Sensors mounted on a sphere surrounding this bottle, in pure regular water, will detect the tiny amounts of Cerenkov light emitted by neutrinos interacting with the heavy water. The Elusive Neutrino Neutrinos are one of the fundamental building blocks of nature, along with other subatomic particles called quarks, electrons, and muon and tau particles. Small numbers of neutrinos are produced by some naturally occurring radioactive materials, but vast quantities of them are liberated in the energy-generating fusion reactions in the cores of the sun and other stars. Two hundred trillion trillion trillion neutrinos are created at the sun's core every second. Neutrinos are the only subatomic particles that can penetrate virtually all types of matter.
Neutrinos stream out from the sun's core and into space at the speed of light unaffected by the sun.
Because they reach Earth in minutes compared to the thousands of years it takes other particles to escape from the sun, they are light speed couriers bearing the news of what's happening today inside the sun. However, observing neutrinos has proven to be notoriously difficult and very little is known about their detailed properties. Electron type neutrinos are produced in the sun's core, but a new theory suggests that they change from one type to another as they travel. SNO, with its sensitivity to detect all three types, will check on this theory. We do not know whether they have a mass or whether they are little bundles of energy with no mass, always moving at the speed of light.