Ray Davis Jr.
December 10, 2002—The award of a share of the 2002 Nobel Prize for Physics to Ray Davis Jr. on December 10 for "the detection of cosmic neutrinos" culminates a remarkable program of research supported by the U.S. Department of Energy (DOE) and its predecessor agencies, beginning with the Atomic Energy Commission in 1947. The DOE's Office of Science is the principal supporter of physical sciences in the United States, including the construction and operation of major research facilities, and its support has made possible a large number of discoveries in neutrino research.
It took a quarter of a century to finally "observe" the neutrino after Wolfgang Pauli proposed in 1930 that a very unusual particle must exist to carry away the energy that was missing when an atomic nucleus emits an electron. This hypothetical particle had to have very elusive qualities or it would have been observed along with the electron.
Later, Enrico Fermi gave this particle the moniker of "neutrino" (little neutral one) because the neutrino could have no electric charge, little or no mass, and must interact so weakly that it would be nearly impossible to detect. This weak interaction turned out to be one of the fundamental forces in nature.
Finding such a ghost of a particle was not easy: it was a billion times less likely than an electron to interact with matter. Ray Davis and others tried to detect neutrinos with carbonate chloride solutions placed near nuclear reactors, hoping that a chlorine nucleus struck by a neutrino would be transformed into radioactive argon nucleus, and the argon atoms could then be chemically separated and counted. Unfortunately, a nuclear plant delivers only antineutrinos, which, as this experiment indicated, are distinct from neutrinos and do not induce this reaction.
In 1956, Fred Reines and Clyde Cowan found a way to detect antineutrinos coming from a nuclear reactor, using a mixture of water and cadmium chloride. The antineutrino interacted with a proton to produce a positron and a neutron; both were observed and the neutron was always detected 15 microseconds after the positron, giving an unmistakable signature. Reines won the 1995 Nobel Prize for this discovery.
In 1946, it was suggested that neutrinos might come in two flavors, one that is always associated with with an electron and another that is associated with a muon (a "heavy electron" that had been discovered in 1937). It was later proposed that a neutrino might actually travel as a mixture of these two flavors, oscillating between the two as it moved along.
In 1962, an experiment at DOE's Brookhaven National Laboratory showed that neutrinos associated with muons were indeed distinct from electron neutrinos, and Leon Lederman and Melvin Schwartz won the 1988 Nobel Prize for this discovery.
The Homestake mine tank, 20 feet in diameter and 48 feet long, held 100,000 gallons of perchloroethylene and was located 4,900 feet below ground surface.
Meanwhile, Ray Davis had shown persuasively that neutrinos were distinct from antineutrinos. Realizing that the sun emits neutrinos rather than antineutrinos, he became interested in using the chlorine-argon technique to detect solar neutrinos. In 1964, he proposed using a large tank of dry cleaning fluid (perchloroethylene, which contains chlorine) to detect electron neutrinos coming from the sun. By counting the radioactive argon atoms produced by neutrinos interacting with chlorine atoms, Davis could determine the number of solar neutrinos reaching earth. Only one or two neutrinos per day interacted in this way, though, so it took a long time to accumulate enough data. The experiment was not easy: he would wait a few months while any argon atoms formed in the tank were collected inside a special charcoal filter, then remove the filter from the tank in a South Dakota gold mine, fly it to Brookhaven, and count the number of radioactive argon atoms.
The experiment showed that only one third as many neutrinos as expected reached earth from the sun, and this meant that either our calculations of the fusion processes in the sun were wrong, or neutrinos did oscillate: electron neutrinos leaving the sun were changing to other flavors before they reached earth. The idea seemed very strange-as if you could throw a blackberry at someone, and along the way it turned into a strawberry before it hit its target.
Martin Perl, a professor at Stanford, shared the 1995 Nobel Prize in Physics for “for the discovery of the tau lepton” at SLAC.
