The Homestake experiment was a pioneering neutrino detection experiment conducted by physicist Raymond Davis Jr. and his collaborators in the late 1960s. It took place in the Homestake Gold Mine in Lead, South Dakota, which is one of the deepest mines in North America. The main goal of the experiment was to detect solar neutrinos, which are produced in the nuclear fusion reactions that power the Sun.
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The Homestake experiment, led by Raymond Davis Jr., used electron neutrino capture on chlorine to produce argon subject to radioactive decay. The 1968 paper reported an upper limit, rather than a resolved positive signal.[ref]
Search for Neutrinos from the Sun by Raymond Davis Jr. and others. Abstract: chlorine reaction and upper-limit result
The underground target contained tetrachloroethylene. Circulated helium swept out argon, which was trapped on cooled charcoal, purified, and put into tiny proportional counters. Electron capture back to chlorine supplies the delayed readout. Added stable argon checked extraction recovery; background counting checked spurious pulses. The capture threshold is and the product's half-life is about 35 days. The chemical extraction retains neither the original neutrino's arrival time nor its direction.[ref]
Nobel Lecture: A half-century with solar neutrinos by Raymond Davis Jr.. pp. 986, 988–991; Figs. 9–14
The reaction selects electron neutrinos, and its threshold excludes the lowest-energy part of the solar neutrino spectrum. A low count alone cannot distinguish a low source neutrino flux, a changed neutrino flavor mixture, or an underestimated loss in extraction or counting. This is why chemical recovery tests and an independent water Cherenkov neutrino reconstruction are scientifically valuable.