Design and construction of the Helium and Lead Observatory for supernova neutrinos.

摘要

The Helium and Lead Observatory (HALO) is a dedicated supernova neutrino detector under construction at SNOLAB in Sudbury, Ontario. HALO will utilize 76 tonnes of lead blocks in order to take advantage of the high neutrino cross-section and low neutron absorption cross-section of lead. Charged and neutral current neutrino interactions in lead expel neutrons from the lead nuclei making a burst of detected neutrons the signature for the detection of a supernova.;This thesis also focuses on minimizing lead contamination in the SNOLAB facilities, a class 2000 clean room. Lead is a toxic metal that can have harmful effects on almost all body systems. Lead carbonate can become suspended in air as fine particles through handling. Not only does this represent an unacceptable contaminant, the presence of lead represents a health hazard. In order to mitigate this hazard the HALO lead blocks were painted. A program to determine the optimal paint and application method was performed. It was determined that Tremclad Rust Paint in green had the optimal properties for this application.;Finally, Monte Carlo studies were performed to optimize the design of the HALO experiment and determine its sensitivity for the detection of neutrinos from galactic supernovae. Detailed studies of material selection and geometric optimization demonstrated that graphite is the optimum material to act as a reflector. The addition of a 15 cm graphite reflector on five sides of the lead array with water and polypropylene shielding improves the neutron detection efficiency by ∼21.5%. The detection efficiency was also optimized by selecting 3He proportional counter supports that can also act as moderators within the experiment. This simulation set demonstrated that for a shielded HALO detector with a graphite reflector the highest feasible efficiency of detected neutrons occured for a moderator geometry of four 3.0" HDPE tubes with 1.02 cm wall thickness.;Monte Carlo simulations were also performed in order to determine the expected neutron capture rate in HALO due to background neutrons. Studies to optimize the geometry of the shielding indicated that 1 ft3 water boxes surrounding five sides and an 0.20 m thick polypropylene base layer reduced the detection of external neutrons from 2286 neutrons/day to 649 neutrons/day. Furthermore, the effect of gamma radiation from radioactive decay of the paint coating on the HALO blocks was shown to give an energy below the neutron energy window.;The focus of this thesis is three fold. The primary purpose is to determine the secondary creep characteristics of the lead blocks in HALO. The goal is to provide input to the engineering design of the lead array and its supporting superstructure. The secondary creep rate was established for four lead blocks at varying loads. The creep behaviour of lead was extrapolated beyond the test times in order to predict the behaviour over ten years. The predicted creep behaviour demonstrated that several layers in the HALO lead array required structural reinforcement in order to protect the structural integrity of the experiment and the sensitive equipment within the bores of lead. In order to mitigate the creep process steel support rings have been inserted in the bore of each block in layers 1-5.