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Located in a mine 1,000 m underground, Super-Kamiokande was designed with a bold idea: to observe space from underground. The bedrock blocks unwanted particles (cosmic rays) pouring down from space, while a stainless steel tank, 41.4 m in total height, 39.3 m in diameter, and filled with 50,000 tons of ultrapure water, is used to detect neutrinos.

The 50,000 tons of ultrapure water are needed because even slight impurities in water would block the faint Cherenkov light produced when neutrinos interact with the water. Organo's mission was to provide a comprehensive water treatment solution, including producing the most ultrapure water possible, designing equipment used deep underground, optimizing piping routes, and performing sophisticated system management. Chosen for this task of great responsibility was Hiromori Mochizuki, then a new employee.

"Organo had supplied ultrapure water for the previous 'Kamiokande' project, and the 'Super-Kamiokande' project was launched with the goal of improving observation accuracy even further," recalls Mochizuki. "I had just joined the company, and I was suddenly assigned this very important sales role. I still vividly remember visiting the laboratory at the University of Tokyo, where papers and documents were stacked high and young Professor Kajita sat at the back of the room. We had many discussions between those stacks of papers."

That "Professor Kajita" was none other than Takaaki Kajita of the University of Tokyo, who would go on to discover neutrino oscillation during the Super-Kamiokande project and ultimately be awarded the Nobel Prize in Physics.
"Professor Kajita is a very kind and gentle man, and he's very passionate about space. He really opens up when talking about it, and he's got a lot to say," Mochizuki continues. "He'll repeatedly break down difficult concepts so that you can understand them, and this really inspired me to do everything I could to help him to unravel the mysteries of the universe. We'd both chant, 'Let's do this, let's make this project a success,' and I think that this shared determination really helped to build momentum."

Removing all impurities from 50,000 tons of water to create the perfect screen. Considering that standard ultrapure water equipment typically handle anywhere from several tons to several hundred tons of water, it is easy to imagine how difficult a challenge it would be to create such a water screen.

This would be a major challenge even for Organo, which has a track record of supplying ultrapure water in many fields. Just how different would this ultrapure water be, in terms of scale and quality?
"The water quality standards required for Super-Kamiokande were simply unparalleled at that time," explains Mochizuki. "The greatest challenge was removing radioactive substances. There are many kinds of radioactive substances, such as thorium, floating in the air in a mine. Radon is the greatest threat. It slowly seeps out from the cracks in the bedrock into the tunnels and inside water tanks. If it breaks down in water and releases radiation, it generates unwanted light that can obscure the faint Cherenkov light produced by neutrinos. Another challenge lay in the degassing process used to produce ultrapure water. During this process, unneeded components dissolved in water are removed using air. However, if we use the air inside a mine to do this, all the impurities contained in that air will simply be reintroduced into the water. Accordingly, we can't meet the requirements unless we control not only the water but also the air as well. This requires some truly impressive technology, far beyond what we use for typical projects."

Organo would need to develop an "ultrapure water screen to meet Super-Kamiokande specifications," by reducing impurities, most importantly radioactive substances, but also ion components, organic matter, and fine particles, to within acceptable limits. It seemed an impossible task. Whenever he felt discouraged at the scale of this unprecedented initiative, Mochizuki would press on, with Professor Kajita's words, 'Let's do this, let's make this project a success,' echoing in his mind.

Creating an ultrapure water screen would require technology capable of removing radon and other radioactive substances from the air. It was a completely new challenge for Organo. The question was how to design and validate such a system? It was a difficult task.

"We knew what had to be done," Mochizuki says. "We had to reduce the amount of radon, which interferes with observations, to at least one-hundredth of the original level. I sent a request to the people at our lab, and they conducted verifications and experiments to reach our water quality target. Whenever we made progress in the lab, I'd go visit Professor Kajita with one of our engineers, and the three of us would travel to Kamioka in Gifu to verify our findings at the site. We traveled back and forth between the lab and the site over and over until we got the results we wanted. We really put a lot of work into developing this ultrapure water equipment."

The successful completion of the water screen was made possible by the firm commitment and solidarity shared by Professor Kajita and other researchers with the Organo project team. It was an initiative that would provide a clear view of outer space and significantly improve observational accuracy. It would also determine the success or failure of Super-Kamiokande itself.

Another challenge for the Super-Kamiokande project was the construction of the 50,000-ton water tank. Materials for the construction of the water tank were transported through a two-kilometer horizontal tunnel leading to a vast underground cavern excavated by a specialized contractor.

"It was like a scene from 'Indiana Jones.' They'd ride mine carts into the mine and set off explosives to blast through the rock. Seeing it all firsthand, the sheer scale of it was so impressive that I was stunned," recalls Mochizuki. "Transporting enormous heavy machinery and the material for the water tank through a relatively narrow tunnel calls for completely different methods than you'd use above ground. We kept running into problems. For example, the heavy machinery you'd usually use for this kind of work wouldn't fit in the tunnel. We just had to solve each problem as it happened, but this involved a lot of traveling back and forth and getting in touch with the right people, which was often very difficult. However, it was also a great opportunity to experience the challenges and rewards of project management," explains Mochizuki when reflecting back on those difficult days.

Construction for Super-Kamiokande lasted a total of four years and three months, including one year building the water tank and several months filling the tank with ultrapure water. At last, humanity had gained a new "eye" with which to detect neutrinos, and unravel the mysteries of the universe.

In 1998, Super-Kamiokande observed, for the first time in the world, the phenomenon of "neutrino oscillation," where a flying neutrino transforms into a different type of neutrino while moving. This historical discovery proved that neutrinos have mass, going beyond the conventional wisdom of particle physics at the time and significantly advancing research related to the evolution of the universe and the origin of matter. In 2015, Professor Kajita of the University of Tokyo was awarded the Nobel Prize in Physics for his discovery of neutrino oscillation.

Super-Kamiokande has also played an important role in observing solar neutrinos coming from the Sun and neutrinos from supernova explosions, and today continues to provide data crucial in elucidating various space-related phenomena. These groundbreaking revelations about the universe would not have been possible without the researchers' insatiable thirst for knowledge, the ultrapure water screen that allows light from neutrinos to reach sensors, and the 13,000 photomultiplier tubes used to capture faint flashes of light.

"The sense of achievement I felt from producing a truly unique level of water quality, which met the requirements of Professor Kajita and the other researchers, was unlike anything I've ever experienced," reflects Mochizuki. "It was very rewarding to receive such high praise for our work. I learned a lot from Professor Kajita, and I also realized that you can accomplish anything, as long as everyone shares the same determination. My experiences participating in this project are truly valuable to me, even today."

In 2020, researchers added a small amount of gadolinium to the 50,000-ton ultrapure water tank in order to conduct even more advanced observations. Thus would begin a new era of observations at Super-Kamiokande. Among its many benefits, this would significantly improve the efficiency of detecting antineutrinos from supernova explosions.

This project would be a brand-new challenge for Organo, as well. Gadolinium must be dissolved uniformly throughout the water while maintaining an extremely high level of purity. At the same time, the quality of the ultrapure water had to be maintained so that new impurities were not introduced in this process. Organo began developing a new water treatment technology in close collaboration with the Institute for Cosmic Ray Research, the University of Tokyo. The result was an "ion exchange resin" specially designed to remove trace amounts of impurities while allowing the gadolinium itself to remain in the water.