Raymond Davis Jr., 91; Won Nobel for Work Showing Sun Is Powered by Nuclear Fusion
Raymond Davis Jr., who won the Nobel Prize in physics in 2002 for capturing solar neutrinos in a tank deep underground in a South Dakota mine, thereby providing the final, conclusive evidence that the sun is powered by nuclear fusion, died Wednesday at his home in Blue Point, N.Y.
He was 91 and had been suffering from Alzheimer’s disease for at least four years.
His achievement in capturing the wispy, extraordinarily elusive solar neutrinos was, according to the Swedish Royal Academy of Sciences, “considerably more difficult” than sifting through the entire Sahara Desert to find a few key grains of sand.
When Davis received word of his prize, he said that he was “embarrassed” to be singled out because the work had required the efforts of a large number of physicists, chemists and others, like virtually any other major physics project undertaken today.
Colleagues, however, noted that Davis provided the key inspiration for the project and was the driving force in keeping it alive and thriving for 30 years.
The first hints that the sun was nuclear-powered came in the 1920s when experiments showed that a helium atom, which contains two protons and two neutrons, has less mass than four hydrogen atoms -- essentially four protons.
British astrophysicists concluded that the fusion of four hydrogen atoms into a helium atom in the interior of the sun could release substantial amounts of energy, plus two neutrinos -- small particles that travel at nearly the speed of light, have little or no mass and no charge and that interact only extremely rarely with other matter.
Researchers calculated that only one in a trillion solar neutrinos that reached Earth would actually strike an atomic nucleus; the rest simply passing through unnoticed. The vast majority of researchers thought detecting them would be impossible.
Davis was virtually the only one who thought otherwise. He based his work on calculations by Italian physicist Bruno Pontecorvo who showed that if a neutrino struck an atom of chlorine, the target atom would be converted into radioactive argon, which could be detected.
Davis’ first attempt at detecting the elusive particles involved placing a large tank of perchloroethylene -- commonly used as dry-cleaning fluid -- 2,300 feet underground in the Barberton Limestone Mine in Ohio in 1961. Burying the detector deep beneath the Earth’s surface was supposed to shield it from cosmic rays and other sources of radiation; but some cosmic rays leaked through at this depth, overwhelming any potential signal from neutrinos.
Later that decade, Davis installed a 100,000-gallon tank of perchloroethylene 4,850 feet below the ground in the Homestake Gold Mine in Lead, S.D. There, too, his initial results were unrewarding and other researchers scoffed at his efforts, arguing that he could not even detect the small number of neutrinos expected.
To prove them wrong, Davis synthesized 100 atoms of radioactive argon, added them to the perchloroethylene, then successfully re-extracted them.
Eventually, observation techniques were refined and the detector began observing neutrinos. Over the 30 years of experimentation, about 2,000 neutrino events were observed, demonstrating conclusively the occurrence of nuclear fusion in the sun.
But the number of neutrinos was only about a third of the total expected by scientists, creating what came to be known as the “solar neutrino problem.” Ultimately, other researchers concluded that about two-thirds of the solar neutrinos changed into a different type of neutrino during their passage to Earth -- a type that could not interact with chlorine.
Those neutrinos were ultimately also observed, but no one would have looked for them had Davis not demonstrated that neutrinos could be detected in the first place.
Raymond Davis Jr. was born on Oct. 14, 1914, in Washington, D.C., the son of a self-educated man who became a prolific inventor while working at what was then called the National Bureau of Standards. Davis often cited his father’s influence in leading him to the design of his own experimental equipment.
He received bachelor’s and master’s degrees in chemistry from the University of Maryland and a doctorate in physical chemistry from Yale University in 1942. Upon graduation, he entered the Army and spent most of the war years observing chemical weapons tests at the Dugway Proving Ground in Utah.
After the war, he worked for Monsanto until 1948, when he was invited to join the newly created Brookhaven National Laboratory on Long Island, where he spent the rest of his active career. Shortly after his arrival, he met his future wife, Anna Torrey, who worked in the biology department at Brookhaven.
They married that year and lived in the same house on Long Island for more than 50 years.
Upon his arrival at Brookhaven, Davis went to the chairman of the chemistry department and asked what he was expected to do. “To my surprise and delight,” he later wrote, “I was advised to go to the library, do some reading, and choose a project of my own, whatever appealed to me. Thus began a long career of doing just what I wanted to do and getting paid for it.”
He spent most of that career studying neutrinos but got sidetracked from time to time, studying radioactive isotopes in meteorites and examining rocks returned from the moon.
During the processing of the rocks from Apollo 12 in Houston, one of the glove boxes that researchers were using leaked “and I had the interesting experience of being quarantined with the astronauts and a few other unlucky scientists for two weeks until it was clear that we were not infected with any lunar diseases.”
He officially retired from Brookhaven in 1984 but wasn’t ready to give up measuring solar neutrinos, so he transferred administration of the project to the University of Pennsylvania and became a research professor there.
The work continued until the late 1990s, when the Homestake Mine closed.
On the occasion of Davis’ Nobel Prize, the late astrophysicist John Bahcall said that Davis was “not only an extraordinary scientific person, but also an extraordinary human being. Ray treats the janitor in the laboratory with the same friendliness and respect that he does the most senior scientist. And for that, he is loved by his colleagues.”
Davis was an avid outdoorsman and rifleman in high school and college and won several medals for marksmanship. He abandoned the pursuit, however, “having concluded that the world would be a better place with fewer sharpshooters.”
Once ensconced in Long Island, Davis built his own boat, a 21-foot sloop called the Halcyon, and was an active member of the South Bay Cruising Club. The durable boat is now plying the bay under its third owner.
Davis is survived by his wife Anna; three sons, Andrew of River Forest, Ill., Roger of Center Moriches, N.Y., and Alan of Seattle; two daughters, Martha Kumler of Honeoye Falls, N.Y., and Nancy Klem of Webster Grove, Mo.; and 11 grandchildren.
