Calibration and commissioning of the Helium and Lead Observatory

Abstract

The Helium And Lead Observatory (HALO) is a dedicated supernova detector at SNOLAB consisting of 79 tonnes of lead instrumented with 128 3He-filled neutron counters. A burst of neutrinos from a supernova will interact with the lead and result in a burst of neutrons, detectable by the counters. This burst can be identified as a supernova signal. The previously existing HALO Monte Carlo simulation was revised to better represent the detector and evaluate its supernova response. The composition of the paint used on the lead blocks was estimated using new and previous measurements. Other geometry updates were checked with neutron capture simulations to verify their implementation. To verify the detection efficiencies of the Monte Carlo simulation, a 252Cf neutron source was deployed in the 40 copper calibration tubes in the detector. The high neutron multiplicity in Cf fissions allowed for the source strength to be determined along with the neutron capture efficiency by an analysis of the relative population of the detected multiplicities. This verified the Monte Carlo simulation’s results and gave an overall efficiency for the detection of supernova-induced neutrons of (27.61 0.17)%. Backgrounds to the detection of supernovae include neutrons leaking into the detector from the lab, the spontaneous fission of uranium inside the detector, and muon-induced spallation events in and around the detector. These factors along with the false positive rates specified by the Supernova EarlyWarning System (SNEWS) limit the trigger threshold to 4 neutrons detected in two seconds, giving a detection range of about 13.7 kpc for supernovae with an average neutrino energy of 18 MeV and a pinching parameter of 2.