sensitive only to electron-type neutrinos (i.e., they specify uniquely the neutrino type) and provide a single number for the rate of detection of all neutrinos above an energy threshold, a number that is averaged over a period of time comparable to the mean life (typically weeks or months) of the radioisotope that is produced.

This section describes the three active and one radiochemical solar neutrino detectors that are under construction. The active detectors will have several thousand events per year, sufficient to test accurately the constancy of the rates predicted by the standard solar model. The expected high event rates will permit the observation of the expected seasonal variation of the solar neutrino fluxes due to the orbital eccentricity of the earth (~7 percent effect, peak to peak).

The Sudbury Neutrino Observatory: SNO

Deuterium (“heavy” hydrogen, with a neutron and a proton in its nucleus) is an excellent target for neutrinos. Two neutrino interactions can occur [23]:

νe + 2H → p + p + e; (4a)

νx + 2H → p + n + νx. (4b)

Reaction (4a) can be induced only by electron-type neutrinos, whereas reaction (4b) can be caused by neutrinos of different types. If more neutrinos are detected via reaction (4b) than by reaction (4a), that would be direct evidence that some electron-type neutrinos have oscillated into neutrinos of some other type.

Deuterium is a rare isotope of hydrogen, but the Canadian nuclear power industry requires vast quantities of heavy water, nearly pure D2O. Also, one of the deepest mines in the Western Hemisphere (a nickel mine belonging to INCO Limited) is located near Sudbury, Ontario. A large cavern has been excavated 2070 m underground to hold a detector consisting of 1000 tonnes of heavy water. The heavy water, contained in a transparent acrylic vessel, is to be surrounded by a shield of 7000 tonnes of ultrapure light water. The flashes of Cerenkov light will be recorded by more than 9000 photomultipliers, specially constructed of materials selected for low radioactivity. When the Sudbury Neutrino Observatory begins operation with heavy water in 1996, solar neutrinos are expected to be detected at the rate of more than 10 counts per day via reactions (4a) and (4b). In addition to the unique reactions of neutrinos on deuterium, SNO will observe the same neutrino-electron scattering process that is detected by Kamiokande. The SNO experiment is a collaboration between experimenters from Canada, the United States, and Great Britain.

If electron-type neutrinos oscillate into one of the other known neutrino types as they travel from the interior of the sun to the terrestrial detector, this will be revealed by the comparison of the rates in the two deuterium reactions, and, if the oscillation parameters are favorable, also by its distinctive effect on the shape of the observed electron energy spectrum in reaction (4a). These potential signatures of new physics are independent of solar models and solar physics.

Superkamiokande

The Superkamiokande experiment [24] is a natural extension of the current Kamiokande experiment with an upgraded performance as well as a much better sensitivity. A large detector of approximately 50,000 tonnes of ultrapure ordinary water is under construction in Japan. The principles of this detector are similar to those of the currently operating Kamiokande detector (see the section above on the Kamiokande detector), although the volume of water used will be much increased and the threshold for neutrino detection will be lowered (perhaps to 5 MeV). By comparison with the Kamiokande experiment, this immense detector will, beginning in 1996, provide a 30-fold increase in the observed rate of neutrino-electron scattering events [24]. The change in the shape of the neutrino energy spectrum predicted by the MSW effect may be observable in this experiment via the spectrum of recoil energies of the scattered electrons. Physicists from several U.S. institutions are collaborating with Japanese colleagues in the Super-kamiokande experiment.

BOREXINO

BOREXINO [25] will be the first real-time detector capable of observing the intense flux of low-energy neutrinos produced by electron capture on beryllium nuclei (see the 0.86-MeV 7Be



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