The first solar neutrino experiment conducted by Raymond Davis, Jr. of Brookhaven National Laboratory in 1970. It consisted of a 400,000 tank containing C₂Cl₄ (perchloroethylene) cleaning fluid. The tank is located 1500 m deep in an abandoned gold mine in Homestake, South Dakota, and detected high energy neutrino interacting with chlorine and producing an argon atom via

for 0.81 MeV.

The presence of Ar was detected by flushing the solution with He. Six events out of 10²¹ neutrinos were predicted per day, but only two were observed. The frequency of neutrino events is sometimes given in solar neutrino units (SNUs), which equal 10^-36 neutrino captures per target atom per second. Possible explanations for the observed shortfall include neutrino oscillations between states, insufficient understanding of solar fusion, variable burning by the sun, and a lower central temperature in the sun than predicted.

A new measurement of solar neutrinos was made using a gallium detector (1990). The experiment is being carried out at the Baksan underground laboratory in the Soviet Union and has some U. S. collaborators. Another gallium experiment is also operated in Italy. Unlike the chlorine detector used for many years by Davis and co-workers, the gallium detector is sensitive to low energy neutrinos produced by the p-p reaction. The expected neutrino flux is therefore almost model independent, given the known solar luminosity. The expected neutrino flux is 132 SNU, but the initial measured value is less than 70 (and consistent with zero!). The indication is that the neutrino discrepancy is a property of nuclear reactions rather than wrong solar models (Smith 1990).

Electron Neutrino, Neutrino Oscillation, Tau Neutrino