The efforts under way to address these challenges are described below.
Stars, to sustain themselves against the force of gravity, must maintain the pressure of their gases by constantly producing energy. In our Sun, this energy is generated in a series of nuclear fusion reactions in which four hydrogen atoms are converted into helium. These reactions take place deep in the solar core, where temperatures are sufficiently high to allow nuclear fusion to occur. Although we cannot see into the solar core by conventional means, these reactions do produce one form of radiation, neutrinos, to which the Sun is transparent. Passing through the cooler outer layers of the Sun without scattering, these neutrinos carry, in their flux and energy distribution, a detailed record of the reactions by which they were produced. Thus, they offer a unique opportunity to view the nuclear processes that power stars like our Sun.
But the reason neutrinos can pass so easily through the Sun—their remarkably weak interactions with matter—also means that detecting them on Earth is a formidable experimental challenge. After almost three decades of effort, the tools to answer that challenge may be in hand. In the summer of 1965, a group of nuclear scientists began excavations for the first experiment, deep within the Homestake gold mine in Lead, South Dakota (see Box 5.1). With a detector filled with 610 tons of cleaning fluid, the experimentalists patiently waited for rare reactions of neutrinos that would convert a chlorine atom into argon. Because