Solar Neutrino Experiments
Neutrinos are ghostlike particles that were postulated by Wolfgang Pauli in 1930 on purely theoretical grounds and, until recently, were believed to have zero mass. They are thought to be produced in the nuclear reactions that provide the sun's energy. They rain down on each square inch of the earth at the rate of about 400 billion per second.
Raymond Davis Jr. started investigating neutrinos that were produced in Brookhaven's Graphite Research Reactor and at a reactor at the Savannah River Plant in South Carolina, in the 1950s. But these experiments were really the prelude to Davisís major triumph, which came in the early 1970s, when he successfully detected solar neutrinos in a new experiment based in Lead, South Dakota (image at right). See more images of the detector.
A solar neutrino was expected to produce radioactive argon when it interacts with a nucleus of chlorine. Davis developed an experiment based on this idea by placing a 100,000-gallon tank of perchloroethylene, a commonly used dry-cleaning chemical and a good source of chlorine, 4,800 feet underground in the Homestake Gold Mine in South Dakota and developing techniques for quantitatively extracting a few atoms of argon from the tank.
The chlorine target was located deep underground to protect it from cosmic rays. Also, the target had to be big because the probability of chlorine's capturing a neutrino was ten quadrillion times smaller than its capturing a neutron in a nuclear reactor. Despite these odds, Davis's experiment confirmed that the sun produces neutrinos, but only about one-third of the number of neutrinos predicted by theory could be detected.
This so-called "solar neutrino puzzle" gave birth to different experiments by scientists around the world, all working to confirm the solar neutrino deficit. First came Kamiokande in Japan, then SAGE in the former Soviet Union, GALLEX in Italy, and then Super Kamiokande. Finally, in 2001-2002, scientists working at SNO, the Sudbury Neutrino Observatory in Ontario, Canada, found strong evidence that the neutrino has the ability to oscillate, or change form, among its three known types: the electron, muon and tau neutrinos.
1967 Brookhaven National Lab press release, "Solar Energy Generation Theory Being Tested in Brookhaven Neutrino Experiment" (PDF)
1967 Brookhaven Bulletin story, "Solar Neutrinos Are Counted at Brookhaven" (PDF)
Brief video featuring comments by Ray Davis on his neutrino research. (Note: This is a streaming video file. You must have RealPlayer installed.)
A "question and answer" interview with Dr. Davis, about 9 min. (RealPlayer required.)