Homestake experiment

The Homestake experiment (sometimes referred to as the Davis experiment) was an experiment headed by astrophysicists Raymond Davis, Jr. and John N. Bahcall in the late 1960s. Its purpose was to collect and count neutrinos emitted by nuclear fusion taking place in the Sun. Bahcall did the theoretical calculations and Davis designed the experiment. After Bahcall calculated the rate at which the detector should capture neutrinos, Davis's experiment turned up only one third of this figure. The experiment was the first to successfully detect and count solar neutrinos, and the discrepancy in results created the solar neutrino problem. The experiment operated continuously from 1970 until 1994. The University of Pennsylvania took it over in 1984. The discrepancy between the predicted and measured rates of neutrino detection was later found to be due to neutrino "flavour" oscillations.

The experiment took place in the Homestake Gold Mine in Lead, South Dakota. Davis placed 1,478 meters (4,850 feet) underground a 380 cubic meter (100,000 gallon) tank of perchloroethylene, a common dry-cleaning fluid. A big target deep underground was needed to prevent interference from cosmic rays, taking into account the very small probability of a successful neutrino capture, and, therefore, very low effect rate even with the huge mass of the target. Perchloroethylene was chosen because it is rich in chlorine. Upon interaction with an electron neutrino, a chlorine-37 atom transforms into a radioactive isotope of argon-37, which can then be extracted and counted. The reaction of neutrino capture is

${\displaystyle \mathrm {\nu _{e}+\ ^{37}Cl\longrightarrow \ ^{37}Ar+e^{-}.} }$

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