The theory of stellar structure and evolution gives a fairly good account of the observed properties of stars. Part of the theory is concerned with the generation of nuclear energy inside stars, and it appears that during nuclear fusion processes a substantial flux of NEUTRINOS is produced. Neutrinos have no electrical charge and scarcely any interaction with matter. The Sun is thought to produce vast numbers of neutrinos at this moment (~1015m-2s-1) and these are passing straight through you, this book and the entire Earth without being absorbed or deflected in any way.

One material which does have a weak but significant interaction with the neutrino flux is chlorine-37 (37 Cl an isotope of chlorine with an atomic mass of 37). The absorption of a neutrino by a chlorine-37 atom produces a single argon-37 (37A) atom. R. Davis, Jr. of the Brookhaven National Laboratory, USA, has constructed a vast tank, 6 m in diameter and nearly 15m long which he fills with nearly 400000 litres of perchloroethylene (C2C14) a dry-cleaning fluid. This liquid contains a considerable quantity of chlorine, and Davis has developed equipment for flushing the tank regularly to look for newly-created argon atoms. In order to ensure that other sources of radiation, such as cosmic rays, do not con¬tribute to the number of argon atoms found, Davis has constructed his experiment at the bottom of the Homestake gold mine near the town of Lead, South Dakota, USA. The tank is about 1500m underground.

Davis has found significantly fewer argon atoms than would have been expected from the theory, which implies that the details of the nuclear energy production mechanisms in the Sun are not as well understood as astronomers had believed.

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