The significance of neutrino observation
The ultimate aim of research being carried out at Kamioka Mine is to verify the Grand Unified Theory (GUT) in particle physics. In turn, GUT is aimed at theoretically unifying the basic forces that exist in the natural world into one. It was this same theory that predicted proton decay, referring to the decay of protons, the lightest basic constituent element in all matter. GUT is also closely related to the theory that there were ultra-high temperatures and density levels during the early days of the universe. Research aimed at verifying GUT is basically a case of unraveling the mysteries of how the universe was created.
The key to verifying GUT lies in observing subatomic particles called neutrinos. Confirming the existence and properties of neutrinos is expected to provide clues that will prove the theory. Neutrinos come from space and hardly interact with other matter at all. This means that they pass straight through the earth, making them extremely difficult to observe. As a result, it is necessary to install a vast facility to increase the efficiency of observation, within an environment that blocks out other cosmic rays that get in the way of observation. The facility also has to be in a location with access to pure water, which is another essential component for observation.
Kamiokande and Kamioka Mine
As research continued, the University of Tokyo Honorable Emeritus Professor Masatoshi Koshiba, who won the Nobel Prize in Physics in 2002, singled out Kamioka Mine as one of a number of potential locations that would be ideally suited to neutrino research. Following a request from the University of Tokyo in 1981, Mitsui Kinzoku wasted no time in agreeing to build a neutrino observatory, and immediately started work on creating a space for the facility inside the mine.
The end result was Kamiokande, which was completed in 1983. Located 1,000 underground, we had built what was an incredibly large space at the time.
The mining technology we have built up to date here a Mitsui Kinzoku enabled us to achieve new feats of rock engineering as part of this project.
Needless to say, the tremendous achievements stemming from research at Kamiokande following the start of neutrino observation in 1983 can be summed up by the fact that Honorable Emeritus Professor Koshiba won the Nobel Prize in Physics. In 1987, he became the first person in the world to successfully observe neutrinos from a supernova explosion in the Large Magellanic Cloud. That was cited as the reason for his Nobel Prize.
Having witnessed one of the century’s great discoveries, Kamiokande completed its mission in 1996. These days, the space that once housed Kamiokande is home to Tohoku University research facility KamLAND, which continues to engage in neutrino observation albeit using different techniques.
From Kamiokande to Super-Kamiokande
Observation at Super-Kamiokande began in 1996, after Kamiokande had fulfilled its purpose. As with its predecessor, Super-Kamiokande was built 1,000 m underground beneath Kamioka Mine. This time however, it required a large cylindrical underground chamber (measuring 39.3 m in diameter and 41.1 m in height) boasting approximately 11 times the capacity of Kamiokande, in order to significantly improve performance. As Hida gneiss is roughly five times harder than concrete however, we had some extremely solid ground to excavate.
Nonetheless, we brought together Mitsui Kinzoku’s rock engineering expertise and built a vast chamber to house Super-Kamiokande. This enabled the installation of a photodetector holding approximately 50,000 tons of pure water, more than 10 times larger than Kamiokande.
Based on observation of muon neutrinos contained in atmospheric neutrinos incoming from space, and of muon neutrinos coming out the other side of the planet, in 1998, results indicated that only half were making their way through to the other side of the earth. The reason for this is that muon neutrinos change into tau neutrinos as they pass through the center of the planet. These observation results led to the discovery of neutrino oscillation, thereby proving that neutrinos do have mass, however small. As it was previously thought that neutrinos had no mass at all, this was a landmark discovery that turned the accepted theory on its head.
It was as a result of this that Professor Kajita won the 2015 Nobel Prize in Physics.
Kamioka Mine has become an international focal point for research. It is in the process of being reborn, transforming from a mine that produces mineral resources into a mine at the cutting edge of cosmic ray research.
Brief history of particle physics and astrophysics research at Kamioka Mine
||Work started on Kamiokande (Kamioka Underground Observatory, Institute for Cosmic Ray Research, The University of Tokyo)
||Successful observation of neutrinos from a supernova explosion in the Large Magellanic Cloud (world first)
||Work started on Super-Kamiokande (observatory as part of the above facility)
||Kamioka Observatory (Institute for Cosmic Ray Research, The University of Tokyo) established
||Super-Kamiokande completed (observation discontinued at Kamiokande)
||Then Assistant Professor Takaaki Kajita (The University of Tokyo) produced evidence that neutrinos have mass
||KamLAND built (observatory as part of the Tohoku University Research Center for Neutrino Science)
||Emeritus Professor Masatoshi Koshiba (The University of Tokyo) wins the Nobel Prize in Physics
||Professor Takaaki Kajita wins the Nobel Prize in Physics