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An experimental researcher whose joint work with Raymond Davis Jr. and Kenneth C. Hoffman used a nuclear conversion to search for solar neutrinos. A neutrino captured by chlorine can produce radioactive argon. Detecting that product provides an indirect way to measure an extremely weakly interacting particle flux, allowing observations on Earth to constrain nuclear processes inside the Sun.^[ref]

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]

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 $0.814\mathrm{MeV}$ 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]

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.

A researcher whose team developed a radiochemical search for solar neutrinos. Their 1968 report with Raymond Davis Jr. and Don S. Harmer converted a limit on argon production in a chlorine detector into a bound on the incoming neutrino flux. The measurement illustrates how a detector’s target nuclei and reaction cross section connect a rare nuclear event with an astronomical inference.^[ref]