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

Abstract

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.