Homestake experiment

Its purpose was to collect and count neutrinos emitted by nuclear fusion taking place in the Sun.

After Bahcall calculated the rate at which the detector should capture neutrinos, Davis's experiment turned up only one third of this figure.

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.

Upon interaction with an electron neutrino, a 37Cl atom transforms into a radioactive isotope of 37Ar, which can then be extracted and counted.

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.

The results of the experiment, published in 2001, revealed that of the three "flavours" between which neutrinos are able to oscillate, Davis's detector was sensitive to only one.

After it had been proven that his experiment was sound, Davis shared the 2002 Nobel Prize in Physics for contributions to neutrino physics with Masatoshi Koshiba of Japan, who worked on the Kamiokande and the Super Kamiokande (the prize was also shared with Riccardo Giacconi for his contributions to x-ray astronomy).