The Super-Kamiokande detector consists of about 50000 tons of pure water filled in a cylindrical water tank (diameter 39.3m and height 41.4m), a water and air purification system, photomultiplier tubes (PMT), electronics and online data acquisition system, and offline computer facilities. Fig 1 shows a schematic view of the detector. It is located 1000m underground (2700m of water equivalent) in the Kamioka mine in Gifu Prefecture, Japan. Its latitude and longitude are 36 25' N and 13718' E, respectively.
The reason why the detector is underground is to shield against cosmic ray muons. Compared to ground level, the intensity of muons is reduced by about 1/100000 at the depth of the Super-Kamiokande detector. The muon rate in Super-Kamiokande is 1.88Hz.
The wall of the cave is covered by ``Mineguard'', a polyurethane material made by the Canadian company Urylon, which blocks radon emanating from the rock.
The water tank is made of stainless steel, and it is divided into an inner part and an outer part. These are summarized in Table 1. The reason for the division is mainly to identify remaining muon events coming from outside the tank and also to make the inner part free from gamma rays and neutrons from the rock.
The tank is sealed tightly to keep the mine's radon rich air outside. Radon gas is the most serious background for the solar neutrino analysis. The concentration of radon gas in the dome is about 1500 Bq/m in summer time and 30 Bq/m in winter time (see Fig.1 in the section Radon free air system). This large difference is caused by the flow of air in the mine.
Air blows into the mine in the winter and out of the mine in the summer.
Construction started in 1991. First, the cavity was excavated until the middle of 1994. After that, water tank construction started, and it finished in the middle of 1995. From June of 1995, we installed PMTs and set the electronics system simultaneously. This continued until the end of 1995. Water filling was from January to March in 1996. After a 1 month test run, we started normal data taking from April 1st in 1996.
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