Koshiba, Masatoshi (1926-2020), was a Japanese physicist. He won a share of the 2002 Nobel Prize in physics for work that gave rise to a field of science known as neutrino astronomy. The prize was also awarded to the American physical chemist Raymond Davis, Jr., and the Italian-born American physicist Riccardo Giacconi.
In the late 1930’s, scientists developed theories of how a process called nuclear fusion creates the tremendous energy of the sun and other stars. According to the theories, that process also produces subatomic particles known as neutrinos. In an experiment that began in the 1960’s, Davis first detected neutrinos that undoubtedly came from the sun. In 1987, Koshiba began an experiment that confirmed Davis’s results, thus verifying the fusion theories.
Koshiba used a huge detector called Kamiokande that he had designed and built for another experiment. The detector was constructed in the early 1980’s in a zinc mine in Kamioka, Japan. In 1986, Koshiba finished a modification that enabled it to detect neutrinos.
The main part of the detector was a tank that held 2,360 tons (2,140 metric tons) of water. Attached to the tank’s inner surface were about 1,100 devices known as photomultiplier tubes. When a neutrino struck an atomic nucleus in the water, the nucleus emitted (gave off) an electron. As the electron traveled through the water, it emitted a cone-shaped pattern of light. The tubes sensed the light, and a computer identified its pattern and indicated the direction from which the neutrino had come. By analyzing light emitted by many electrons, Koshiba showed that the sun emits neutrinos.
Koshiba also verified Davis’s discovery that the number of a certain type of neutrino detected on Earth is less than the number predicted by the American physicist John N. Bahcall. Physicists currently account for the “missing” neutrinos by saying that neutrinos can change: There are three types of neutrinos. All the neutrinos that leave the sun are of one type, but two-thirds of them change their type on the way to Earth.
Koshiba needed to use an enormous amount of water because neutrinos rarely interact with matter. Neutrino detectors are built underground because other particles can produce the same effects used to identify neutrinos. The other particles are plentiful at and near Earth’s surface, but most cannot penetrate far into the ground.
In 1987, the Kamiokande device detected neutrinos that had been emitted by a supernova about 170,000 years ago. A similar device in the United States confirmed this discovery.
Koshiba completed a larger detector, called Super-Kamiokande, in 1996. That detector has shown that neutrinos can change their type as they travel from the sun to Earth. To do so, neutrinos must have mass (an amount of matter). Neutrinos were once thought to be massless, so the discovery of neutrino mass—though the amount of mass is unknown—is extremely important to scientists’ understanding of subatomic particles.
Masatoshi Koshiba was born in Toyohashi, Japan, on Sept. 19, 1926. He studied at the University of Tokyo and, in 1955, received a Ph.D. degree from the University of Rochester in New York. In 1970, he became a professor of physics at the University of Tokyo. He retired in 1987. Koshiba died on Nov. 12, 2020.
See also Astronomy (Neutrino astronomy) ; Davis, Raymond, Jr. ; Neutrino .
