector will be filled with liquid scintillator (LS) making the detector sensitive to antineutrino interactions. Finally, Telluric acid will be loaded into the scintillator to begin the final phase.
Context of PhD Research
The research carried out during this PhD will predominantly focus on the characterisation and monitoring of radioactive backgrounds within the detector. Radon isotopes act as a background to several of the studies through the radioactive decays of daughter isotopes; this is of particular importance due to the high presence of radon in the mine air. Solar neutrino and nucleon decay studies are susceptible to these backgrounds (during the water phase) as well as reactor, geoneutrino and neutrinoless double beta decay studies (during the LS and Tellurium loaded stages).
Techniques to identify bismuth 214 decays will be employed to characterise radon backgrounds during the Cherenkov phase following the processing of the detector data. Furthermore, a prompt-delay discrimination technique to identify alpha-neutron reactions will be developed to supress the background signal caused by polonium 210. As the detector is filled with scintillator, this technique will be tested, enhanced and employed to analyse this background during the second and third detector phases.
Time will also be spent at the detector site to aid in advancing the detector into the LS stage. During this time, contributions will be made to preparing and assembling detector hardware. This will focus on the installation of an umbilical retrieval mechanism (for the deployment of calibration sources). Furthermore, investigations will be preformed to optimise cleaning techniques for the umbilicals that will deployed within the scintillator.
Reference
S. Andringa et al., "Current Status and Future Prospects of the SNO+ Experiment", Advances in High Energy Physics, vol. 2016, pp. 1-21, 2016. Available: 10.1155/2016/6194250 [Accessed 15/11/19].