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 height50 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.
