HALO at SNOLAB

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The Helium and Lead Observatory (HALO) is a supernova neutrino detector currently under construction at SNOLAB. HALO is a long term, low cost, high livetime, and low maintenance, dedicated supernova detector. A core-collapse supernova is powered by the energy liberated in the gravitational collapse of the core of a massive star. The core is transformed to a hot proto-neutron star that rapidly cools by the emission of neutrinos of all flavours. In fact, about 99% of the total energy is released in the form of ~ 10 MeV neutrinos, in some tens of seconds. The direct measurement of the time evolution of the luminosity, flavour partition, and average neutrino energies are our best experimental window into the challenging field of supernova dynamics which is expected to lead to an improved understanding of supernovae and of fundamental neutrino properties. Lead is particularly interesting, as a medium for supernova detection, due to its low cost, high neutrino cross-section and low neutron absorption cross-section. Neutrino interactions in lead expel neutrons from the lead nuclei making a burst of detected neutrons the signature for the detection of a supernova. Conceptually HALO is a volume of lead instrumented with neutron detectors and shielded from environmental neutrons by water. While other supernova detectors are primarily sensitive to electron antineutrinos the nuclear physics of lead makes HALO dominantly sensitive to electron neutrinos through the charged current interaction and to all flavours of neutrinos through the neutral current interaction. These complementary sensitivities ensure that supernova data from HALO will be vital to unravelling the supernova neutrino signal and extracting physics from the next galactic supernova. The public investment required for the construction of HALO was kept modest by having access to 79 tonnes of lead, from the decommissioned Deep River Cosmic Ray Station, and more than 600 meters of He-3 neutron detectors from the final phase of the SNO experiment. These assets, valued at approximately seven million dollars, have been employed in the construction of HALO in SNOLAB.