In the summer of 1965, workers deep in the Homestake gold mine, Lead, South Dakota, completed the excavation of a 30 × 60 × 32 ft³ cavity at a depth of 4,850 ft. This was the first step in bringing to life a new detector proposed by Ray Davis, Jr., and his Brookhaven National Laboratory collaborators. The cavity was soon occupied by an enormous tank filled with 610 tons—the contents of 10 railway tankers—of chlorine-bearing cleaning fluid. The purpose of this detector was to make the first attempt to verify that our Sun produces its energy by converting four protons into ⁴He in a sequence of nuclear reactions called the pp chain (Figure 5.1.1). This prediction was the basis of our understanding of stellar evolution. Three years later Davis, Harmer, and Hoffman announced an upper bound on the solar neutrino flux that was a factor of two and one-half below the theoretical expectation. Davis was consoled by one of the miners, who pointed out that it had been, after all, an unusually cloudy summer.

FIGURE 5.1.1

The ³⁷Cl detector located in the Homestake gold mine, Lead, South Dakota. The steel vessel contains 0.6 kilotons of percloroethylene. During use, the cavity is filled with water to provide additional shielding from neutrons and other radioactivity produced in the surrounding rock walls. (Courtesy of Brookhaven National Laboratory.)