Kamiokande Neutrino Telescope Begins Operation

Date 1986

The Kamiokande neutrino telescope, the first instrument to determine the direction and time of arrival of neutrinos from space, established that the Sun was the source of most of the neutrinos recorded in earlier detectors, demonstrating that the Sun generates its energy by nuclear fusion.

Locale Kamioka-cho, Gifu, Japan

Key Figures

• Masatoshi Koshiba (b. 1926), Japanese physicist
• Raymond Davis, Jr. (1914-2006), American chemist
• Wolfgang Pauli (1900-1958), Austrian theoretical physicist

Summary of Event

The Kamiokande neutrino telescope was designed to record the time, energy, and direction of arrival of subatomic particles called neutrinos, providing scientists with the first opportunity to associate neutrinos with a specific source in space. This was a significant improvement over earlier neutrino detectors, which recorded only the numbers of neutrinos that arrived over periods of weeks, providing no information on their source.

Only a few months after the Kamiokande neutrino telescope began operating in 1986, it made its first major discovery. In February, 1987, scientists detected a pulse of neutrinos from a supernova that occurred in the Large Magellanic Cloud, a small galaxy that orbits around our Milky Way galaxy. This was the first confirmed detection of neutrinos from an object outside Earth’s solar system.

The neutrino telescope consists of a large tank containing 2,140 tons of ultrapure water situated 1,000 meters (3,281 feet) below the surface in the Mozumi Mine of the Kamioka Mining and Smelting Company located in Kamioka-cho, Gifu, Japan. The tank of water is surrounded by detectors that record the faint pulses of light produced when neutrinos interact with the water. A typical neutrino can travel through water a distance of about ten times the distance from Earth to the Sun before interacting. A huge amount of water thus detects only a very small fraction of the neutrinos that pass through it.

Wolfgang Pauli, an Austrian physicist, first proposed the existence of the neutrino in 1930 to explain why the emission of electrons in radioactive decay appeared to violate the long-established physical principles of energy and momentum conservation. Pauli suggested that an undetected particle, now called the neutrino, carried off the energy and momentum required to achieve balance. The neutrino is extremely difficult to detect because it has no electric charge and almost no mass. Pauli said: “I have done a terrible thing. I have postulated a particle that cannot be detected.”

Given that science is based on the principle that an idea is not accepted until it has been verified by experiment, physicists began a careful search for Pauli’s elusive particle. It was not until 1956, when Frederick Reines and Clyde Cowan devised an experiment to detect the neutrinos produced by nuclear reactions at the Savannah River Plant in South Carolina, that Pauli’s elusive neutrino was found. Even then, although more than 1,000,000,000,000,000 neutrinos passed through their detector every second, so few interacted that they reported a detection rate of only 3 neutrinos each hour.

Nuclear reactions, which generate the same kind of energy as the Sun, were expected to produce abundant neutrinos. In the 1960’s, Raymond Davis, Jr., a chemist at Brookhaven National Laboratory, constructed the first detector designed to investigate the neutrinos from the Sun. His detector, located a mile underground at the Homestake Gold Mine in South Dakota, was a tank containing 615 tons of carbon tetrachloride. Most neutrinos simply passed through the tank, but rarely one would interact with a chlorine atom, transforming it into argon. Once every few weeks, Davis collected these argon atoms. By counting the argon atoms, Davis could determine the number of neutrinos that had passed through the tank. As the Sun was expected to be the dominant source of neutrinos, Davis’s detection of neutrinos indicated that the Sun generated energy by “fusion reactions,” a process in which two light atoms come together to become one heavier atom in a process that releases energy. Davis detected fewer neutrinos than predicted by the theory, however.

Masatoshi Koshiba and his team of researchers in Japan designed an improved neutrino detector, called a neutrino “telescope” because the instrument would record the time, energy, and direction of arrival of each neutrino. The Kamioka Underground Observatory, set up in 1983, was originally established to determine if the proton was unstable, decaying to other subatomic particles, using an instrument named Kamiokande, for the Kamioka Nucleon Decay Experiment.

