Masatoshi Koshiba who won the Nobel Prize in Physics in 2002 for studies of the ghostly cosmic particles known as neutrinos, died on Thursday in Edogawa Hospital in Tokyo. He was 94.
His death was announced by the University of Tokyo, where he had been a longtime professor. No cause was given.
Dr. Koshiba, widely known as “Toshi,” was a driving force in molding high-energy physics in Japan as it emerged from a postwar cocoon in the latter part of the 20th century.
“He’s responsible for putting Japan on the map in a big way,” said one of his collaborators, Dr. Eugene Beier, a physicist at the University of Pennsylvania.
Colleagues described Dr. Koshiba as a man who could get things done. Starting in the 1970s, using part of the Kamioka zinc mine in Japan, he spearheaded the building of a series of enormous underground tanks filled with ultrapure water and lined with phototubes for the purpose of detecting neutrinos, which can float through ordinary matter like moonlight through a window.
Deep in that cavern, shielded from outside interference by thousands of feet or rock, neutrinos streaming from the Sun and elsewhere in the cosmos could be identified and some of the subtlest interactions of nature could be studied.
In 1987, one of these chambers, called Kamiokande, with its half-million gallons of pure water, recorded neutrinos streaming from a supernova explosion in a nearby galaxy, the Large Magellanic Cloud, confirming theories about how such explosions occurred and giving birth to a new field of neutrino astronomy.
This year, neutrino measurements at a successor chamber, Super Kamiokande, made headlines around the world for producing the first evidence that certain properties of neutrinos might explain why there is matter instead of empty space in the universe.
John Learned, a longtime collaborator and neutrino expert at the University of Hawaii, wrote in an email, “Koshiba represented for many the great tradition of the Samurai warrior in science, having the long view, fiercely competitive and amazingly competent.”
Masathoshi Koshiba was born on Sept. 19, 1926, in the coastal city of Toyohashi, in central Japan, the second child of Toshio and Hayako Koshiba. His mother died when he was 3, and his father, a military officer, married his deceased wife’s elder sister. They had two sons.
Dr. Koshiba grew up in Yokosuka, a city on Tokyo Bay, and attended an elite high school in Tokyo. He had been thinking of studying German literature at the University of Tokyo until he overheard his physics teacher, who had given him a flunking grade, denigrate his abilities.
“That statement made me furious, so I started studying physics,” Dr. Koshiba said in an oral history interview for the American Institute of Physics in 1997. “After one full month of concentrated work, I passed the physics department requirement, while the favorite student of the professor failed.”
After graduating from the university and two years of graduate study there, he went to the United States and dashed through the University of Rochester, earning his Ph.D. in physics in 1955 after only a year and eight months of study. He had hurried because he wanted the higher salary that a doctorate could command ($400 a month at the time) so that he could have more money to send back to his family in Japan.
After three years as a postdoctoral fellow at the University of Chicago, Dr. Koshiba returned to the University of Tokyo. Shortly after moving back, he entered into a marriage, brokered by physicist friends, with Kyoto Kato, an art museum curator. She survives him along with a son, Shyun; a daughter, Mari Fujii; and two grandchildren.
Dr. Koshiba’s early research involved studying cosmic rays — high-energy particles flying from outer space — by placing detectors on high-altitude balloons. But he was to make his mark in the other direction — namely, underground.
He originally built Kamiokande in hopes of recording the disintegration of protons, the building blocks of ordinary matter, as predicted by an ambitious “grand unified theory” of particle physics. Phototubes lining the tank would detect flashes of light produced in the water by high-energy fragments flying away.
Neither Kamiokande, completed in 1983, nor a competing experiment in Michigan called I.M.B., saw any protons decay in the predicted time, scuttling the grand unified theory.
Dr. Koshiba and his collaborators then decided to convert Kamiokande to a neutrino detector.
Neutrinos had captured physicists’ imaginations since 1930, when Wolfgang Pauli predicted their existence out of desperation to account for a discrepancy in the process of radioactive decay. According to the theory worked out by Dr. Pauli and Enrico Fermi, neutrinos were virtually massless ghost riders of the sky, traveling at the speed of light and rarely interacting with other matter.
Yet space should be flooded with them. According to theory, nuclear reactions in the sun alone send 65 billion neutrinos through every square inch of Earth every second. But in the 1960s, when Raymond Davis, a researcher at the Brookhaven National Laboratory on Long Island, set out to catch solar neutrinos in an underground vat of dry-cleaning fluid in an old gold mine in South Dakota, he counted only about a third as many neutrinos as theory had predicted.
If the Davis experiment was right, there was something wrong with theories of neutrinos or how the Sun worked. Dr. Davis was always confident of his method and measurements, but his experiment could not tell when the neutrinos arrived or where they were coming from.
That is where Dr. Koshiba’s Kamiokande came in. It would be able to track the time and direction of incoming neutrinos.
The conversion of the chamber to neutrino detector took two years, partly because the Kamioka zinc mine was full of radon gas, which would contaminate the detector.
The conversion was completed toward the end of 1986. Dr. Koshiba’s team had just started running their neutrino experiment when, on Feb. 23, a few dozen neutrinos from a supernova explosion 160,000 light years away in the Large Magellanic Cloud tickled the detector, illuminating for the first time the inner life of an exploding star.
“The lucky thing was that it happened one month before my retirement,” Dr. Koshiba recalled.
By 1990, the Kamiokande experiment had confirmed that neutrinos were indeed coming from the sun, and that there were only about a third as many as there should be, confirming the Davis experiment.
Dr. Koshiba and Dr. Davis shared the 2002 Nobel Prize in Physics with Riccardo Giacconi, an X-ray astronomer.
Meanwhile, experiments led by Dr. Koshiba’s protégé Takaaki Kajita at Kamiokande’s successor chamber, Super Kamiokande, and by Arthur McDonald at the Sudbury Neutrino Observatory in Ontario, had confirmed that the apparent solar neutrino deficit was the fault of some bizarre property of the neutrinos. They come in three kinds and can oscillate from one form to the other as they travel along and become undetectable, behavior that astronomers and physicists think could be the key to many cosmic mysteries.
Dr. Kajita and Dr. McDonald went on to share the 2015 Nobel Prize in Physics. Many physicists believe that there are more Nobel prizes yet to be handed out in this field.
Dr. Koshiba, upon retirement, joined Tokai University in Tokyo for a couple of years and devoted himself to philanthropic and educational activities.
He was named distinguished university professor by the University of Tokyo in 2005. In a statement released by the university announcing Dr. Koshiba’s death, Sachio Komamiya, director of the International Center for Elementary Particle Physics, lauded him for nurturing talent.
“With his sharp intuition, passion for research and outstanding planning ability and leadership,” the statement said, “he has established the foundations of the related fields of his research in Japan.”
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