Some 70 years ago the Nobel Prize-Winning scientist Wolfgang Pauli proposed that whenever a neutron decayed into a proton plus an electron, some tiny unseen particle stole away with some of the energy. Enrico Fermi later christened this particle “neutrino,” meaning “little neutral one.” But studying this elusive creature has been difficult. Neutrinos carry no charge and only interact with other particles via the weak nuclear force. They can pass through a huge thickness of matter without being affected by it. While they were finally detected in the 1950s, their properties remained a mystery. Three kinds of neutrinos were discovered by their different interactions with matter, yet why nature needed exactly three types is still not understood. Why not just one, or two?  Do neutrinos have mass? For a long time it was believed that neutrino mass was exactly zero. The Standard Model, the present theory of particle interactions, assumes such. But accumulated experimental observations indicate that neutrinos can be massive, yet with a mass so tiny it challenged experiments for several decades. Beyond that, with three types of massive neutrinos it is possible that each type can be converted into another type. This phenomenon is called neutrino flavor oscillation and is possible only if all three types (flavors) of neutrino have different masses.  Although the existence of neutrino oscillations has not been established with 100 percent confidence, evidence from the detection of solar, atmospheric and accelerator-produced neutrinos indicate its possibility. Deep in the mountains of Japan, UT’s Dr. Yuri Kamyshkov and his colleagues work to reveal the mysteries of these fundamental particles. What they find will have significant implications for fundamental physics. The Solar Neutrino Problem In 1959 scientists Clyde Cowan and Fred Reines proved the neutrino did in fact exist. A decade later, experiments designed to detect electron neutrinos produced by the thermonuclear energy of the sun reported that less than half the predicted neutrinos were observed. This became known as the “solar neutrino problem” and prompted the theory that the missing electron neutrinos may have transformed into another type (flavor); the tau or muon neutrino. Experiments sprang up around the world to test this hypothesis. In 1982 the University of Tokyo began building the Kamiokande detector in Kamioka, a city of 12,000 in Japan’s Northern Alps. The experiment was built deep under Mount Ikenoyama, in an unused portion of the Kamioka Mine. “It attracted scientists to do their underground experiments because it has horizontal tunnels,” which makes moving people and equipment much easier, Dr. Kamyshkov explained. Neutrinos are difficult to intercept because they so rarely interact with other particles. Because they have no charge, they will penetrate deep into the earth, where scientists can catch them more easily with highly sensitive detectors. The earth’s rotation also varies the distance from the detector to the sun and allows researchers to test the neutrino oscillation hypothesis for different regions of neutrino mass difference. The Kamiokande (Kamioka Nucleon Decay Experiment) was built to detect the decay of the proton (nucleon), but instead made several interesting measurements with neutrinos. The detector required 3,000 tons of pure water and 1,000 photomultiplier tubes. The tubes collected a pale blue light called Cherenkov Light, which is emitted by charge particles coursing through the water detector faster than the speed of the light in the water. The Kamiokande experiment confirmed the dearth of neutrinos from the sun, detected atmosperic neutrinos and successfully detected neutrinos from a supernova explosion in 1987. The success of the first experiment evolved into Super-Kamiokande, a next-generation detector using 50,000 tons of pure water and 11,200 photomultiplier tubes. Completed in 1996, the Super-K experiment reported strong evidence for neutrino oscillation for atmospheric-produced neutrinos, which result from the decay of particles as they fall to earth in atmospheric cosmic ray showers. The significance in the findings is that if they oscillate, neutrinos must have non-zero mass. Solar neutrino data from Kamiokande and Super-K experiments, together with data obtained from other experiments exploring various neutrino oscillation possibilities, led experimentalists to conclude that oscillations could be responsible for the dearth of solar neutrinos if the mass difference between different types of neutrinos is contained within a certain region called the large-mixing-angle (LMA) MSW solar neutrino solution. (MSW stands for Mikheev, Smirnov and Wolfenstein who first considered certain resonance mechanisms of solar neutrino oscillations.) The problem with this, the only remaining solution of the solar neutrino puzzle, is that it is practically impossible to test it with solar neutrinos. KamLAND: The Next Step Japanese physicists decided to test the LMA solution region with anti-neutrinos from nuclear plant reactors. The Tohoku University group proposed a third generation of experiments at Kamioka mine called KamLAND: the Kamioka Liquid Scintillator Anti-Neutrino Detector. “This experiment was created as an idea in 1994 and was approved in 1997,” Dr.Kamyshkov