Some of the neutrons produced from the dissociated deuterons make their way through the acrylic vessel into the light water jacket surrounding the heavy water, and since light water has a very large cross section for neutron capture, these neutrons are captured very quickly.
The direction of the gamma ray is completely uncorrelated with the direction of the neutrino. Heavy water has a small cross section for neutrons, but when neutrons are captured by a deuterium nucleus, a gamma ray ( photon) with roughly 6 MeV of energy is produced. The neutrino continues on with slightly less energy, and all three neutrino flavours are equally likely to participate in this interaction. In the neutral current interaction, a neutrino dissociates the deuteron, breaking it into its constituent neutron and proton. The electrons produced in this reaction are emitted in all directions, but there is a slight tendency for them to point back in the direction from which the neutrino came. The proton which is produced does not have enough energy to be detected easily. The emitted electron carries off most of the neutrino's energy, on the order of 5–15 MeV, and is detectable. Solar neutrinos have energies smaller than the mass of muons and tau leptons, so only electron neutrinos can participate in this reaction. The neutrino is absorbed in the reaction and an electron is produced. In the charged current interaction, a neutrino converts the neutron in a deuteron to a proton. Most of the facility is Class 3000 (fewer than 3,000 particles of 1 μm or larger per 1 ft 3 of air) but the final cavity containing the detector is an even stricter Class 100. Along the drift are a number of operations and equipment rooms, all held in a clean room setting. The observatory is located at the end of a 1.5-kilometre-long (0.9 mi) drift, named the "SNO drift", isolating it from other mining operations.
The cavity housing the detector was the largest in the world at such a depth, requiring a variety of high-performance rock bolting techniques to prevent rock bursts. The heavy water was viewed by approximately 9,600 photomultiplier tubes (PMTs) mounted on a geodesic sphere at a radius of about 850 centimetres (28 ft). The detector cavity outside the vessel was filled with normal water to provide both buoyancy for the vessel and radiation shielding. The SNO detector target consisted of 1,000 tonnes (1,102 short tons) of heavy water contained in a 6-metre-radius (20 ft) acrylic vessel. As relativistic electrons travel through a medium, they lose energy producing a cone of blue light through the Cherenkov effect, and it is this light that is directly detected.Ī wide-angle view of the detector interior (Courtesy of SNO) The experiment observed the light produced by relativistic electrons in the water created by neutrino interactions. At the time it competed with TRIUMF's KAON Factory proposal for federal funding, and the wide variety of universities backing SNO quickly led to it being selected for development. The SNO collaboration held its first meeting in 1984. It was quickly identified as an ideal place for Chen's proposed experiment to be built, and the mine management was willing to make the location available for only incremental costs. The Creighton Mine in Sudbury is among the deepest in the world and, accordingly, experiences a very small background flux of radiation. A location in Canada was attractive because Atomic Energy of Canada Limited, which maintains large stockpiles of heavy water to support its CANDU reactor power plants, was willing to lend the necessary amount (worth Can$330,000,000 at market prices) at no cost. Thus, such a detector could measure neutrino oscillations directly. Unlike previous detectors, using heavy water would make the detector sensitive to two reactions, one reaction sensitive to all neutrino flavours, the other reaction sensitive to only electron neutrino. In 1984, Herb Chen of the University of California at Irvine first pointed out the advantages of using heavy water as a detector for solar neutrinos.
All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and yielded little to no information on muon neutrinos and tau neutrinos.
Over several decades many ideas were put forward to try to explain the effect, one of which was the hypothesis of neutrino oscillations. As several experiments confirmed this deficit the effect became known as the solar neutrino problem. The first measurements of the number of solar neutrinos reaching the Earth were taken in the 1960s, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted by the Standard Solar Model.
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