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New detector to monitor neutrinos from nuclear reactors

A new detector, now nearing completion in central Japan, will help physicists determine whether elementary particles called neutrinos can oscillate from one "flavor" to another. And in one of the first practical applications of neutrino physics, technologies developed for the detector may make it easier to keep track of the world's supply of nuclear fuel.

KamLAND (the Kamioka Liquid scintillator Anti-Neutrino Detector), which will measure neutrinos emitted by Japan's nuclear reactors, is expected to start collecting data next month. If it finds evidence for neutrino oscillation, it will have helped solve one of the puzzles left unsolved by the Standard Model, the dominant theory describing elementary particles and the forces that govern them.

One of the leaders of the KamLAND project is Giorgio Gratta, an associate professor of physics at Stanford, who spoke about the project Nov. 7 at the New Horizons in Science Briefing, a workshop in Tempe, Ariz., organized by the Council for the Advancement of Science Writing.

Gratta and his collaborators hope to find further evidence that neutrinos, which interact so infrequently with other particles that they are almost impossible to detect, have mass. Only if neutrinos have mass would it be possible for them to oscillate from one of the three neutrino flavors electron, muon or tau to another.

For the past several years, a group of physicists from Japanese and American universities have been building KamLAND on the ashes of an older neutrino detector named Kamiokande. Kamiokande made groundbreaking discoveries in the 1980s, but was decommissioned in the mid-1990s when a larger, more sensitive detector was built nearby. Kamiokande's old chamber, a three-story, cylinder-shaped room buried 1,000 meters underground, was then taken over by KamLAND.

The motivation for KamLAND comes, in part, from observations made at Super-Kamiokande, the neutrino detector that rendered Kamiokande obsolete. Researchers at Super-Kamiokande, who were monitoring electron neutrinos emitted by the sun, detected only one-third to one-half of the neutrinos predicted by models of fusion in the sun. That meant one of two things: Either something was wrong with the models, or the sun's electron neutrinos were somehow turned into muon or tau neutrinos, which are undetectable by Super-Kamiokande, between their generation in the sun and detection on Earth.

If KamLAND can show that neutrinos generated in nuclear reactors oscillate just like neutrinos from the sun, it will strengthen the growing consensus among particle physicists that neutrinos have mass. Because the fusion reactions that take place in nuclear reactors are well understood, KamLAND's researchers will know exactly how many neutrinos they should be seeing, and any deviation from the expected number will be strong evidence that neutrinos naturally oscillate.

Once KamLAND is up and running, researchers expect to see about 2 reactor neutrinos a day, so it may take as long as three years before they have collected enough data to tell whether neutrinos are oscillating, says Gratta.


Filling a 1-million-gallon bathtub slowly

At the heart of KamLAND is a 13-meter-diameter plastic balloon that contains about 1,000 tons of scintillator a mix of purified mineral oil, a solvent and another chemical called 2,5-diphenyloxazole, or PPO. When a neutrino strikes the mixture of solvent and PPO, the PPO fluoresces, or scintillates, giving off a tiny flash of light. Those flashes can be detected by the 2,000 photomultiplier tubes arranged on the walls of the metal sphere surrounding the balloon. The sphere, in turn, is contained inside the cylinder-shaped Kamiokande chamber, which is filled with water.

This summer, researchers at KamLAND spent four months slowly filling the fragile balloon with scintillator. Because the photomultiplier tubes, which are located outside the balloon, have to be able to detect tiny amounts of light produced when neutrinos hit the scintillator, the balloon is made of transparent plastic only a tenth of a millimeter thick about the width of a human hair. A break in the balloon could have cost millions of dollars in lost scintillator and damaged equipment, so researchers filled it slowly to avoid a disastrous burst.

"It was a painfully slow process," says Nikolai Tolich, a Stanford graduate student who joined the team in Japan in September. Tolich built KamLAND's trigger system, a crucial piece of electronics that signals when flashes of light are detected. He and Jason Detwiler, a Stanford graduate student who is on his second stint at KamLAND, are now working with a team of Japanese and American physicists to prepare the detector for initial testing later this month.

KamLAND is located in the Mozumi Mine near the center of Honshu, Japan's largest island. A crucial factor in the decision to locate the project there was the need for a high concentration of nuclear reactors at the right distance about 150 to 200 kilometers from the detector to detect oscillations of the electron flavor of neutrino. Only Japan, Europe and the East Coast of the United States have a high enough concentration of reactors, and of the three locations, only Japan's reactors are arranged in a way that maximizes the chance of detecting neutrino oscillation. Because the country has few inland rivers that can be used to cool nuclear reactors, all of its reactors are located on the coasts in a rough circle at the center of which, as luck would have it, is the old Kamiokande site. The reactors that provide KamLAND's neutrinos also generate "as a byproduct," jokes Gratta about 60 gigawatts of power, or 4 percent of the world's total power supply.


'Applied neutrino physics'

People tend to laugh when he talks about "applied neutrino physics," says Gratta, because neutrinos are so difficult to detect and manipulate. But despite their elusive nature, neutrinos are already making their way into mainstream applications.

The first is a method for monitoring commercial nuclear reactors to account for plutonium that could be used in weapons if it were to fall into rogue hands. Currently, nuclear plant operators are forced to shut down reactors so inspectors can safely remove and examine the fuel. But shutting down a reactor for inspection costs operators as much as $1 million a day in lost revenue, says Gratta.

Sandia National Laboratories in Albuquerque, N.M., is exploring a KamLAND-inspired solution: a mini-detector that identifies the distinctive neutrino "fingerprint" of a reactor that is being used to produce weapons-grade plutonium. A test of Sandia's prototype is scheduled to begin early next year at the San Onofre nuclear power plant near San Diego. If the test is successful, mini-KamLANDs could be built and sold to reactor operators as a security precaution for as little as $100,000, says Gratta.

Another application is a side benefit of KamLAND's exquisite sensitivity. KamLAND will be one of the first detectors that can detect neutrinos produced by radioactive minerals in the earth, which are thought to account for as much as 40 percent of the earth's internal heat. The decay of uranium and thorium, which generates about 90 percent of that heat, produces neutrinos with distinctive energy patterns. By counting those neutrinos, KamLAND will allow geophysicists to measure the exact amount of uranium and thorium in the Earth for the first time.


Future projects

Although KamLAND is the biggest project that Gratta and his students are currently working on, other large-scale neutrino projects are on the horizon. Graduate students Sam Waldman and Jesse Wodin, for instance, are working on a detector that could be used to study a phenomenon called "neutrinoless double beta decay," which involves the radioactive decay of xenon into barium. If successfully detected, the phenomenon would provide a precise measurement of neutrino mass.

Another project would use an underwater microphone system built by the U.S. Navy in the Caribbean to monitor naval exercises. The idea of the proposed project is to use the microphones to monitor the ocean for the sound of collisions between high-energy cosmic neutrinos a trillion times more energetic than those detected by KamLAND and molecules of water. The amount of noise in the ocean may prove to be a problem, however; snapping shrimp, for instance, sound a lot like high-energy neutrinos. But if not, the project could turn hundreds of square miles of the Caribbean Sea into the world's largest neutrino detector.


Etienne Benson is a science writing intern at the Stanford News Service.


By Etienne Benson

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