On the measurement of optical scattering and studies of background rejection in the SNO+ detector

Thesis

SNO+ is a liquid scintillator experiment designed to study a wide range of neutrino-related physics goals, such as solar neutrinos, reactor and geo-neutrinos, and neutrinoless double beta decay. The success of the experiment depends in part on the ability to accurately characterise the detector's optical properties, and also develop effective methods to suppress contributions from unwanted backgrounds.

This thesis presents one of the central calibration systems which will be used to make measurements of the detector's optical scattering properties. The hardware and its integration into the wider detector will be discussed. A simplifed analysis of simulated water data is also presented, in order to show that the scattering properties can be accurately measured. The reconstruction of photons' scattering position, length and angles can be achieved to a high degree of accuracy, with a small reconstruction bias and resolution in comparison to the scale of the detector itself.

This thesis also presents a study into the rejection of two important radioactive backgrounds that will be encountered during the scintillator phases of SNO+: the (²¹²Bi + ²¹²Po) and (²¹⁴Bi + ²¹⁴Po) beta-alpha chains, where both components of each chain occur within a single detector trigger window, creating what is known as a BiPo pileup event. Two methods have been considered: one using a comparison between the cumulative time residual distributions of BiPo pileup and double-beta signal events, and the other using a log-likelihood difference method. Both methods perform extremely well, with the first being capable of rejecting > 95% of BiPo pileup events for a 1% loss of signal, and the second rejecting > 97% of backgrounds for the same signal loss.

Authors: Majumdar, K

• DOI: 10.5287/ora-rvy5wbxe8
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Particles (Nuclear physics); Neutrinos--Experiments