HALO

HALO (the Helium and Lead Observatory) is one of the most patient experiments on Earth, poised to detect neutrinos from an exploding star.  

The experiment is one of five TRIUMF-SNOLAB collaborations, involving some of the world’s most advanced experiments in neutrino science (SNO+, nEXO), and dark matter detection (DEAP-3600, SuperCDMS). SNOLAB is the deepest underground laboratory in North America, based two skilometers under Sudbury, Ontario in order to shield experiments from cosmic ray background. 

When a massive star explodes as a supernova it first collapses on itself and 99% of the energy of the collapse is radiated away in neutrinos. The neutrinos escape within seconds because they interact so weakly with other matter that a wall of lead 400 light-years thick would be needed to block a neutrino’s free passage. However, since photons interact strongly with matter, the light produced in the explosion is contained for hours until the dying star expands and becomes less dense at which point light escapes.  

Thus, by detecting supernova neutrinos HALO would make a direct observation of the nuclear physics occuring in a supernova’s core and provide an early warning signal enabling astronomers to observe the dying star’s entire visible sequence.  

HALO is a member of the international SuperNova Early Warning System (SNEWS), which also includes neutrino detectors in China, Japan, Antarctica, and Italy. It’s estimated that a supernova occurs in the Milky Way once every 30 to 50 years; the last one occurred in 1987 and its neutrino flux lasted about ten seconds.  

The HALO detector is a rare case of a cosmic particle detector built primarily from re-used materials. The lead used in HALO is from a decommissioned Canadian cosmic ray detector and its helium-3 (³He) gas detectors are repurposed from the 2015 Nobel Prize-winning Sudbury Neutrino Observatory experiment.  

HALO is analogous to a massive Geiger counter, detecting the neutrons ejected from lead nuclei as the result of collisions with neutrinos from a nearby supernova.  

The detector has two core parts: 79 tons of lead; and ³He neutron detectors. 

The lead is in the form of 864 hollow, ring-shaped blocks, each weighing about 82 kg, arranged in stacks of 32 three-meter long columns. Inside each of the lead columns are four, three-meter long nickel tube ³He-filled detectors, each wrapped in a thin polyethylene liner. The ³He detectors are made from special purified, low-radioactivity nickel and filled with gaseous ³He. The entire lead-³He structure is surrounded in a bath of 30 cm-thick water to shield HALO from background environmental neutrons. 

Lead has a very high neutrino cross section, or probability that a nearby neutrino will interact with a neutron in the lead nucleus. During a neutrino flux from a supernova, a miniscule fraction of the trillions of neutrinos that pass-through HALO will make a direct hit on a neutron in the detector’s lead nuclei. The energized neutrons will ricochet, pinball-like within the lead and those that escape it will first hit the polyethylene liner around the ³He detectors. The polyethylene is a neutron moderator, slowing the neutrons to room-temperature (also known as thermal or slow neutrons), enabling them to collide with ³He atoms and trigger a Geiger counter-like electrical signal. It’s estimated that a supernova detection would produce several dozen such signals. 

From its inception, in collaboration with Laurentian University, TRIUMF has provided a combination of scientific and technical expertise for HALO’s design, construction and operation. 

This includes TRIUMF’s design of a fail-safe data acquisition electronics system. Since a supernova’s neutrino signature only lasts 10-20 seconds and only occurs about every half-century, it’s essential that HALO operate 100% of the time. The fail-safe system divides HALO into two separately powered halves so that if one fails the other is still operational. Similarly, HALO has a dual computer system designed so that if the operating one fails, data is immediately routed to the second, waiting computer. A TRIUMF scientist also designed and installed the neutron detector calibration system to determine the ³He detector’s detection efficiency. 

TRIUMF’s Science Technology Group made the high-voltage signal cables that connect the detector to its computers, and the Machine Shop built the test stands used for the bench-testing of the ³He neutron detectors at SNOLAB prior to their installation in HALO. 

TRIUMF scientists are also involved in the international proposal for a 12-times larger version of HALO that would be able to detect supernova with greater resolution and up to three-times further away.