While experimenters continued to run their neutrino experiments, the late John Bahcall of the Institute for Advanced Study in Princeton, N.J., and other theorists improved the models used to predict the rate of solar neutrino production. With no understanding of why the predictions and the measurements were so different, physicists had to put on hold the original goal of studying the solar core by observing neutrinos. The number of neutrinos arriving from the sun was always significantly less than the predicted total, in some cases as low as one third, in others as high as three fifths, depending on the energies of the neutrinos studied. (In 2002 Davis shared the Nobel Prize with Masatoshi Koshiba of the University of Tokyo for pioneering work in neutrino physics.) Thirty years of experiments following Davis's all found similar results despite using a variety of techniques. But rather than seeing one atom of argon created each day, as theory predicted, Davis observed just one every 2.5 days.
The detector contained 615 metric tons of liquid tetrachloroethylene, or dry-cleaning fluid, and neutrinos transformed atoms of chlorine in this fluid into atoms of argon.
Davis's experiment, located in the Homestake gold mine near Lead, S.D., detected neutrinos by a radiochemical technique. THE FIRST SOLAR NEUTRINO EXPERIMENT, conducted in the mid-1960s by Raymond Davis, Jr., now at the University of Pennsylvania, was intended to be a triumphant confirmation of the fusion theory of solar power generation and the start of a new field in which neutrinos could be used to learn more about the sun. We now understand the workings of the sun better than we do the workings of the microscopic universe. Ironically, the confirmation of our best theory of the sun exposes the first flaw in the Standard Model of particle physics-our best theory of how the most fundamental constituents of matter behave. Using this ability, SNO has demonstrated that the deficit of solar neutrinos seen by earlier experiments resulted not from poor measurements or a misunderstanding of the sun but from a newly discovered property of the neutrinos themselves. The additional neutrons allow SNO to observe solar neutrinos in a way never done before, by counting all three types, or flavors, of neutrino equally. But unlike most of the other experiments built over the previous three decades, SNO detects solar neutrinos using heavy water, in which a neutron has been added to each of the water molecules' hydrogen atoms (making deuterium). Like all underground experiments designed to study the sun, SNO's primary goal is to detect neutrinos, which are produced in great numbers in the solar core. It was not until 2002, with the results from the underground Sudbury Neutrino Observatory (SNO) in Ontario, that physicists resolved this conundrum and thereby fully confirmed Eddington's proposal.
English physicist Arthur Eddington suggested as early as 1920 that nuclear fusion powered the sun, but efforts to confirm critical details of this idea in the 1960s ran into a stumbling block: experiments designed to detect a distinctive by-product of solar nuclear fusion reactions-ghostly particles called neutrinos-observed only a fraction of the expected number of them. Yet that has turned out to be the key to unlocking a decades-old puzzle about the physical processes occurring inside the sun. Building a detector the size of a 10-story building two kilometers underground is a strange way to study solar phenomena.