HALO at SNOLAB

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The Helium and Lead Observatory (HALO) is a long-term, low cost, high livetime, and low maintenance dedicated supernova detector running at SNOLAB. Among the world’s supernova neutrino detectors the choice of lead as the target material makes HALO uniquely sensitive to electron neutrinos and will provide complementary information on neutrino fluxes and energies from the next galactic supernova. HALO will also participate in the SuperNova Early Warning System (SNEWS) increasing the odds that the astronomical community is promptly alerted to a galactic supernova. A type II supernova is powered by the energy liberated in the gravitational collapse of the core of a massive star. The initially very hot proto-neutron star cools primarily by the emission of neutrinos, of all flavours equally partitioned, in some tens of seconds. In fact, about 99% of the gravitational potential energy is released in a sharp burst of ~10MeV neutrinos. Conceptually, HALO is an 80 tonne mass of lead instrumented with Helium-3 neutron detectors. In HALO the energetic and intense pulse of neutrinos from a supernova can excite, via both charged current (CC) and neutral current (NC) weak interactions, the lead nuclei to states from which one or two neutrons are ejected. The subsequent detection of these neutrons in HALO’s Helium-3 neutron detectors signals a galactic supernova. The direct measurement of the time evolution of the luminosity, flavour partition, and average neutrino energy are our only direct window into supernova dynamics though a gravitational wave is also anticipated. The nuclear physics of lead provides for a dominant sensitivity to CC interactions of the electron neutrino flux; complete insensitivity to CC interactions of the electron anti-neutrino flux; and minor sensitivity to all flavours through NC excitation channels. A detailed understanding of supernova dynamics is at the forefront of large-scale computational physics efforts and, when combined with observational data, offers the possibility of extracting fundamental neutrino properties as well as advancing our understanding of the supernova mechanism, heavy element nucleosynthesis, and other astrophysical questions. Supernova neutrino fluxes and interaction cross-sections are such that detectors with masses between 100 tonnes and 100 kilotonnes are only sensitive to supernovae occurring in the Milky Way Galaxy. The rate of galactic supernovae is estimated at ~3 per century which underlines the importance of capturing the next such event with as many detectors of diverse characteristics and sensitivities as possible. A great deal was learnt from the 20 events observed from SN 1987A. Much more should be possible with the next galactic supernova and the current generation of detectors, however the majority of these detectors are high cost, high maintenance, primarily aimed at other physics objectives, and are subject to appreciable downtime due to extended calibration runs, repairs, major modifications, etc. and consequently may be relatively short-lived. By employing a robust technology for neutron detection, attention to detail, and by recycling available equipment and materials, HALO fulfills its objectives of a being a long lifetime, low cost and low maintenance supernova detector. Since May 8th 2012 HALO has been operating in a mode where it is fully populated with Helium-3 detectors with those detectors being read out nearly continuously. Although some important work remains, to complete and fully commission HALO, the collaboration wishes to invest some effort in the possibility of measuring neutrino-lead cross-sections at the ORNL Spallation Neutron Source, as well as to investigate technologies that may prove appropriate for larger-scale lead-based supernova detectors.