Remembering the first SNO result

It’s already 20 years since the Sudbury Neutrino Collaboration (SNO) announced the first results from the heavy water detector experiment in northern Ontario: that neutrinos have a small but finite mass, and therefore their flavours oscillate. That is, that some fraction of electron-type neutrinos produced from the beta-decay of 8B in the thermonuclear chain reaction inside the sun were promptly (8 min or so later) detected as muon-type neutrinos on Earth. Though it was expected, this was a big deal: for one, it confirmed that we understood the H-H fusion process inside the sun (i.e., the standard solar model, mostly worked out by John Bahcall), as well as the root of the “solar neutrino problem” (i.e., the disparity between Bahcall’s prediction and the results of the Homestake Mine experiment by Ray Davis); namely, that neutrinos oscillate due to mass-flavour eigenstate mixing, as first proposed by Bruno Pontecorvo. Also, it suggested an upper limit to the neutrino mass that permitted an estimate of their contribution to dark matter (confirming that neutrinos alone were insufficient to explain it and boosting the now favoured dark energy hypothesis). Finally, in addition to confirming the standard solar model, it failed to break the Standard Model. SNO’s results, as well as those from other neutrino detector experiments in Japan and Italy, almost certainly raised the profile of the researchers (Ray Davis, Matatoshi Koshiba, and Riccardo Giacconi) who won the 2002 Nobel Prize in Physics.

Art McDonald, the former SNO director and emeritus Queen’s physics professor who (along with Takaaki Kajita) was awarded the 2015 Nobel Prize in Physics, has written a Commentary in Nature Reviews Physics that is worth checking out. (Also see the Arthur B. McDonald Institute website for more information on astroparticle physics.)

I was just a Master’s student at Queen’s University in Canada, where the SNO Calibration Group was based; and I had the dumb luck to (1) fall into a successful experiment as it was jolted with sudden international fame, and (2) be the warm body who was available to run (as my MSc thesis project) an important background radiation calibration for the detector. I feel very lucky to have been among so many smart, indefatigable experimental physicists, engineers, technicians, and machinists who pulled the whole thing off. Among these were my own supervisors and committee members, Axel Hallin, Tony Noble, as well as Hamish Leslie and Barry Robertson, emeritus now but profiled here.

The SNO detector was made of some 10k photomultiplier tubes, submerged in ultra-clean water (H2O, looking in on a transparent acrylic sphere of $200 million-worth of ultra-pure heavy water (D2O) on loan from Atomic Energy Canada, and monitored 24/7 to find dim flashes of light. The light was produced by fast particles radiating Cherenkov photons to dump energy as they transited through the D2O at relativistic speeds greater than that of light in water, analogous to a sonic boom with sound waves in air. From the orientation of the light cones and the number of photons produced, we could reconstruct the types of particles. Because there are so many high-energy charged particles produced on the Earth’s surface, the detector was buried in a specially excavated and sealed chamber some 2100+ meters (7000 ft) underground. Yet it required constant close observation, maintenance, and repairs. Researchers and technicians went underground at all hours, from 2 AM in winter with temperatures below -30ºC at the surface, down to the 7000 level where the ambient rock temperature would have been 40ºC absent the massive chillers (heat-pumps) and mine-air control. Naturally radioactive elements in the rock dust were abundant at that level, so we entered the underground lab through a positive air-pressure door and disrobed completely to pass through a shower, and then put on low-lint underclothes, coveralls, and hairnets, to enter. The lab itself contained a clean refuge station, built to withstand severe underground tremors (rock-bursts), and ultra-clean caverns for the water system (that circulated the light water and monitored the system for leaks, which would have been disastrous), the control room, and the detector chamber itself — these behind air cleaning stations to mitigate any dust we might have picked up. In the control room and detector chamber, it was as radio-clean as a microchip production site.

The depth and extraordinary measures SNO took for radio-cleanliness were necessary to shield the detector and reduce its background radiation. On paper, a solar neutrino looks like a photon with a touch of mass, but it behaves very differently: neutrinos are almost indifferent to ordinary matter — usually they pass through it as easily as empty space, and only occasionally collide with other particles. In the SNO detector, its kilotonne of heavy water and trillions of electrons and neutrons (both with small likelihoods of scattering solar neutrinos) we expected only about 1-2 neutrino events a day. We saw many more high-energy events, like muons (heavy electrons) produced by cosmic rays in the upper atmosphere easily punching through 2 km of rock above the detector, and occasionally 12.8 km of crust, mantle, and core below the detector. We also stood ready with a hotline to a network of observational astronomers prepared to task their telescopes in the direction of a supernova should we have received a sudden neutrino spike (thermonuclear activity within the sun releases a lot of neutrinos, but a Type II supernova releases almost inconceivably more, and we expected the sky to fill with supernova neutrinos).

Commentary:
McDonald, A.B. Neutrino physics over the two decades since the first SNO result. Nat Rev Phys (2021). https://doi.org/10.1038/s42254-021-00339-w

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