Super-Kamiokande
Super-Kamiokande, or Super-K for short, is a neutrino observatory in Japan. The observatory was designed to study solar neutrinos and atmospheric neutrinos, search for proton decay, and detect neutrinos from a supernova anywhere in our galaxy.
Super-K is located 1,000 m underground in Mozumi Mine of the Kamioka Mining and Smelting Co. in Hida city (formerly Kamioka town), Gifu, Japan. It consists of 50,000 tons of pure water surrounded by about 11000 photomultiplier tubes. The cylindrical structure is 40 m tall and 40 m across. A neutrino interaction with the electrons or nuclei of water can produce a particle that moves faster than the speed of light in water (although of course slower than the speed of light in vacuum). This creates a cone of light due to Cherenkov radiation which is the optical equivalent to a sonic boom. The distinct pattern of this flash provides information on the direction and in the case of atmospheric neutrinos, the flavor of the incoming neutrino. The difference in time between the top of the cone reaching the detector wall and the bottom can be used to calculate the direction the particle is coming from; the bigger the difference, the greater the angle from the horizontal of the particle's path. From the sharpness of the edge of the cone the type of particle can be inferred. A muon is very penetrative and so interacts rarely with the water giving a sharp cone. An electron will regulary interact causing extra particle showers to occur and therefore a fuzzier cone to be detected by a series of photomultiplier tubes.
History
Construction of Kamioka Underground Observatory, the predecessor of the present Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo began in 1982 and was completed in April, 1983. The purpose of the observatory was to detect the proton decay, one of the most fundamental questions of elementary particle physics.
The detector, named KAMIOKANDE for Kamioka Nucleon Decay Experiment, was a tank which contained 3,000 tons of pure water and had about 1,000 photomultiplier tubes (PMTs) attached to the inner surface. The size of the tank was 16.0 m in height and 15.6 m in diameter. An upgrade of the detector was started in 1985 to allow the detector to observe solar neutrinos. As a result, the detector (KAMIOKANDE-II) had become sensitive enough to detect neutrinos from a supernova explosion which was observed from in the Large Magellanic Cloud in February 1987. Solar neutrinos were observed in 1988 adding to the advancements in neutrino astronomy and neutrino astrophysics. The ability of the Kamiokande experiment to observe the direction of the electron produced in a solar neutrino interaction allowed the experimentors to demonstrate for the first time that the sun really did produce neutrinos.
Despite its success in neutrino observation, Kamiokande did not detect proton decay, its first aim. Also, even higher sensitivity was needed to observe neutrinos with high statistical confidence. This led to the construction of Super-Kamiokande, with ten times more water volume and PMTs than Kamiokande. Super-Kamiokande started observation in 1996.
Super-Kamiokande Collaboration announced the first evidence of neutrino oscillations in 1998, consistent with the theory that the neutrino has non-zero mass. Until this, all observational evidences were consistent with neutrinos being massless, although theorists had speculated on the possibility of neutrinos having non-zero mass for many years.
On November 12, 2001, several thousand photomultiplier tubes in the Super-Kamiokande detector imploded, apparently in a chain reaction as the pressure waves from each imploding tube cracked its neighbours. The detector has been partially restored by redistributing the photomultiplier tubes which did not implode, and by adding protective acrylic shells that are hoped would prevent another chain reaction from recurring.