Neutron-Based Measurements in Large Neutrino Detectors and Characterization of Water-Based Liquid Scintillator

• Main Author: Callaghan, Edward Joseph
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2023
• Subjects: Astrophysics; Nuclear physics; Physics
• ISBN: 9798381704051

This dissertation presents neutron-based measurements made using the water-Cherenkov SNO detector filled with heavy water, and the scintillator-based SNO+ detector filled with liquid scintillator, as well as lab-scale R&D measurements of water-based liquid scintillator (WbLS), a candidate material for achieving hybrid Cherenkov/scintillation technology. The results are of interest for the design of future detectors, from the perspectives of both background prediction and reconstruction capabilities, as well as improving the understanding of the nucleus and its interactions and the nature of dark matter, and the cosmos at large. Neutrons produced by cosmic-ray muons in the SNO detector are characterized, and their production rate is measured to be, in units of 10^-4 cm / (g · μ), 7.28 +/- 0.09 (stat.) +1.59/-1.12 (syst.) and 7.30 +/- 0.07 (stat.) +1.40/-1.02 (syst.) in pure deuterium and deuterium loaded with NaCl at 0.02% by weight, respectively. A comparison of high-level observables in the accumulated data and a set of GEANT4-based simulations reveals generally accurate modeling of the production and transport physics in heavy water, but may indicate shortcomings in high-energy interactions with sodium and chlorine nuclei, which warrants further investigation. A preliminary search for extraterrestrial antineutrinos with SNO+, which would manifest as an excess of tens-of-MeV inverse beta-decay (IBD) events and be indicative of new astrophysical phenomena, potentially shedding light on the nature of dark matter, has yielded no significant signal. Statistical analysis in a Bayesian framework has produced a 90% credible limit on the flux of astrophysical antineutrinos of approximately 10^3 cm^-2 s^-1 MeV^-1, further decreasing as a function energy. Sensitivity projections for an updated analysis on 5 years worth of data indicate that SNO+ would achieve limits comparable to the current world-leading limits, which would be further improved by extensions to reconstruction techniques to adapt to a higher-energy regime. Also presented is a characterization of the response of both WbLS and LAB+PPO, a conventional scintillator cocktail, to MeV-scale protons via a broad-spectrum neutron beam, and electrons and alpha particles via radioactive sources. The results of this characterization are relevant for measurements made in a low-energy regime, including antineutrinos from nuclear reactors and solar neutrinos, and demonstrate timing-based Cherenkov/scintillation discrimination in electron interactions at the benchtop scale.