Super-Kamiokande is a 50,000 ton ring-imaging water Cerenkov detector located at a depth of 2700 meters water equivalent in the Kamioka Mozumi mine in Japan. It is used mostly for the search for proton decay (nucleon decay in general), observation of neutrinos (solar, atmospheric, from super-nova, …) and cosmic rays (mostly muons: downward going muons created by cosmic ray particles in the atmosphere, and upward going muons created by neutrino interaction in the Earth beneath the detector).
Some of the things that the detector can see, but we don’t want it to see (a so called background to interesting signals) is natural radioactivity in the surrounding rocks and water (mostly radon gas), and to some extent cosmic muons. The detector is located deep underground in order to shield it from cosmic ray muons by the rock above it. Super-Kamiokande is a big tank of very clean water. It is a cylinder of roughly 40 m diameter and 40 m height. On all the walls (side, top and bottom) there are many (about 13000) photomultiplier tubes (PMT’s). They are very sensitive light detectors and they can detect single photons. They are ‘looking’ toward the inner volume of water.