In 1977, another heavy electron called the tau was discovered at the Stanford Linear Accelerator Center (SLAC). The tau would have its own sibling neutrino, making three flavors, so maybe the strawberry could then change to a blueberry, and that fickle berry would just keep oscillating among the three flavors as it moved along-electron, muon, and tau.
If neutrinos do change flavors, then at least one of them must have mass. Physicists had hypothesized that neutrinos have exactly zero mass, an assumption that simplified the theory and was consistent with all evidence until Davis' observation. After his "solar neutrino puzzle" appeared, other experiments were mounted to study neutrino oscillations in many ways, using neutrinos from the sun or produced by accelerators.
The DOE Office of Science has supported research in this challenging field for decades, staying the course in spite of many difficulties and slow progress. State-of-the-art particle accelerator facilities are a major strength of the Office of Science, providing proton or electron beams at the highest available energies and intensities, and using sophisticated apparatus to detect and measure the properties of particles produced by collisions of the beams.
Neutrino investigations were among the first experiments carried out with the high energy proton beams at Fermi National Accelerator Laboratory when it was completed in 1971 and were also conducted with the intense proton beams at Argonne and Brookhaven national laboratories.
The Office of Science continued to support a substantial program of research in neutrino physics for three decades, with steadily improving knowledge of their interactions. The last few years have seen major advances.
In 2000, the tau neutrino was directly observed for the first time, at Fermilab. In another Fermilab experiment, measurements of neutrino interactions in 2001 indicated a tiny but significant discrepancy in the current theory of the weak interaction. Such small effects are often the harbinger of new advances in physics.
Experiments at underground detectors, SuperKamiokande in Japan and the Solar Neutrino Observatory (SNO) in Canada, have confirmed Davis' finding of too few neutrinos reaching earth from the sun and have strongly indicated that neutrinos do oscillate into other flavors. DOE-supported physicists participate in SuperK and in SNO, which is sensitive to all three flavors of neutrinos.
Americans provided the precise timing for the first Kamiokande experiment in Japan, which allowed another 2002 Nobel Prizewinner, Masatoshi Koshiba, to use that detector to observe neutrinos from a supernova in 1987 and from the sun in 1988.
First results from KamLAND, an underground detector in Japan, indicate that antineutrinos from reactors also undergo flavor oscillations. (see KamLAND, the Case of the Vanishing Neutrinos).
Two new Fermilab accelerator experiments—MiniBooNE and MINOS—will study neutrino oscillations. The MINOS experiment uses two detectors, one at Fermilab and one now being assembled in a deep mine 450 miles away in northern Minnesota. It will follow up on the SuperK results that muon neutrinos generated by cosmic rays in the atmosphere oscillate into other flavors. The MiniBooNE experiment at Fermilab has recently started taking data, following up on indications from a previous experiment at DOE's Los Alamos National Laboratory that muon neutrinos can change into electron neutrinos. Physicists are investigating the feasibility of providing extremely intense beams of neutrinos with dedicated accelerators called neutrino factories.
Neutrinos have played an important role in the universe since the Big Bang. They are extremely abundant (hundreds of them in every cubic centimeter of space) and thus, even with their tiny masses, must constitute a small part of the dark matter that appears to form about one third of the universe and affects the motions of galaxies. They play a key role in the nuclear fusion burning of stars and their explosions as supernovae.
The DOE Office of Science program of neutrino research over the years has provided fundamental insight into the nature of the matter, energy, and the universe.—by Neil Baggett
Story of the Neutrino: Historic Events in Neutrino Physics
The First Detection of the Neutrino by Frederick Reines and Clyde Cowan
The Scientific Background to the Discovery of the Tau Lepton
MiniBooNE Web page
Upward muon marks a MINOS first
Live video webcasts of the December 10, 2002, Nobel Prize Awards ceremonies in Oslo, Norway and Stolkholm, Sweden
Pauli and the Neutrino
Discovery of the Muon Neutrino
Discovery of Cosmic Neutrinos