Cosmic muon-induced neutrons in the SNO+ water phase

PhD ThesisThe SNO+ experiment is the successor to the Nobel prize winning SNO experiment. SNO+ will ultimately search for neutrinoless double beta decay in 130Te. 1.3 tonnes of 130Te will be dissolved into 780 tonnes of organic liquid scintillator (LAB). 9300 photo-multiplier tubes (PMTs) will observe the loaded scintillator held in an acrylic vessel of 6m radius. This final stage of operation is proceeded by a number of preparation periods, during which other physics may be probed. This thesis reports the status of a physics measurement in the first phase of operation with a detector filled with ultra-pure water. The work aims to measure the production of cosmic muon induced neutrons that spallate in water and capture on hydrogen releasing a 2.2MeVgamma ray. This process presents as a large background in many weak signal searches, such as dark matter searches, knowledge of the induced neutron production rate will inform both theoretical and experimental considerations. All components of the analysis developed to date are detailed. A low level PMT anomaly detection system is motivated and implemented, it is shown to remove high noise and inactive PMTs from data, applicable to general SNO+ analyses but especially important for low energy events. Muon event reconstruction is implemented, resulting in a track length error of ≤5% for tracks lengths > 10 m. A muon selection focused on a purity is detailed, producing a sample of muon candidates consistent with the rate observed in SNO. A neutron candidate selection with a purity of ≈99% is also defined. After applying both selections to a custom data processing, the neutron capture time is measured to be 134±16 μs which is inconsistent with the expected capture time of 206.03±0.44 μs as measured by an 241Am9Be calibration source in SNO+. This inconsistency is shown to be a result of electronic instability at short times after a cosmic muon event. This electronic instability is a previously unknown effect, leaveing the analysis incomplete and requiring future work