Professor Raymond Davis

Raymond Davis, physical chemist: born Washington, DC 14 October 1914; staff, Monsanto Chemical Co 1945-48; Senior Scientist, Brookhaven National Laboratory 1948-84; Adjunct Professor of Astronomy, University of Pennsylvania 1984-2006; Nobel Prize for Physics 2002; married 1948 Anna Torrey (three sons, two daughters); died Blue Point, New York 31 May 2006.

The Nobel laureate Raymond Davis was the chemist who first detected the elusive neutrinos produced by the Sun. At his death, he was Research Professor at the University of Pennsylvania and a research collaborator in chemistry at Brookhaven National Laboratory in Upton, New York.

Davis was admired throughout the scientific community for his pioneering experiment on neutrinos, conducted a mile underground in the Homestake Gold Mine in Lead, South Dakota, and for his patient and humble defence of a result that, for many years, seemed to contradict the standard theories of stellar evolution and particle physics.

Raymond Davis's Homestake experiment started one of science's most intriguing mystery stories, the case of the missing solar neutrinos. Neutrinos are particles produced in the core of the Sun as a by-product of the nuclear reactions responsible for solar energy. Unlike most other sub-atomic particles, neutrinos can pass through solid matter almost as if that matter were transparent. Consequently, neutrinos travel from the core of the Sun to the Earth at the speed of light, unaltered by the Sun's cooler outer envelope. Davis recognised that, by measuring the number of neutrinos arriving at Earth, he could determine conditions in the solar core, including the Sun's central temperature. Theory predicts that that temperature would be about 10 million degrees.

Born in 1914, Davis grew up in the Washington, DC, area, attending public schools and obtaining a degree in chemistry from the University of Maryland in 1938. After a year of working for Dow Chemical Company, he returned to school, earning his master's degree from Maryland and his doctorate in physical chemistry from Yale University in 1942. He then entered the army as a reserve officer, working at the Dugway Proving Ground in Utah until his discharge in 1945.

Following the Second World War, Davis spent three years with the Monsanto Chemical Company before joining the Chemistry Department of the newly established Brookhaven National Laboratory, remaining there until his retirement in 1984, when he joined the faculty at Penn.

According to his Nobel Prize autobiography, Davis was inspired to do neutrino experiments by a scientific paper he read shortly after his arrival at Brookhaven. He developed a neutrino detector based on a suggestion that the Italian theorist Bruno Pontecorvo had made a few years earlier. Neutrinos can interact with atoms of chlorine, converting them to argon atoms. Davis's first detector consisted of a 1,000-gallon tank of carbon tetrachloride, the chlorine source, and chemical procedures to extract from the tank the few argon atoms that might be produced in rare neutrino reactions. He also began the development of miniaturised devices for counting the argon atoms, exploiting the fact that they are radioactive.

Davis's initial experiment was performed at the Brookhaven research reactor, then repeated later at one of the larger reactors located at Savannah River, South Carolina. Neither effort succeeded. It was not fully established at the time that nuclear reactors produce antineutrinos, not neutrinos like those produced by the Sun. Pontecorvo's chlorine detection scheme is only sensitive to neutrinos. Indeed, Davis's experiments helped to confirm that reactor antineutrinos and solar neutrinos must be different.

Following the Savannah River experiments, Davis began to think about detecting neutrinos from the Sun. Davis mounted a 1,000-gallon solar-neutrino pilot experiment in the Barberton limestone mine, near Akron, Ohio, in 1960, but it was expected at that time that the Sun was too weak a neutrino source to make neutrino detection feasible. This expectation changed by the end of the decade with the development of more refined models of the Sun that predicted significant numbers of higher energy solar neutrinos, neutrinos that could be more readily detected by Davis's chlorine detector.

In 1964 Davis, supported by his close friend and theoretical colleague John Bahcall, proposed a full-scale solar neutrino detector, 100 times larger than the pilot detector. Because the solar neutrino signal could be obscured by the interactions of cosmic rays in the detector, Davis knew that the detector would have to be placed very deep underground, beyond the reach of cosmic rays. The Homestake Gold Mine, a historic mine in the Black Hills that was established shortly after General Custer lost his battle with the Sioux, was identified as the site.

The Homestake Corporation excavated a large cavity for Davis's steel tank in 1965, on the mine's 4,850ft level. The following year it was filled by 100,000 gallons of chlorine-bearing cleaning fluid, a volume equivalent to 10 railway tank cars. Davis and his experiment colleagues, Don Harmer and Ken Hoffman, announced their first results in 1968 - a rate of neutrino interactions less than half that expected.

This puzzling result began the field of solar neutrino astronomy. The chlorine experiment continued to operate for 35 years, determining with increasing accuracy the solar neutrino rate, a rate that proved to be about one-third that predicted by theory. Other experimentalists joined the field, mounting experiments to verify and extend Davis's results. Some suspected that the result reflected some misunderstanding of solar theory, others some flaw in experimental procedures.

Only recently, with observations made in a new solar neutrino observatory mounted in Canada's Inco nickel mine, in Sudbury, Ontario, was the mystery finally resolved: the missing solar neutrinos were shown to have transformed into a new type on their way from the sun, a type that could not interact with chlorine. This transformation, called neutrino oscillations, requires neutrinos to have properties not allowed in the so-called "standard model" of particle physics. One such property is a non-zero mass.

The discovery of neutrino mass has profound implications for physics, including the existence of "dark matter" in our universe. Space is filled with an invisible sea of neutrinos left over from the Big Bang. Davis's work, and that of subsequent neutrino experimenters, has established that the total mass in this sea of neutrinos is comparable to that of all of the visible stars.

For his patient and careful experiments, Davis was frequently honoured. President George W. Bush presented him with the National Medal of Science in 2001. In 2002 he shared the Nobel Prize in Physics with Masatoshi Koshiba of the University of Tokyo and Riccardo Giacconi, now Professor at Johns Hopkins University in Maryland. In 1982 Davis was elected to the National Academy of Sciences.

Davis's pioneering work and modest demeanour won him many friends. Ken Lande, a long-time colleague at the University of Pennsylvania, explained,

Ray was not only an outstanding scientist but also a kind, caring, sensitive, and warm person. Having him as a friend and colleague for three decades was extraordinarily enriching.

Wick Haxton