Directional reconstruction in liquid scintillator neutrino detectors

Thesis

Direction reconstruction can be used as a powerful background rejection tool in neutrino experiments, notably for the isolation of highly directional sources such as the Sun. While the high light-yield of scintillator detectors leads to precise energy resolution suitable for low energy studies, the light emission is isotropic with no directional information. However, directionality might be provided by Cherenkov light, if it could be sufficiently isolated from the overwhelming scintillation signal.

The SNO+ detector is a large multipurpose neutrino detector, housed 2 km underground at SNOLAB in Sudbury, Ontario. During commissioning, the target medium of the detector was gradually changed from ultra-pure water to liquid scintillator, creating an interface between the two materials. The concentration of the primary fluor (PPO) was gradually increased as the detector was filled. There were two stable periods of data acquisition during commissioning where the concentration of PPO was 0.6 g/L, lower than the intended target of 2.2 g/L. This lower concentration created a slower scintillation profile. As time-based separation of Cherenkov light had already been demonstrated in slow scintillators on a bench-top scale, these data sets were examined for directional information. This thesis presents the first demonstration of event-by-event direction reconstruction in a high-yield large-scale liquid scintillator detector using data from these periods, with a significance of > 5.7σ.

Future prospects of directionality in scintillator will benefit from multiple Cherenkov separation techniques. However, simulated studies of 0.6g/L PPO in the solvent LAB show that significant improvements can be made only by using modern photon-detection capabilities, without reducing the detected light yield. By prioritising improvements to photon-sampling, the next generation of liquid scintillator neutrino detectors may be able to effectively reject directional backgrounds at low energies, improving the sensitivities of studies such as reactor anti-neutrinos, CNO solar neutrinos, and the search for neutrinoless double beta decay.

Authors: Paton, J

• DOI: 10.5287/ora-kzbemygg5
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: hybrid detectors; neutrino detectors; neutrino; reconstruction; Cherenkov; solar neutrino; directionality; scintillation
• Subjects: Solar neutrinos; Liquid scintillators; Neutrino Detectors