Engineer devises tiny optical sensor after Nobel triumph

by Tatsuya Tsujimura


Hiroyuki Kyushima, an engineer at Hamamatsu Photonics K.K. who helped create a particle detector that enabled physicist Masatoshi Koshiba to win the Nobel Prize in physics in 2002, has developed an ultra-small photomultiplier tube together with his fellow workers.

“This is a break-proof, ideal PMT with functions comparable to those of the Super-Kamiokande,” Kyushima, 56, said.

The original Kamiokande, short for the Kamioka Nucleon Decay Experiment neutrino physics laboratory, was built underground near the Kamioka section in the city of Hida, Gifu Prefecture, in the early 1980s under the leadership of Koshiba, 87, a professor of physics at the University of Tokyo at the time.

For the project, Koshiba asked Hamamatsu Photonics to develop a PMT with a diameter of 50 cm, the largest in the world.

A PMT is a highly sensitive optical sensor in a vacuum tube with a photoelectron surface that emits negatively charged electrons, a multiplier that accelerates electrons for their collision to create a stream of more electrons, and an electrode to accept the flow.

Assigned to the task, Kyushima and his supervisor, Masuo Ito, 67, made a full-size model and filled it with water. They set up an aluminum board in the model to play the role of a photoelectron surface and energized the board. When a needle was inserted into the water, it moved by electricity, enabling them to examine how electrons moved.

Through trial and error, including delicate adjustments added to the curve of the board, Kyushima and Ito developed a photoelectron surface with a shape matching Koshiba’s request, and Hamamatsu Photonics developed a PMT based on the model.

Having 1,000 such PMTs lined up in a tank containing 3,000 tons of water, the Kamiokande observed thermal neutrinos that were produced by a supernova roughly 160,000 light-years away and arrived at the Earth in 1987. Koshiba won the 2002 Nobel Prize in physics for the achievement.

Born in Hamamatsu, Shizuoka Prefecture, Kyushima was intrigued by electricity when he was a small boy, wondering “why an invisible power can move things,” he recalled.

Kyushima graduated from a local high school in 1975 and joined a small company that would develop into Hamamatsu Photonics, a renowned optical technology company. Like many other plant workers, Kyushima studied at the junior college of technology in Shizuoka University after working for three years.

After the Kamiokande triumph, Kyushima took charge of developing a PMT for the Super-Kamiokande, which replaced the Kamiokande in 1996.

The Super-Kamiokande is “incomparably more advanced than the Kamiokande thanks to extremely solid work” by Kyushima, said Atsuto Suzuki, 67, director general of the High Energy Accelerator Research Organization and one of Koshiba’s disciples.

Kyushima now heads a team of some 90 engineers at a Hamamatsu Photonics plant in Iwata, Shizuoka Prefecture, that has developed a fingertip-size PMT.

While PMTs are usually hand-made, the new “Micro PMT” is the world’s first device for mass production and is expected to be applicable to medical equipment.

“We’ve taken 13 years to develop (it) since we conceptualized it,” Kyushima said, adding that his greatest pleasure is to develop an idea into the first-ever product of its kind.