Calibration of the SNO+ detector using natural radioactivity and first demonstration of multisite discrimination for solar neutrinos

Thesis

A high-precision measurement of the ⁸B solar neutrino flux in the 2.0 to 6.0 MeV transition region is a key objective in neutrino physics. This region, where the ν_e survival probabilities are highly sensitive to physics beyond the Standard Model, such as non-standard neutrino interactions or additional neutrino flavors, remains largely unexplored. However, challenges arise due to the high rates of internal ²⁰⁸Tl radioactive backgrounds in liquid scintillator detectors. This thesis introduces multisite event discrimination as an effective method to enhance the precision of ⁸B solar flux measurements in this background-dominated energy regime. A Bayesian optimization calibration method was developed using internal ²¹⁴Bi-²¹⁴Po coincidences to extract clean α and β event samples. These samples were used to build precise models of the scintillation response, leading to the creation of high-accuracy probability density functions (PDFs) for ⁸B solar flux fitting. For the first time, multisite event discrimination was applied to liquid scintillator data, yielding a 30% improvement in measurement precision when combined with energy PDF information, as compared to fits performed in energy alone.

Sensitivity projections for the upcoming SNO+ bisMSB phase, featuring improved light yields and faster timing, show a 17% increase in precision over the current full-fill phase, with position resolution enhanced to 6 cm.

While this work marks a significant step towards achieving a high-precision measurement of the ⁸B solar flux in the transition region, future analyses will integrate multisite, energy, and ²¹²Bi-²⁰⁸Tl coincidence tagging into the likelihood functions, maximizing the information extracted to further refine measurement precision.

Authors: Hunt-Stokes, R

• DOI: 10.5287/ora-dezvrkob9
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: solar; multi-site; multisite; physics; SNO+; neutrino; discrimination; neutrinos; particle
• Subjects: Physics; Neutrinos; Solar neutrinos; Neutrino interactions