= Homestake experiment
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{title2=$\nu_e+{}^{37}\mathrm{Cl}\longrightarrow{}^{37}\mathrm{Ar}+e^-$}
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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.<Search for Neutrinos from the Sun by Raymond Davis Jr. and others>[Abstract: chlorine reaction and upper-limit result]{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.<Nobel Lecture: A half-century with solar neutrinos by Raymond Davis Jr.>[pp. 986, 988–991; Figs. 9–14]{ref}
\Image[/homestake-chemical-neutrino-readout.png]
{title=From chlorine capture to delayed radioactive-decay pulses}
{description=Functional schematic of the <Homestake experiment>. The plotted curves are an illustrative <radiochemical neutrino counting model>, not measured data.}
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.
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