STOCKHOLM -- With the high level of technological prowess behind a cutting-edge particle observation facility, Japanese manufacturers played a key role in the studies that led to this year's Nobel Prize in physics.
Takaaki Kajita, the director of the University of Tokyo's Institute for Cosmic Ray Research, was one of this year's Nobel physics laureates. He received the honor for his contribution to the advancement of our understanding of the elusive neutrino particle. This feat was probably not possible without the existence of the Super-Kamiokande facility, one of the world's most advanced particle observation laboratory.
Purest lake in the world
A key element of the facility is an artificial lake that was created 1,000 meters underground, in a mine located in Gifu Prefecture in central Japan. Described by some as the most beautiful body of water in the world, this artificial lake measures roughly 40 meters in depth and diameter. The 50,000 tons of water held there are illuminated in a bright blue color.
A neutrino is one type of particle, one of the smallest building blocks of matter. Trillions of neutrinos are showered onto the earth from space every second, but the vast majority of them simply pass right through the human body and the ground. Only a fraction of these neutrinos interact with materials. It was Masatoshi Koshiba, distinguished professor at the University of Tokyo and a winner of the 2002 Nobel Prize in physics, who came up with a way to observe this hard-to-catch particle. Koshiba was one of Kajita's former teachers.
Neutrinos occasionally collide with electrons and protons in water. When they do, the collision produces a faint blue light. Koshiba's idea was to use the light to observe the existence of the ghost-like particle. But this was not possible with just any water.
If the water contains even a tiny amount of impurities, the faint light from the collision becomes even weaker, making it impossible for sensors to detect it. To avoid this, the water must be extremely pure.
Organo, a company that specializes in water whose main products include ultrapure water production systems for chipmakers, was brought in to produce water that is as pure as possible. The Super-Kamiokande facility uses groundwater from below the mine, and the biggest challenge was to rid the water of radon.
The solution was to first remove impurities by using absorbent materials and various devices, then feed the treated water at a pace of 60 tons per hour through a system designed to help remove the radon. This made it possible to reduce the presence of the radioactive substance, which has a half-life of roughly four days, to a level that would not interfere with the detection of neutrinos.
This was how the world's most beautiful artificial lake, which sports a light transmissibility of more than 90 meters, was created.
Another crucial part of Super-Kamiokande is its optical sensors: giant eyes measuring 50cm in diameter that can detect even the faintest of light. For this, Hamamatsu Photonics, which developed the optical sensors for Kamiokande, the particle observation facility that preceded Super-Kamiokande, was recruited.
Detecting individual photons was impossible at Kamiokande, where Koshiba and other researchers performed their particle studies. Hamamatsu Photonics made detection possible at the new facility by developing an optical sensor that is so strong it can catch light from a flashlight on the moon's surface, under certain conditions.
Kajita assisted on the development of the optical sensor while still a graduate school student.
"He got work done steadily with few words, but he refused to compromise when it came to deciding the specs that determined the capability of the photo-multiplier tube," said Toshikazu Hakamata, a corporate adviser at Hamamatsu Photonics.
In November 2001, Super-Kamiokande suffered a major setback when more than 6,000 of its optical sensors were damaged all at once. The incident, which Kajita recalls as one of the most difficult times, came about because just one sensor at the bottom of the water tank cracked. The shock waves from this single optical sensor cracking were enough to cause a chain reaction that ultimately destroyed 6,000 others.
The fluctuation was produced by water suddenly rushing into the space that had been occupied by the optical sensor before it cracked. The solution was to encase the optical sensors in an acrylic box with multiple tiny holes so water would not rush in all at once even if a sensor cracked.
Kuraray was tapped to produce the acrylic case. The company succeeded in developing a material for the case in May 2002 and began mass production the following month. However, cases made of the material did not deliver the performance required. Over the next month, the company and the University of Tokyo worked together to clarify the issues and fix the problem.
Nowadays, shrinking government grants for basic research are a major challenge confronting the Japanese scientific community. Amid this situation, cooperation between academia and industry is becoming increasingly important to ensure that Japan's research levels remain high enough to produce more Nobel Prize winners in the future.