Director of Kamioka Observatory

Masayuki Nakahata

Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, is located in Hida-city, at the northern end of Gifu prefecture. It is 30 km south of the Toyama railway station and 15km north of the center of Kamioka town. The observatory was established in 1995 to conduct the Super-Kamiokande experiment (SK) but is now used as an inter-university facility for other underground experiments as well. Its underground facilities are located 1000m below the top of the 1369m high Mt.Ikeno-yama and its surface research buildings, including a dormitory for visiting researchers, are located in Higashi-Mozumi, Kamioka-town.


SK is a large neutrino detector containing 50,000 tons of pure water that started taking data in 1996. Neutrino oscillations in atmospheric neutrinos were discovered by SK in 1998 which thereby demonstrated that neutrinos have a finite mass. In 2001 neutrino oscillations were subsequently discovered in solar neutrinos by comparing the results of SK and the SNO experiment in Canada. The K2K (KEK to Kamioka) experiment, which was conducted from 1999 to 2004, confirmed neutrino oscillations using man-made accelerator neutrinos. Furthermore, the T2K (Tokai to Kamioka) experiment, which has been operating since 2009, discovered the third neutrino oscillation mode in 2011. All of these discoveries are world firsts and have established SK as a world leading neutrino experiment. The SK experiment is an international collaboration and about half of its 120 collaborators are from foreign countries. While most of these belong to US institutions, research groups from Canada, Korea, China, Spain and Poland also participate in the experiment.


The reason the SK detector is located underground is to reduce cosmic ray backgrounds. On the Earth’s surface about one cosmic ray particle passes through your hand every second. However, at the SK site this cosmic ray flux is reduced by five orders of magnitude. Such a “quiet” underground site is well suited to experiments searching for rare phenomena. For instance, it is known that 27% of the matter/energy in the universe is comprised of as-yet unknown “dark matter,” which corresponds to five to six times the amount of ordinary matter in the universe. A possible candidate for dark matter is a new elementary particle which only interacts rarely but that may reach underground where it can be detected by experimenters. At Kamioka observatory the XMASS experiment is searching for dark matter using liquid xenon. Additionally, the observatory is used for a double beta decay experiment and for R&D towards a direction sensitive dark matter experiment. Further, a laser strain-meter and a superconducting gravimeter are observing the vibrations and deformations of the Earth for geophysical research purposes.


Accordingly Kamioka observatory continues to conduct frontier research using the low background environment its underground facilities afford. Welcome to Kamioka Observatory


April, 2014

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