Homestake experiment

Methodology

The experiment took place in the Homestake Gold Mine in Lead, South Dakota. Davis placed a 380 cubic meter (100,000 gallon) tank of perchloroethylene, a common dry-cleaning fluid, 1,478 meters (4,850 feet) underground. 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 ³⁷Cl atom transforms into a radioactive isotope of ³⁷Ar, which can then be extracted and counted. The reaction of the neutrino capture is

${\displaystyle {\nu }_{\mathrm{e}}+{}^{37}\mathrm{C}\mathrm{l}⟶{}^{37}\mathrm{A}{\mathrm{r}}^{+}+{\mathrm{e}}^{-}.}$

The reaction threshold is 0.814 MeV, i.e. the neutrino should have at least this energy to be captured by the ³⁷Cl nucleus.

Because ³⁷Ar has a half-life of 35 days, every few weeks, Davis bubbled helium through the tank to collect the argon that had formed. A small (few cubic cm) gas counter was filled by the collected few tens of atoms of ³⁷Ar (together with the stable argon) to detect its decays. In such a way, Davis was able to determine how many neutrinos had been captured.