A Problem Deep Underground
For decades, a major puzzle persisted in physics: experiments consistently detected only about one-third of the neutrinos predicted to arrive from the Sun. This discrepancy, known as the "solar neutrino problem," challenged the understanding of both stellar fusion and the fundamental nature of particles. The solution was found 2.1 kilometers (6,800 feet) below the surface in an active nickel mine in Sudbury, Ontario. This immense depth was necessary to shield a highly sensitive detector from the constant bombardment of cosmic rays that would otherwise obscure the faint signals from neutrinos.
The Sudbury Neutrino Observatory (SNO) was built inside a specially excavated cavern as tall as a 10-story building. At its center was a 12-meter-diameter acrylic sphere containing 1,000 tonnes of heavy water (D₂O). This sphere was surrounded by 7,000 tonnes of ultra-pure regular water for support and further shielding. A geodesic sphere, 17 meters in diameter, held 9,600 photomultiplier tubes (PMTs) pointed inward. These extremely sensitive detectors were designed to register the tiny flashes of blue light, called Cherenkov radiation, that are produced when a neutrino interacts with an atom in the heavy water. The experiment began collecting data in May 1999.
Flavors in Flight
The SNO detector had a unique advantage over its predecessors. Previous experiments were primarily sensitive to only one type of neutrino: the electron neutrino, which is the only flavor produced by the Sun's fusion reactions. The use of heavy water allowed the SNO to perform two types of measurements simultaneously. One reaction was sensitive only to electron neutrinos. A second reaction, where a neutrino breaks a deuterium nucleus into a proton and a neutron, was equally sensitive to all three known neutrino flavors: electron, muon, and tau.
By comparing the results from these two reactions, the SNO collaboration made a definitive measurement. The number of detected electron neutrinos matched the one-third figure seen by previous experiments. However, the total number of neutrinos detected across all three flavors perfectly matched the predictions of the Standard Solar Model. The conclusion was inescapable: the "missing" neutrinos were not missing at all. Instead, the electron neutrinos were changing, or oscillating, into muon and tau neutrinos during their eight-minute travel from the Sun to the Earth.
This discovery, announced in 2001, solved the solar neutrino problem. It also proved that neutrinos have mass, a property not accounted for in the original Standard Model of particle physics. For this work, SNO director Arthur B. McDonald was a co-recipient of the 2015 Nobel Prize in Physics. The SNO experiment officially stopped taking data on November 28, 2006, and the facility has since been expanded into a permanent underground laboratory called SNOLAB.