An upgrade of Kamiokande, to enable it to detect neutrinos, was started in 1985. In 1986, the members of Koshiba’s team obtained their first neutrino measurements using the new detector. Because their instrument was sensitive to arrival direction and arrival time of the neutrinos, they were able to map the arrival direction, providing the first image of the Sun in neutrino light and confirming Davis’s observation of solar neutrinos. Once again, however, the number of neutrinos was only about one-third the amount predicted by the theory.

The Sun is not the major source of neutrinos in the universe. Astrophysics models indicate that there should be about 100 neutrinos per cubic centimeter left from the “big bang,” the event that occurred at the start of the universe, making the neutrino the most abundant type of particle in the universe. The low energy of these neutrinos, however, makes them undetectable in current detectors. If these neutrinos have no mass, as originally suggested by Pauli, then their presence has no significant consequences on the behavior of the universe. There are so many neutrinos, however, that if each has even a tiny mass, together they would contribute significantly to the mass of the universe. The rotation of galaxies indicates the presence of “dark matter,” matter that is undetectable by optical telescopes. If neutrinos have mass, they could contribute significantly to this dark matter.

Theoretical physicists who attempted to explain why the number of neutrinos detected from the Sun was one-third the expected mass proposed the idea that there might be three types of neutrinos and that a neutrino traveling through space would oscillate back and forth among the different types. Only the electron-type neutrino would be detectable in the solar neutrino experiments, so the number of neutrinos that could be detected would be only one-third the expected value. This same theory suggested the neutrino might have a very small mass.

The first major triumph of Kamiokande was the detection of a burst of neutrinos that arrived from a supernova explosion, called Supernova 1987A. Koshiba’s team detected ten neutrinos that arrived within fifteen seconds of each other and determined that their arrival direction was consistent with the Supernova 1987A explosion, which was observed through optical telescopes. These neutrinos, which had traveled 180,000 light-years from Supernova 1987A to Earth, had energies that differed by a factor of three. (A light-year is the distance light travels in a vacuum in one year, approximately 5.88 trillion miles, or 9.46 trillion kilometers.) Neutrinos of different energies should travel at different speeds if they have mass. The short difference in arrival time was interpreted as indicating that the mass of the neutrino was less than one-fifteenth that of the electron.

Kamiokande continued operating until February, 1995, and work on a much larger neutrino telescope, Super-Kamiokande, using 50,000 tons of ultrapure water, was completed in 1995. The first observations using Super-Kamiokande, made in April, 1996, showed that neutrinos change from one type to another as they travel through space, explaining the neutrino deficit observed in earlier experiments.

Models of cosmic-ray interactions suggest that twice as many muon neutrinos as electron neutrinos are produced in Earth’s atmosphere. The Super-Kamiokande instrument measured this ratio as about one to one, however. This result is consistent with electron and muon neutrinos having different masses, providing the best evidence that the mass of the neutrino is not zero.

Significance

Results obtained by neutrino detectors have had significant impacts on scientists’ understanding of energy generated by stars, particle physics, and cosmology. The neutrino sky map showing that the Sun was the source of most of the neutrinos detected by the Kamiokande neutrino telescope confirmed the theory that the Sun and other stars generate their energy through the process of nuclear fusion.

The Kamiokande observation of neutrinos from Supernova 1987A provided an upper limit on the mass of the neutrino. Its successor, Super-Kamiokande, provided the first clear indication that neutrinos are not massless particles, so their small mass coupled with their large abundance may account for much of the dark matter in the universe. Super-Kamiokande observations also demonstrated that a single neutrino oscillates among three different types of particles as it travels through space.

For their “pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos,” Koshiba and Davis shared the 2002 Nobel Prize in Physics.

Bibliography

Bahcall, John N., et al., eds. Solar Neutrinos: The First Thirty Years. 1994. Reprint. Boulder, Colo.: Westview Press, 2002. Collection of scientific papers reports the results of solar neutrino experiments.

Franklin, Allan. Are There Really Neutrinos? An Evidential History. Cambridge, Mass.: Perseus Books, 2001. A physicist uses the search for the neutrino to present an account of how progress is made in science.

Mann, Alfred K. Shadow of a Star: The Neutrino Story of Supernova 1987A. New York: W. H. Freeman, 1997. Well-illustrated nontechnical account of the explosion of Supernova 1987A includes a comprehensive discussion of the measurements made by the Kamiokande neutrino telescope.