Pub Date : 2010-05-01DOI: 10.1142/9781786347725_0006
Peter Bishop
This thesis has explored the use of PMT hit patterns in time and space for reconstruction and particle identification in liquid scintillator, as well as the application of Bayesian methods to 0νββ signal extraction. Using Hamiltonian Markov Chain Monte Carlo, two dimensional fits in event energy and radius were employed, to predict a SNO+ 0νββ half-life sensitivity of T ββ 1/2 > 1.76× 1026 yr, at 90% confidence, after a three year live-time. β±γ events produced by radioactive decay in the scintillator were shown to have non-point-like timing distributions, produced by the multi-site deposition of Compton scattering γ and the time delay caused by ortho-positronium formation. This characteristic signature was used to differentiate internal backgrounds from pointlike 0νββ events using pulse shape discrimination (PSD) parameters. In particular, a PSD parameter, designed to separate 0νββ from poorly constrained 60Co decay, was used to improve the 3σ mββ discovery level from 191meV, which is already ruled out by Kamland-Zen, to 90.5meV, which is allowed by all experiments. Similarly, 40–60% rejection of each of the dominant external backgrounds inside r < 4.2mwas demonstrated using PSD,without significant sacrifice. For these backgrounds, improved rejection was achieved by accounting for the timing correlations and the angular hit distribution of the external backgrounds. Finally, using a simulationof a next-generation slow-scintillator detector, equipped with a high coverage of high quantum efficiency, fast PMTs, it was shown that the angular distribution of Cherenkov light and the timing distribution of scintillation light can be used to reconstruct the position, time and direction of electrons. This information can be used to reject the 8B elastic scattering background, improving the mββ sensitivity of a 8B dominated experiment by 50%, and, in principle, to determine the underlying mechanism of 0νββ.
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