Davis, research professor of physics at Penn and research collaborator in chemistry at the Brookhaven Lab in Upton, N.Y., shares the honor with Masatoshi Koshiba of the University of Tokyo and Riccardo Giacconi of Associated Universities Inc. in Washington.
The Nobel Prize to Davis and Koshiba was awarded in recognition of their groundbreaking research into the emission of neutrinos produced by nuclear fusion reactions in the center of the sun. The observation of these neutrinos demonstrated conclusively that the sun is powered by the fusion of hydrogen nuclei into helium nuclei.
"The awarding of the Nobel Prize to Professor Raymond Davis is a great moment for this extraordinary researcher, for the University and for the world of science," said Penn President Judith Rodin. "His pathbreaking work has given rise to the discipline of neutrino astrophysics, a field that has already told us much about our own sun and other astronomical objects and may yield equally stunning insights into the nature of matter itself.
"Ray Davis is a truly outstanding scientist and an inspiration to his peers worldwide. We offer him and his colleagues our deepest and most heartfelt congratulations."
The source of the sun's energy has challenged scientists for centuries. In the 19th century it was assumed that the sun's energy resulted from its gravitational collapse. But with the advent of radioactive dating in the beginning of the 20th century, the age of the Earth was determined to be roughly 4 billion years. Only nuclear reactions within the sun could supply energy for such a long time; gravitational collapse could provide solar power only for a few tens of million of years, insufficient to have fostered the biological evolution of species on Earth.
"Since the interior of the sun is opaque to all modes of observation other than neutrinos, directly observing these nuclear reactions proved enormously challenging," said Kenneth Lande, a Penn professor of physics who has collaborated with Davis since the 1970s. "Ray Davis conceived, built and ran the first experiment to detect neutrinos from the core of the sun."
Starting in 1967, Davis detected solar neutrinos by observing the neutrino-induced conversion of chlorine atoms into argon atoms. The observed rate was one argon atom produced every two days in a 615-ton neutrino detector Davis constructed a mile underground in the Homestake Gold Mine in Lead, S.D. The subterranean location served to screen out cosmic radiation that would otherwise produce too many distracting signals. Since neutrinos rarely interact with matter, they passed easily though the Earth to reach the detector, essentially a 100,000-gallon tank filled with perchloroethylene, a common, chlorine-rich dry-cleaning fluid that could be manufactured cheaply in large quantities.
The number of neutrinos Davis detected reaching the Earth was only one-third that predicted by detailed models of nuclear reactions within the sun. One of the explanations for this discrepancy was that some electron neutrinos produced in solar fusion reactions convert into other neutrino species -- specifically, muon and tau neutrinos -- during the eight-minute flight from the solar core to the Earth.
Subsequent experiments at the Kamiokande and Superkamiokande detectors in Japan and the Sudbury Neutrino Observatory in Sudbury, Ontario -- research in which Penn scientists have played a pivotal role -- have confirmed this theory.
"This phenomenon of neutrino flavor conversion is one of our first views of a new, previously unknown class of particle interactions that may help in understanding the evolution of the universe," said Lande, who has been responsible for the operation of the South Dakota detector since 1990. "These conversions require that neutrinos have mass and that the masses of the various neutrino species are different. By combining the results of the various neutrino observations, we have established that the masses of these neutrinos are amazingly small, of the order of a billionth the mass of the electron, but not zero as previously thought. However, because of the enormous number of neutrinos in the universe, the total mass of neutrinos is comparable to the total mass of all the visible matter of the universe."
At Lande's behest, Davis joined Penn in 1985 after 37 years at Brookhaven Lab. His research was supported by the Department of Energy from 1965 to 1984; since 1985 his work at Penn and the operation of the Homestake neutrino detector have been supported by the National Science Foundation.
Davis, 87, a resident of Blue Point, N.Y., received a B.S. in 1937 and an M.S. in 1939, both from the University of Maryland. In 1942 he earned a Ph.D. from Yale University. He served in the U.S. Army during World War II and worked at Monsanto Chemical Company for two years before joining Brookhaven Lab in 1948.
Earlier this year Davis received the 2001 National Medal of Science from President George W. Bush. He is also a member of the National Academy of Sciences and the American Academy of Arts and Sciences.
Other researchers who have contributed significantly to Davis' experiment include Kenneth Hoffman, Daniel Harmer, John Evans, John Galvin, Keith Rowley, R.W. Stoenner and Bruce Cleveland at Brookhaven Lab and Kenneth Lande, Paul Wildenhain, James Distel, C.K. Lee, Alicia Weinberger and Timothy Daily at Penn. In addition, Jack Ullman from Lehman College of the City University of New York and Edward Fireman of the Harvard-Smithsonian Observatory participated in these experiments.