Reprinted with permission from Physics Today.
Raymond Davis Jr, who won the 2002 Nobel Prize in Physics for first observing neutrinos emitted from the nuclear-fusion reactions in the core of the Sun, died on 31 May 2006 at his home in Blue Point, New York.
Davis’s observations of solar neutrinos not only were the first detection of neutrinos from outside the Earth, but they also experimentally demonstrated that the Sun was powered by the fusion of four protons into helium-4. The solar-neutrino flux measured by Davis was about one-third the expected flux based on the thermal energy emitted by the Sun. For many years, this discrepancy was known as the “solar-neutrino puzzle.” It was not a puzzle at all, but the first indication of neutrino flavor oscillations, the conversion of one neutrino species into another during the flight from the solar core to Earth.
Davis was born in Washington, DC, on 14 October 1914. In 1938 he received a BS in chemistry from the University of Maryland. After a brief period at the Dow Chemical Co, he pursued graduate work at Yale, where he received a PhD in physical chemistry in 1942. In 1945, after completing his World War II military service, Davis joined the Monsanto Chemical Co. Three years later, he joined the chemistry department at the newly established Brookhaven National Laboratory.
When Davis arrived at Brookhaven in 1948, the existence of neutrinos was still speculative. Looking for a challenging problem that could be addressed with physical-chemistry techniques, he decided to develop and construct a chlorine-based neutrino detector that had been described by Bruno Pontecorvo two years earlier. The concept was that neutrinos interacting with chlorine-37 would produce argon-37, an unstable gas that decays with a 35-day half-life. The use of a chlorine-containing liquid target, perchloroethylene (C2Cl4), made the extraction and measurement of even a small number of argon atoms possible.
After some preliminary tests at Brookhaven, Davis proceeded to construct a 4000-liter perchloroethylene detector at the Savannah River reactor in South Carolina. Concurrently, Frederick Reines and Clyde Cowan were installing a liquid-scintillator antineutrino detector there. The absence of neutrino interaction events in Davis’s chlorine detector at the same time that Reines and Cowan were seeing antineutrino interactions in their detector became the first clear indication that neutrinos and antineutrinos are distinct particles with different lepton numbers.
In 1958, just as Davis’s Savannah River experiment was ending, Harry Holmgren and R. L. Johnson of the Naval Research Laboratory measured the rate at which 3He and 4He combine to form beryllium-7. Until then it had been assumed that the only significant neutrino-producing channel in the solar fusion reactions was the fusion of two protons to form a deuteron, a positron, and a neutrino. With energies less than 440 keV, such p–p neutrinos were below the threshold of most detectors, especially the chlorine detector. Holmgren and Johnson’s measurement showed that the 7Be formation rate was about 100 times larger than previously assumed; a significant fraction of the solar fusion reactions would thus form 7Be, and some of the 7Be would then combine with a proton to form boron-8. Electron capture by 7Be produces 862-keV neutrinos, and 8B decay gives multi-MeV neutrinos; neutrinos from both reactions are thus above the 812-keV reaction threshold of37Cl. Both William Fowler of Caltech and Alastair Cameron of Atomic Energy of Canada Ltd urged Davis to use his detector to look for neutrinos produced by fusion in the solar core.
The question that Davis was about to address, the source of the Sun’s energy, first arose in 1860 when Lord Kelvin challenged the time scales in Charles Darwin’s On the Origin of Species by pointing out that the Sun’s gravitational potential energy could keep Earth warm for only about 30 million years. By the 1930s, with the recognition of the large amount of energy available from nuclear fusion reactions, it had become clear that the Sun was probably powered by the fusion of hydrogen into helium. However, no one had yet experimentally demonstrated that.
To shield the detector from cosmic rays, in 1962 Davis moved his Savannah River apparatus to a PPG Industries limestone mine in Barberton, Ohio, that was 2300 feet (700 meters) deep. The lack of a solar-neutrino signal and the high residual cosmic-ray background at Barberton demonstrated that a much larger and much deeper detector was required.
Together with John Bahcall, who was carrying out detailed calculations of the solar-neutrino flux and spectrum, Davis persuaded the Brookhaven administration to support and fund the construction of a 610-ton C2Cl4 detector. The Homestake Mining Co agreed in 1965 to excavate an experimental area at 4850 feet (1480 meters) deep in its gold mine in Lead, South Dakota, and so began a unique academic–industrial cooperation that lasted for 35 years.
The first Homestake results, announced in 1968, showed that the solar neutrino signal was less than 3 solar neutrino units (1 SNU = 10–36 interactions per target nucleus per second), compared with a predicted rate of about 8 SNU. As soon as those data became available, Pontecorvo speculated that the reduction in observed flux might be due to transitions of electron neutrinos into muon neutrinos (the tau neutrino had not yet been observed). It took almost three decades to experimentally verify Pontecorvo’s astute explanation.
When the Homestake experiment was being designed, the predicted solar neutrino flux was about 30 SNU, or about five conversions of 37Cl to 37Ar per day, and the expected cosmic-ray background in the 610-ton detector was about 1% of that rate. By the time the detector was in operation, the predicted signal had been reduced to 8 SNU, or slightly more than one 37Ar produced per day. The observed signal, 2.5 SNU, corresponded to one 37Ar atom produced every two days, with a cosmic-ray background of about 10% of that signal. Detecting the signal required a series of upgrades and improvements to the argon extraction, sample purification, counting system, and internal calibrations as well as increased rejection of background signals. Those improvements permitted Davis and his team, of which I was a part, to ultimately measure the solar neutrino flux with a statistical precision of 5%.
In 1984 Davis retired from Brookhaven and joined the University of Pennsylvania as a research professor. He remained in that position until his death. The experiment, which transferred with him from Brookhaven to Penn in 1984, continued for another 18 years. In addition to the Nobel Prize, Davis received the National Medal of Science, the Wolf Prize, the Pontecorvo Prize, the National Academy of Sciences’ Comstock Prize, and numerous other honors.
I collaborated with Davis for many years. Despite his renown, he always remained an extremely kind, well-liked, sensitive, and unusually modest person, happiest in his laboratory or with his family or his experimental collaborators. He will be missed.