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Argon-37 is a radioactive isotope with $18$ protons and $19$ neutrons. It undergoes electron capture to chlorine-37, with an approximately $35$-day half-life:The mass number is unchanged while the nuclear charge decreases by one. Atomic vacancies left by the captured electron can produce detectable low-energy radiation during atomic relaxation.

Argon-37 made from chlorine was the counted product in the radiochemical solar-neutrino experiment. The method separated rare radioactive argon atoms from a much larger chlorine target, connecting nuclear counting to a very weak astronomical signal.^[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.

Raymond Davis Jr. developed a chemical method for observing solar neutrinos. In the chlorine experiment, rare interactions converted chlorine-37 into radioactive argon-37; extraction and counting of the argon atoms made a very weak interaction observable. The unexpectedly low measured rate became a central part of the solar neutrino problem.^[ref]

A chemist.^[ref]

Tetrachloroethylene is $\mathrm{C}{\mathrm{l}}_2\mathrm{C}=\mathrm{CC}{\mathrm{l}}_2$, a chlorinated molecular liquid. Its four chlorine atoms supply a large chlorine inventory per molecule.

In the Homestake experiment, tetrachloroethylene served as a target for detecting solar neutrinos through a rare chlorine-to-argon nuclear transformation. The solvent's chemical identity determines the target inventory, while the observed conversion is a nuclear process.