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The Sudbury Neutrino Observatory (SNO) is a heavy water Cherenkov neutrino detector located 6800 feet underground at the active INCO, Ltd. Creighton nickel mine near Sudbury, Canada. It uses a one kiloton heavy water (D2O) primary neutrino target surrounded by 9456 photomultiplier tubes. SNO was designed to test a theory purported to solve the Solar Neutrino Problem, the thirty year-old question of why solar neutrino experiments detected between 30 and 50% of the expected number of neutrinos coming from the sun. The theory claims that the electron neutrinos produced by the sun oscillate between the three neutrino flavor eigenstates, νe, νμ and ντ, as they travel between the sun and the earth. Previous neutrino experiments were either sensitive only to νe, or had a reduced sensitivity to νμ and ντ, and therefore reported unexpectedly small solar neutrino fluxes. SNO detects 8B solar neutrinos through three channels. The first is through elastic scattering of a neutrino with an electron in the D2O: νx + e- → νx + e-. The recoiling electron, traveling at relativstic speeds in the water, produces a cone of Cherenkov radiation that is detected by SNO's photomultiplier tubes which surround the D2O. Previous water Cherenkov neutrino detectors, such as Kamiokande and Super-Kamiokande, relied on this reaction, which has a reduced sensitivity to νμ and ντ.
In addition to elastic scattering, SNO's unique D2O volume allows it to detect neutrinos through a neutral current (NC) reaction and a charged current (CC) reaction. The NC reaction, νx + d → p + n + νx, is equally sensitive to all three flavors of neutrinos. The CC reaction, νe + d → p + p + e-, however, involves only electron-type neutrinos. As a result, by comparing the CC neutrino flux to the NC + ES neutrino flux, the total solar neutrino flux can be determined independent of the Standard Solar Model.
On April 20, 2002, SNO published results which are thought to have solved the Solar Neutrino Problem. The results are shown in Figure 2, and the total solar neutrino flux detected by SNO agrees beautifully with the Standard Solar Model prediction. The total flux, as measured by the NC reaction, was φNC = (6.42 ±1.57 +0.55-0.58) x 106 cm-2 s-1 , while the Standard Solar Model prediction was φSSM = (5.05 +1.01-0.81) x 106 cm-2 s-1 . The results support the neutrino oscillation theory and solved the mystery of the missing solar neutrinos.
SNO is currently in its second phase of operation, the salt phase. NaCl has been dissolved in the D2O to improve the neutron detection efficiency. Chlorine has a higher cross section for neutron absorption, and the signature of neutron capture on chlorine has a higher energy than neutron capture on deuterium. The results from this phase of operation should improve the NC results. The third phase of operation will begin later this year with the installation of 3He proportional counters throughout the D2O volume. Whereas the pure D2O phase and the salt phase both used the photomultiplier tube array to detect neutrons from their capture on deuterium and chlorine respectively, the Neutral Current Detector array will provide a signal which is completely independent of the PMT signals.
This research is supported by the NSERC, Industry Canada, the NRC, the Northern Ontario Heritage Fund Corporation, Inco, AECL and Ontario Power Generation in Canada, the U.S. Department of Energy, and in the U.K., the PPARC.
 Q.R. Ahmad et al., Phys. Rev. Lett. 89, 011301 (2002)