SNO

The Sudbury Neutrino Observatory (SNO) was a neutrino detector located in Vale (near Sudbury), Ontario, Canada. Operated by an international collaboration, it was established to study solar neutrinos and fundamental properties of neutrinos, particularly flavor transformation and mass.

The detector was housed in the Creighton Mine about two kilometers underground to shield it from cosmic rays. It consisted of a large acrylic vessel containing about 1,000 tonnes of heavy water (D2O), surrounded by light water and an extensive array of photomultiplier tubes to detect Cherenkov light produced by neutrino interactions. The design enabled discrimination among different reaction channels and provided sensitivity to all active neutrino flavors.

SNO measured solar neutrinos via three main interaction channels: charged-current interactions, which are sensitive only to electron neutrinos; neutral-current interactions, which are sensitive to all active neutrino flavors equally; and elastic scattering, which is primarily sensitive to electron neutrinos but has a smaller sensitivity to other flavors. By comparing results from these channels, SNO could determine both the electron-neutrino flux and the total flux of active neutrinos from the Sun.

The experiment operated in multiple phases. The initial heavy-water phase established the existence of flavor change by showing a deficit in electron neutrinos relative to solar-model predictions, while the total active-neutrino flux agreed with those predictions. A salt-doped phase enhanced neutral-current detection, and a subsequent phase employed 3He proportional counters to further probe NC interactions. The findings provided strong evidence for neutrino oscillations and established that neutrinos have mass, resolving long-standing questions about the solar neutrino problem and informing the parameters of solar-neutrino mixing.

SNO ceased operations in the mid-2000s, and its detector components contributed to the development of SNOLAB, continuing Canada’s role in underground neutrino research.