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Helium and Lead Observatory
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<p>The Helium And Lead Observatory (HALO) is a <a href="/facts/Neutrino_detector/LwtyrVAc">neutrino detector</a> at <a href="/facts/SNOLAB/qzyPjIvF">SNOLab</a> for the <a href="/facts/Supernova_Early_Warning_System/KcQCvGzY">Supernova Early Warning System</a> (SNEWS). It began engineering operation on May 8, 2012, and joined as an operational part of <a href="/facts/SNEWS/KcQCvGzY">SNEWS</a> in October 2015. </p><p>It was designed to be a low-cost, low-maintenance detector with limited capabilities: 38 sufficient for the burst of neutrinos generated by a nearby <a href="/facts/Supernova/Pfo2J23l">supernova</a>. Its major components are left over from other decommissioned experiments: 76 tons of <a href="/facts/Lead/EYljasJN">lead</a> from an earlier cosmic-ray experiment, and 128 three-metre-long <a href="/facts/Helium-3/SmPC5oT3">helium-3</a> <a href="/facts/Neutron_detector/8BRncp82">neutron detectors</a> from the <a href="/facts/Sudbury_Neutrino_Observatory/jEqnTGaW">Sudbury Neutrino Observatory</a>. </p><p>The idea of using lead to detect <a href="/facts/Supernova_neutrinos/p1w54oDl">supernova neutrinos</a> was originally proposed in 1996 by Cliff Hargrove as the "lead astronomical neutrino detector" (LAND), and in 2004, Charles Duba, then a PhD student working on SNO, proposed re-using them for this purpose, prompting the renaming to HALO. Design of the current detector began in 2007. </p><p>When an <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrino</a> collides with a lead nucleus, it causes a nuclear transmutation that ends with a <a href="/facts/Neutron_emission/lXLQRDCQ">neutron emission</a>. Lead does not absorb neutrons readily since 208Pb it has a "<a href="/facts/Magic_number_(physics)/v9PwXFzQ">magic number</a>" of both protons and neutrons, so the neutrons pass through to the 3He detectors. If enough neutrons are detected in a short time, an alert is generated. </p><p>One limitation of the detector's design is its small size; due to the limited amount of surplus lead available, half of the neutrons generated escape before hitting a neutron detector.: 15 To mitigate this, it is surrounded by a layer of water to reflect some of the neutrons back in. Budget permitting, there are plans for a larger detector using 1000 t of lead and the remaining leftover 3He detectors: 13–18 (Due to lead's high density; 1000 t is a cube 4.45 m (14.6 ft) on a side, not an impractical size for underground installation.) </p>