Study of Neutron Reflector for the HALO-1kT Supernovae Neutrino Detector

Patel, Divyaben Ashvinkumar
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Faculty of Graduate Studies and Research, University of Regina

The Helium and Lead Observatory 1 Kilotonne (HALO-1kT) is a lead-based detector to study electron neutrinos emitted from core-collapse supernovae. It is proposed to follow the same purpose as the HALO detector located at SNOLAB but with higher detection efficiency. The sensitivity to electron neutrinos makes HALO-1kT (and also the current HALO detector) unique in the sense that all other detectors with capability to detect supernova neutrinos are sensitive to anti-electron neutrinos through charged- current inverse beta-decay such as the Super-Kamiokande, LVD, IceCube and KamLAND. HALO-1kT's sensitivity to supernova neutrinos is larger than that for HALO due to its proposed 12-fold target-mass increase relative to HALO and a more e cient neutron detection. The detector will consist of 1 kT of lead. Neutrinos from a supernova will interact with the lead via inverse beta-decay process producing bismuth or lead in highly-excited states (the excitation states depend on the incoming neutrino flavour). The daughter nuclei emit neutrons during de-excitation, which are then detected by 3He proportional counters. The layer of the detector immediately following the lead volume consists of a graphite reflector to recover neutrons that would otherwise escape the detector fiducial volume. The main goal of this thesis is to develop simulation studies for the design of a graphite layer which will serve as a neutron moderator and reflector, redirecting escaping neutrons back into the detector lead volume. The reflector layer will increase the detection efficiency by up to 50% relative to the 28% efficiency achieved in HALO. Geant4 simulations have been used to assess the optimal thickness and grade of the graphite to be used. It was found that a graphite material of 15 cm thickness is the favourable choice for a neutron reflector. As for the graphite grade, it should, ideally, be nuclear-reactor quality. However, costs involved in the acquisition of such high grade material should be considered. It was found that a < 1.0 ppm concentration level of boron in the graphite layer is an acceptable compromise between cost and effectiveness.

A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Science in Physics, University of Regina. xiv, 75 p.