Detector

Detector outline

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.

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Figure 1: The detector appearance. Inset at bottom right shows the location within the mountain. (cutaway view)

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.

Tank Dimensions
Volume
39.3m in diameter 41.4m in height
50 kton
Outer part Thickness

Volume
Num. of PMT
2.6m (7.2 R.L. and 4.3 N.L.) on top and bottom
2.75m (7.6 R.L. and 4.6 N.L.) on barrel
18 kton
302 (top), 308 (bottom) and 1275 (barrel)
Inner part Dimensions
Volume
Num. of PMT
33.8m in diameter 36.2m in height
32 kton
1748 (top and bottom) and 7650 (barrel)
Fiducial area Thickness
Volume
2m (5.5 R.L. and 3.3 N.L.) from the inner wall
22kton
Table 1:
Several parameters of Super-Kamiokande detector. R.L and N.L mean radiation length and nuclear interaction length, respectively.

Air blows into the mine in the winter and out of the mine in the summer.
We use 11146 20-inch PMTs in the inner detector and 1885 8-inch PMTs in the outer detector, as summarized in Table 1. The characteristics of the PMTs are described elsewhere.
In the inner detector, PMTs are placed at intervals of 70 cm, and the ratio of PMT area to all area (photo coverage) is 40.41%. The wall of the tank is covered with black polyethylene terephthalate sheets (called ``black sheet'') behind the inner PMTs.
In the outer detector, there are 2 outer PMTs just behind each set of 12 inner PMTs, and they face outward from the support structure. Around each outer PMT, there is wave length shifter to increase the photo coverage. In order to increase the light detection efficiency, all surfaces of the outer tank are covered with white tyvek sheets with a reflectivity of above 80%.
The inner part and outer part are optically isolated using black sheets and tyvek sheets. There are complicated stainless steel frames for supporting the PMTs and so on in this optically insensitive region. The schematic view of these structures is shown in Fig 2.

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Figure 2: The schematic view of the frame which supports PMTs. For a clearer view, enter here.


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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revised on 1999/09/27 webmaster@suketto.icrr.u-tokyo.ac.jp