said. “KamLAND is the largest detector built so far to detect reactor neutrinos.” KamLAND’s principal goal will be to investigate the possibility of neutrino oscillations by studying the flux and energy spectra of neutrinos produced by Japanese commercial nuclear reactors. The detector and the reactor neutrino sources are ~100 miles apart, just within the limits of making the experiment work. “This distance is just right to observe the oscillation effect in the LMA region with the man-controlled source of neutrinos. Thus, neutrinos produced on the Earth will help to solve the neutrino problem in the sun,” Dr. Kamyshkov said. The heart of KamLAND is “the sphere,” a stainless steel tank measuring 18 meters in diameter. It will contain a kiloton of active very-high-purity mineral-oil-based liquid scintillator in a transparent balloon, surrounded by two kilotons of passive mineral oil buffer that play the role of radioactivity shield. The outside of the tank is further shielded by two kilotons of pure water. Unlike previous experiments at Kamioka that used water-filled detectors and Cherenkov light, KamLAND’s scintillator will be sensitive enough to detect energies of neutrino interaction products within a fraction of a mega electron volt. The underground location and shielding will provide the very low background needed to detect rear neutrino interactions. “We expect signal-to-noise ration of 20 to 1,” Dr. Kamyshkov said. KamLAND is international in scope, with collaborators from Lawrence Berkeley National Laboratory, CalTech, Drexel University, Stanford, UT and Tohoku University, to name a few. Tohoku University in Sendai has taken the lead on the project, with Japanese researchers orchestrating their efforts through the Institute of Neutrino Physics. While Japan has four professors and 25 graduate students and post-docs working on the experiment, the U.S. has about 25 major professors with 25 graduate students and post-docs. Japan’s part is well-funded with about $25 million. Although Dr. Kamyshkov and other U.S. researchers wanted to join the collaboration upon its approval, it took three years to win funding from the U.S. Department of Energy. They finally succeeded in winning $4 million, which is administered by the University of California at Berkeley. “The U.S. contribution to KamLAND is relatively small,” Dr. Kamyshkov said, “but we were glad to contribute because our contribution is important for the success of this project in Japan.” Scintillation light in KamLAND will be detected with 2,000 photomultipliers. The Japanese contingent supplied 1,300 photomultipliers, with the UT group refurbishing and installing the remaining old ones from the Kamiokande experiment. To install the photomultipliers, the sphere was filled with water and a styrofoam floor was constructed. Floating on the false floor while trying to work presented a unique challenge for the professors and students involved. “If you’re not careful you can fall in, Dr. Kamyshkov said. “And some people did.” But putting the tubes in place was far from the most daunting task, as he went on to explain. “The installation was the simple part,” he said. “Cleaning was the difficult part.” It took 25 people four months to clean several thousand square feet using Kleenex napkins and alcohol. The surfaces had to be cleaned several times over until the napkins were clean. Undergraduates Thomas Gadfort and Stephen Wilson from UT spent most of last summer working on the project, as reported in an earlier issue of Cross Sections. Although their work wasn’t the most glamorous, the UT group, including Dr. Kamyshkov, Dr. Yuri Efremenko, Dr. Bill Bugg, Dr. Hans Cohn, post-doc Achim Weidemann and engineers Steve Berridge, Brad Dallas and Roger Gearhart, did their part with time to spare. “We were the first team to finish our assignment,” Dr. Kamyshkov said. Now come the actual oscillation studies, which begin in the fall and will take a few years. Researchers will investigate the large mixing angle of oscillation and will find the answer to the long-standing solar neutrino problem. “It is no-loser game; whatever we get will have an important impact on modern science,” Dr.Kamyshkov said. If their work shows that neutrinos do not oscillate, solar models will need major repair. If they find that neutrinos do in fact oscillate, the Standard Model, which says they have zero mass, will need to be corrected. But UT’s group doesn’t plan to stop there. In the future, they would like to use the low-threshold detector to see what’s going on inside the nucleus and search for the nucleon decay processes. Due to its low threshold and large mass of scintillator, the KamLAND detector will also be very efficient in detecting neutrinos from supernova explosions, should one occur within the next several years, as well as in the detection of antineutrinos from the Earth’s radioactive ores. Dr. Kamyshkov explained interested graduate students are always welcome to help with investigations of this nature. “We believe our students will have an opportunity to do a unique Ph.D. on problems of primary importance for fundamental physics,” he said. Cross Sections, Spring/Summer 2001 Issue, Contents Page UT Physics News & Notes Page UT Physics Home Page This page was last updated on June 25, 2001. Please send comments to cal@utk.edu.