Kamioka Observatory
The Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo is a neutrino and gravitational waves laboratory located underground in the Mozumi mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. Multiple neutrino experiments have taken place at the observatory over two decades. The experiments have contributed to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation.
The mine
The Mozumi mine is one of two adjacent mines owned by the Kamioka Mining and Smelting Co..The mine is famous as the site of one of the greatest mass-poisonings in Japanese history. From 1910 to 1945, the mine operators released cadmium from the processing plant into the local water. This cadmium caused what the locals called itai-itai disease. The disease caused weakening of the bones and extreme pain.
Although mining operations have ceased, the smelting plant continues to process zinc, lead and silver from other mines and recycling.
While current experiments are all located in the northern Mozumi mine, the Tochibora mine 10 km south is also available. It is not quite as deep, but has stronger rock and is the planned site for the very large Hyper-KamiokaNDE caverns.
Past experiments
KamiokaNDE
The first of the Kamioka experiments was named KamiokaNDE for Kamioka Nucleon Decay Experiment. It was a large water Cherenkov detector designed to search for proton decay. To observe the decay of a particle with a lifetime as long as a proton an experiment must run for a long time and observe an enormous number of protons. This can be done most cost effectively if the target and the detector itself are made of the same material. Water is an ideal candidate because it is inexpensive, easy to purify, stable, and can detect relativistic charged particles through their production of Cherenkov radiation. A proton decay detector must be buried deep underground or in a mountain because the background from cosmic ray muons in such a large detector located on the surface of the Earth would be far too large. The muon rate in the KamiokaNDE experiment was about 0.4 events per second, roughly five orders of magnitude smaller than what it would have been if the detector had been located at the surface.The distinct pattern produced by Cherenkov radiation allows for particle identification, an important tool for both understanding the potential proton decay signal and for rejecting backgrounds. The identification is possible because the sharpness of the edge of the ring depends on the particle producing the radiation or electrons produce fuzzy rings due to the multiple scattering of the low mass electrons. Minimum ionizing muons, in contrast, produce very sharp rings as their heavier mass allows them to propagate directly.
Construction of the Kamioka Underground Observatory began in 1982 and was completed in April, 1983. The detector was a cylindrical tank which contained 3,000 tons of pure water and had about 1,000 50 cm diameter photomultiplier tubes attached to the inner surface. The size of the outer detector was 16.0 m in height and 15.6 m in diameter. The detector failed to observe proton decay, but set what was then the world's best limit on the lifetime of the proton.
KamiokaNDE-I operated 1983–1985.
KamiokaNDE-II
The KamiokaNDE-II experiment was a major step forward from KamiokaNDE, and made a significant number of important observations. KamiokaNDE-II operated 1985–1990.Solar neutrinos
In the 1930s, Hans Bethe and Carl Friedrich von Weizsäcker had hypothesized that the source of the Sun's energy was fusion reactions in its core. While this hypothesis was widely accepted for decades, there was no way of observing the Sun's core and directly testing the hypothesis. Ray Davis's Homestake Experiment was the first to detect solar neutrinos – strong evidence that the nuclear theory of the Sun was correct. Over a period of decades, the Davis experiment consistently observed only about 1/3 the number of neutrinos predicted by the Standard Solar Models of his colleague and close friend John Bahcall. Because of the great technical difficulty of the experiment and its reliance on radiochemical techniques rather than real time direct detection, many physicists were suspicious of his result.It was realized that a large water Čerenkov detector could be an ideal neutrino detector, for several reasons. First, the enormous volume possible in a water Čerenkov detector can overcome the problem of the very small cross section of the 5-15 MeV solar neutrinos. Second, water Čerenkov detectors offer real time event detection. This meant that individual neutrino-electron interaction candidate events could be studied on an event-by-event basis, starkly different from the month-to-month observation required in radiochemical experiments. Third, in the neutrino-electron scattering interaction the electron recoils in roughly the direction that the neutrino was travelling, so the electrons "point back" to the Sun. Fourth, neutrino-electron scattering is an elastic process, so the energy distribution of the neutrinos can be studied, further testing the solar model. Fifth, the characteristic "ring" produced by Čerenkov radiation allows discrimination of the signal against backgrounds. Finally, since a water Čerenkov experiment would use a different target, interaction process, detector technology, and location it would be a very complementary test of Davis's results.
It was clear that KamiokaNDE could be used to perform a fantastic and novel experiment, but a serious problem needed to be overcome first. The presence of radioactive backgrounds in KamiokaNDE meant that the detector had an energy threshold of tens of MeV. The signals produced by proton decay and atmospheric neutrino interactions are considerably larger than this, so the original KamiokaNDE detector had not needed to be particularly aggressive about its energy threshold or resolution. The problem was attacked in two ways. The participants of the KamiokaNDE experiment designed and built new purification systems for the water to reduce the radon background, and instead of constantly cycling the detector with "fresh" mine water they kept the water in the tank allowing the radon to decay away. A group from the University of Pennsylvania joined the collaboration and supplied new electronics with greatly superior timing capabilities. The extra information provided by the electronics further improved the ability to distinguish the neutrino signal from radioactive backgrounds. One further improvement was the expansion of the cavity, and the installation of an instrumented "outer detector". The extra water provided shielding from gamma rays from the surrounding rock, and the outer detector provided a veto for cosmic ray muons.
With the upgrades completed, the experiment was renamed KamiokaNDE-II, and started taking data in 1985. The experiment spent several years fighting the radon problem, and started taking "production data" in 1987. Once 450 days of data had been accumulated, the experiment was able to see a clear enhancement in the number of events which pointed away from the Sun over random directions. The directional information was the smoking gun signature of solar neutrinos, demonstrating directly for the first time that the Sun is a source of neutrinos. The experiment continued to take data for many years and eventually found the solar neutrino flux to be about 1/2 that predicted by solar models. This was in conflict with both the solar models and Davis's experiment, which was ongoing at the time and continued to observe only 1/3 of the predicted signal. This conflict between the flux predicted by solar theory and the radiochemical and water Čerenkov detectors became known as the solar neutrino problem.