The Sudbury Neutrino Observatory is a collaborative effort among physicists from Canada, the U.K., and the U.S. Using 1,000 tons of so-called heavy water and almost 10,000 photon detectors, they measure the flux, energy, and direction of solar neutrinos, which originate in the sun. SNO, located 6,800 feet underground in an active Ontario nickel mine, can also detect the other two types of neutrinos, muon neutrinos and tau neutrinos. In 2001, just two years after the observatory opened, physicists at SNO solved the 30-year-old mystery of the "missing solar neutrinos." They found that the answer lies not with the sun—where many physicists had suspected that solar neutrinos undergo changes—but with the journey they take from the core of the sun to the Earth.
2001-2002: Proof of solar neutrino oscillation
The Sudbury Neutrino Observatory (SNO), the first neutrino detector that can pick up all three known types of neutrinos, resolves conclusively that, in the case of the missing solar neutrinos, the neutrinos are not, in fact, missing. SNO finds that the total number of neutrinos from the sun is remarkably close to what John Bahcall predicted three decades earlier. Ray Davis's experimental work is vindicated as well, because SNO finds that only about a third of the solar neutrinos that reach Earth are still in the same state that Davis could measure. Roughly two-thirds change type—or oscillate—during the journey.
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