Kamioka Observatory

The Kamioka Observatory, Institute for Cosmic Ray Research (神岡宇宙素粒子研究施設, Kamioka Uchū Soryūshi Kenkyū Shisetsu, Japanese pronunciation: ) 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. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades. All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation. The Kamioka Observatory, Institute for Cosmic Ray Research (神岡宇宙素粒子研究施設, Kamioka Uchū Soryūshi Kenkyū Shisetsu, Japanese pronunciation: ) 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. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades. All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation. The Mozumi mine is one of two adjacent mines owned by the Kamioka Mining and Smelting Co. (a subsidiary of the Mitsui Mining and Smelting Co. Mitsui Kinzoku).:1The 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.:2,6–7 While current experiments are all located in the northern Mozumi mine, the Tochibora mine 10 km south:9 is also available. It is not quite as deep, but has stronger rock:22,24,26 and is the planned site for the very large Hyper-Kamiokande caverns.:19 The first of the Kamioka experiments was named KamiokaNDE for Kamioka Nucleon Decay Experiment. It was a large water Čerenkov 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 (the source of the protons) 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 Čerenkov 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 Čerenkov radiation allows for particle identification, an important tool both understanding the potential proton decay signal and for rejecting backgrounds. The ID is possible because the sharpness of the edge of the ring depends on the particle producing the radiation. Electrons (and therefore also gamma rays) 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 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 detector was a cylindrical tank which contained 3,000 tons of pure water and had about 1,000 50 cm diameter photomultiplier tubes (PMTs) 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.