Neutrino research and nuclear reactors go back a long way. The first neutrinos ever detected, in a 1956 Nobel Prize-winning experiment by physicists Clyde Cowan and Frederick Reines, emanated from a nuclear plant. But since then the relationship has cooled. In recent years, physicists trying to understand these elusive particles have targeted the high-energy neutrinos coming from space or from accelerators at high-energy physics labs because of the logistical problems of siting detectors at the right distance from enough reactors. Now the old flame is reviving in a Japan-United States collaboration that is building a massive underground snare for neutrinos emitted by Japan's nuclear power plants, which may hold the key to neutrino puzzles that are hard to unlock with other approaches.
Called KamLAND (Kamioka Liquid scintillator Anti-Neutrino Detector) and located beneath the mountains of central Japan, the detector will catch antineutrinos--the antimatter counterparts of neutrinos--from the country's 51 nuclear power reactors, as well as neutrinos directly from the sun. By studying how the neutrinos behave on their way to the detector, the project members hope to add to recent evidence that neutrinos--assumed until recently to be massless--do have mass. And because nuclear reactors produce neutrinos in similar energy ranges to those produced in the sun, KamLAND may help physicists explain the so-called solar neutrino deficit: the shortfall--by up to one-half--in the observed versus expected number of neutrinos from the sun. As a bonus, KamLAND could also yield clues to the distribution of radioactive elements in Earth's crust and how their decay contributes to the heat generated within the planet.
"KamLAND is a great experiment," says John Bahcall, a neutrino expert at the Institute for Advanced Study in Princeton, New Jersey. He is particularly excited about the ability to investigate the solar neutrino anomaly under what amounts to laboratory conditions, that is, knowing the conditions under which the neutrinos were created: "I never expected to live to see a laboratory test of a solar neutrino explanation."
KamLAND is a collaboration of three Japanese and 10 U.S. institutions, led by the Research Center for Neutrino Science of Tohoku University in Sendai. It uses a mine cavern occupied by Kamiokande, an earlier neutrino detector that has been succeeded by Super-Kamiokande, now running in a separate cavern in the same mine. The detectors made worldwide headlines last year by offering evidence of mass for at least one of the three flavors, or types, of neutrinos. Both of these detectors consisted of huge tanks of water outfitted with photomultiplier tubes, which pick up the flash of light generated when an occasional high-energy neutrino interacts with a proton in the water.
In contrast, KamLAND will use 1200 cubic meters of a liquid scintillator, a chemical soup that luminesces in response to neutrinos at lower energies. The liquid is confined in a 13-meter-diameter spherical balloon surrounded by layers of inert oil and water intended to cut background noise. With 1280 photomultiplier tubes to pick up the luminescence, KamLAND will cost an estimated $20 million, all coming from Japan's Ministry of Education, Science, Sports, and Culture (Monbusho). U.S. collaborators have asked the Department of Energy for $7.8 million to provide another 650 photomultiplier tubes, which would increase the sensitivity of the detector.
After it starts taking data in 2 years, KamLAND could bolster the neutrino mass claims from Super-Kamiokande. Those claims were based on signs that muon neutrinos made by cosmic rays colliding with air molecules were "oscillating," or changing into another type, on their way to the detector--something the laws of quantum mechanics forbid if both particles are massless. But Super-Kamiokande's case for oscillations had a weak point, because it relied in part on calculations of how efficiently cosmic rays should produce neutrinos in the atmosphere.
A number of so-called long-baseline experiments are attempting to remove the uncertainty by sending streams of neutrinos generated in accelerators through a near detector to a far detector so the neutrinos can be counted at both ends of their trip. These experiments, however, are aimed at the muon neutrino and energy ranges associated with atmospheric neutrino oscillation. KamLAND will focus on electron antineutrinos and the solar neutrino anomaly.
Atsuto Suzuki, a professor of physics at Tohoku University and head of the collaboration, says there's no need to place a detector at the source because the neutrino-producing reactions of commercial nuclear reactors are well understood. Instead, Suzuki and his colleagues will simply compare the number of electron antineutrinos detected at KamLAND with the number made by the reactors to determine whether some of them are oscillating into undetectable muon antineutrinos. "It's an amazing coincidence that Kamioka is just the right distance from these reactors" for the oscillations to show up if neutrinos do indeed have mass, says Stuart Freedman, a physicist at Lawrence Berkeley National Laboratory in California and one of the U.S. spokespersons for the collaboration.
Evidence of oscillations may shed light on the solar neutrino deficit. The current favorite explanation for the deficit is that the missing solar neutrinos, on their way to Earth, are oscillating into flavors not seen by the detectors. But theorists have four different scenarios for how this might happen. Suzuki says that KamLAND will be able to investigate all four, using the reactor neutrinos for one and its observations of solar neutrinos to examine the others. KamLAND also will be sensitive to critical neutrino energies that have eluded previous detectors.
In addition, KamLAND will be looking downward at Earth's own internal processes. The decay of radioactive isotopes of uranium and thorium is one of the major sources of Earth's internally generated heat, but nobody knows just how much heat this source produces or how the uranium and thorium are distributed within the crust and mantle. Fortunately, the low-energy antineutrinos generated by this decay fall within KamLAND's range of sensitivity, and their signature can be distinguished from reactor antineutrinos. By tracking neutrinos coming from the deep Earth to their origins, investigators hope to get a better fix on the nature and location of the planet's internal heat source.
Suzuki expects KamLAND to yield most of its useful data within the first few years, although the experiment is capable of running for a decade or longer. If it succeeds, it will add another link to the chain that connects neutrinos with nuclear reactors.