A giant trap has been set deep underground to catch a few of the neutrinos that theory predicts should be pouring out of the sun. Their capture would prove that the sun runs on thermonuclear power.

Most physicists and astronomers believe that the sun’s heat is produced by thermonuclear reactions that fuse light elements into heavier ones, thereby converting mass into energy. To demonstrate the truth of this hypothesis, however, is still not easy, nearly 50 years after it was suggested by Sir Arthur Eddington. The difficulty is that the sun’s thermonuclear furnace is deep in the interior, where it is hidden by an enormous mass of cooler material. Hence conventional astronomical instruments, even when placed in orbit above the earth, can do no more than record the particles, chiefly photons, emitted by the outermost layers of the sun.

Of the particles released by the hypothetical thermonuclear reactions in the solar interior, only one species has the ability to penetrate from the center of the sun to the surface (a distance of some 400,000 miles) and escape into space: the neutrino. This massless particle, which travels with the speed of light, is so unreactive that only one in every 100 billion created in the solar furnace is stopped or deflected on its flight to the sun’s surface. Thus neutrinos offer us the possibility of « seeing » into the solar interior because they alone escape directly into space. About 3 percent of the total energy radiated by the sun is in the form of neutrinos. The flux of solar neutrinos at the earth’s surface is on the order of 1011 per square centimeter per second. Unfortunately the fact that neutrinos escape so easily from the sun implies that they are difficult to capture.

Nevertheless, within the past year a giant neutrino trap has begun operating in a rock cavity deep below the surface in the Homestake Gold Mine in Lead, South Dakota, 1478 m underground.  This experiment proposed by Raymond Davis, Jr. and John N. Bahcall in 1968, was trying to observe neutrinos emitted by nuclear reactions inside the Sun. The neutrino trap is a tank filled with 100,000 gallons of tetrachloroethylene (C2Cl4), an ordinary cleaning fluid.

The experiment is being conducted by Raymond Davis, Jr., of the Brookhaven National Laboratory, with the assistance of Kenneth C. Hoffman and Don S. Harmer. In 1964 Davis and I showed that such an experimental test of the hypothesis of nuclear burning in stars was feasible. The idea was strongly supported by, among others, William A. Fowler of the California Institute of Technology, Richard W. Dodson, chairman of the Brookhaven chemistry department, and Maurice Goldhaber, the director of Brookhaven. Subsequently the Homestake Mining Company contributed valuable technical help.

The initial results published by Davis and his co-workers have left astronomers and astrophysicists somewhat puzzled because the neutrino flux rate seems low. It is less than half the theoretical value one obtains by assuming certain « standard » values for quantities used in constructing theoretical models of the solar interior. I shall discuss the range of theoretical predictions later. The important initial fact is that one can now use the results of the experiment to improve our knowledge of the sun’s thermonuclear furnace.

Neutrinos from the Sun, by John N. Bahcall, Scientific American, Volume 221, Number 1, July 1969, pp. 28-37