A detector in a mine
Deep inside Vale's Creighton Mine, an active nickel mine near Sudbury, Ontario, lies a unique physics laboratory. The original Sudbury Neutrino Observatory (SNO) was constructed 2,100 meters (6,800 feet) underground. This depth is important. The two kilometers of solid norite rock overhead shield the sensitive detector from the constant barrage of cosmic rays that bombard the Earth's surface. This rock barrier reduces the rate of cosmic ray muons passing through the lab to just 0.27 per square meter per day, a tiny fraction of the surface rate.
The center of the SNO experiment was a 12-meter diameter acrylic sphere, which contained 1,000 tonnes of heavy water (D₂O). This sphere, assembled underground from 122 panels, was suspended within a massive, barrel-shaped cavity 34 meters high and 22 meters wide. Surrounding the acrylic vessel was a geodesic support structure holding 9,600 photomultiplier tubes (PMTs), each designed to detect the faintest flashes of light. The entire assembly was submerged in 7,000 tonnes of ultra-pure regular water to provide buoyancy and further shielding against trace radiation from the cavern walls. The whole facility is maintained as a Class-2000 cleanroom, meaning there are fewer than 2,000 particles of 0.5 micrometers or larger in any cubic foot of air.
Solving the solar neutrino problem
For decades, physicists faced a puzzle known as the solar neutrino problem. Experiments consistently detected only about one-third of the neutrinos that solar models predicted the Sun should be producing. The SNO experiment, which began collecting data in May 1999, was designed to solve this mystery. Its use of heavy water was the key element.
The detector could observe three different types of neutrino interactions. One, the "charged-current" interaction, was only sensitive to electron neutrinos, the type produced by the Sun. A second, "neutral-current" interaction, was equally sensitive to all three known neutrino flavors: electron, muon, and tau. By comparing the rates of these two reactions, SNO could determine both the number of electron neutrinos arriving from the Sun and the total number of all neutrinos.
In 2001, the SNO collaboration published its first results. The data showed that while the number of electron neutrinos was low, matching previous experiments, the total number of all three neutrino flavors matched the solar models' predictions perfectly. This was conclusive evidence that the "missing" electron neutrinos had not disappeared; they had changed flavor, or "oscillated," during their journey from the Sun to the Earth. For this to happen, neutrinos must have mass, a property not included in the Standard Model of particle physics. This discovery revolutionized the field and led to Arthur B. McDonald, the project director, co-receiving the 2015 Nobel Prize in Physics. The original SNO experiment was turned off on November 28, 2006, but the facility has since been expanded into the permanent SNOLAB, hosting a new generation of physics experiments.