In the last four decades of the twentieth century, the detection of neutrinos from the Sun became a reality. Using a detection scheme beginning with a chloroethylene-filled tank in the Homestake mine in South Dakota, Raymond Davis and his collaborators (Davis, Harmer, & Hoffman, 1968) established upper limits on the fluxes of neutrinos made in the Sun by the reactions 8B→8Be* + e+ + ve and 7Be + e−→ 7Li + ve and reaching the Earth as electron-flavor neutrinos. The experiment relied on the reaction 37Cl(ve, e−)37 A, which has a threshold (at 0.814 MeV) approximately twice as large as the 0.43 MeV maximum energy of the neutrino emitted in the pp reaction, slightly smaller than the 0.861 MeV energy of the neutrino emitted in the 7Be + e−→ 7Li +ve reaction, and much smaller than the maximum energy of the neutrino emitted in the 8B(e+ve)8Be* reaction.

The limits established by Davis et al. were an order of magnitude smaller than fluxes which had been predicted on the basis of solar models that incorporated the then best guesses as to the appropriate input physics. Among other consequences, the discrepancy, commonly referred to as the solar neutrino problem, led to a re-examination of available data on relevant nuclear cross sections, revisions and new measurements of these cross sections, and to a refinement over time in the solar models.