Cosmic rays: Japan's great leap forward

Cosmic rays: Japan's great leap forward
Clockwise from left: The Dec. 24, 1945, issue of the U.S. magazine Life shows U.S. forces dumping RIKEN’s particle accelerator, known as a cyclotron, in Tokyo Bay; the Cosmic Ray Observatory of The University of Tokyo, which was built on Mt. Norikura in 1953 (courtesy of the Institute for Cosmic Ray Research University of Tokyo); renovations at Kamiokande, which started operations in 1983.

In November 1945, US forces seized particle accelerators known as cyclotrons from three facilities, including Tokyo-based RIKEN, and dumped them in Tokyo Bay and other places.

The General Headquarters Supreme Commander for the Allied Powers (GHQ) had been told that Japan was banned from nuclear research and it should destroy all related equipment.

Among the three facilities, RIKEN and Kyoto University had become bases for research on atomic bombs during the war.

Atomic bomb production was set back by problems acquiring enough uranium, although such world-famous physicists as Yoshio Nishina were taking part.

"GHQ knew about Japan's capabilities, so that's probably why they were anxiously monitoring nuclear research," said Fumitaka Sato, a professor emeritus at Kyoto University.

However, accelerators are not actually key tools for atomic bomb production, although their loss was a blow to physicists in Japan.

"Dumping the accelerators was basically saying, 'Don't do anything' to physicists trying to solve the fundamental question of what physics means," said Ken Kikuchi, former deputy director of the National Laboratory for High Energy Physics. Experiments including those involving accelerators continued to be banned until the Treaty of Peace with Japan was enforced in 1952, leading to a large gap with the West.

1950s: A string of results

"Despite being unable to conduct accelerator experiments, some physicists were forging new paths," recalled Akira Masaike, professor emeritus at Kyoto University. Masaike was referring to research on cosmic rays.

In space, all kinds of particles, including protons, whiz around at high speeds. Some of them rain down on Earth, colliding with other materials in the atmosphere to form new particles - nature's accelerator experiments, so to speak.

These particles are known as cosmic rays, which can be detected without the need of complex observation equipment by using photographic film known as nuclear emulsion plates.

"Many physicists set out to research cosmic rays," said Jun Nishimura, 88, professor emeritus at the University of Tokyo.

"[1965 Nobel Prize winner] Shinichiro Tomonaga once told senior Finance Ministry officials, 'We should pour our strength into cosmic ray research and step up our results,' during talks to secure funding."

In 1953, the Cosmic Ray Observatory of The University of Tokyo was built on Mt. Norikura. Two years later, the University of Tokyo Institute of Particle and Nuclear Studies was inaugurated, including a cosmic ray department.

There were several notable results by the 1950s, including an analysis of air showers - a phenomenon in which cosmic rays strike the nuclei of oxygen in the atmosphere to form new particles.

"We started pretty much from scratch, but now we've soared to become world-class in the field of cosmic rays," said Nishimura.

Taking on neutrinos

"We couldn't do other research, so we had little choice," recalled Masatoshi Koshiba, who became a graduate student at the University of Tokyo in 1951. He too had chosen to research cosmic rays. In 1979, he decided to tackle the challenge of observing subatomic particles known as neutrinos, which are some of the most difficult cosmic rays to detect.

A device called Kamiokande was built 1,000 meters underground in Kamioka mine in Gifu Prefecture. Filled with 3,000 tons of water, Kamiokande detects light that is produced when neutrinos collide with water using sensors known as photomultiplier tubes.

"I heard it was built out of a desire to conduct experiments by combining different kinds of budgets, rather than one large science budget," recalled Takaaki Kajita, director of the Institute for Cosmic Ray Research University of Tokyo. Kajita was stationed at Koshiba's research lab at the time.

The United States was pressing forward with observations using large-scale water tanks. "[At this rate] we're in trouble. Let's increase sensitivity by using much bigger bulbs [referring to intensifier tubes]," Koshiba said in talks with maker Hamamatsu TV, now known as Hamamatsu Photonics K.K. Intensifier tubes at the time were only made with diameters of up to 13 centimeters, so they decided to make a big leap and shoot for 50 centimeters.

"Just do it. You should all be taking on challenges as part of your jobs anyway," the firm's former President Teruo Hiruma told his technical team, trying to encourage them.

Plans started being drafted at the end of 1979. By July 1982, 1,050 orders were filled.

Able to think ahead of his time, Koshiba became the first in the world to observe neutrinos born from a supernova explosion. He went on to win a Nobel Prize in 2002.

Leading the world

Japan has had a string of successes in neutrino research since then, and now leads the world in this field.

"Japan's cosmic ray research was able to come this far thanks to strong curiosity and undeterred determination," said Masayuki Nakahata, who leads research on neutrinos at Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo.

He was expressing his respect for his predecessors who managed to overcome such hurdles as equipment and funding shortages.

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