with a dome shape was excavated to use for the structure of the water tank. The cavity was plastered with 40 - 50 cm thick reinforced concrete on the side wall and the floor, then laid with 3 - 4.5 mm thick stainless steel. The size of the tank is 42 m in hight and 39 m in diameter. The total volume is 50,000 m**3. 26 sets of Helmholz coils are arranged in the tank to eliminate the effect of geo-magnetic field on PMT's. The tank is filled up with pure water in early 1996.
The Super-Kamiokande is a water Celencov detector. It consists of two layers of detectors, a inner counter and a outer counter. Support structures for PMT's, which divided the tank between inner and outer counter, are built 2.7 m apart from the tank wall. 11,146 20-inch PMT's are placed uniformly over the entire inner surface. The photo-cathode coverage of the PMT's is 40%, which is twice of that of the present Kamiokande. The gap between PMT's is covered with black polyetylene sheets. The total mass of water inside the PMT surface is 32,000 tons.
Another black polyetylene sheets are set behind the inner PMT's to optically isolate between the inner and the outer counter. The outer part, which has 2 m in thickness, is used for anti-couters. All the surface of the outer counter is covered with white polyethylen sheets with high reflection. 1,857 8-inch PMT's are used for the outer counter. On each PMT window, a 60-cm squre acyliric plate containg wave-length shifter is equipped to enlarge the light-correction-efficiency.
The 20-inch PMT was first developed by HAMAMATSU photonics Co. for the present Kamiokande. Sufficient performance for a large water Celencov detector has been demonstrated during its long-run. For Super-Kamiokande, several improvements in the dynodes shape, bleeder chain, and etc. enable us to obtain good timing and energy resoluton.
A typical gain of the PMT is 10**7 with about 2000 V. The quantum efficiency is typically 20 %. A typical spread of the transit-time is about 3 ns. These performance is enough to carry out experiments in Super-Kamiokande.