Development of Water Cherenkov Detector for Observation of 300GeV Gamma-Ray Bursts at Yangbajing in Tibet

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We have studied methods to detect small air showers using water Cherenkov detector, aiming at a detection of Gamma-ray burst in the energy region from 100 GeV through 1 TeV.Test was performed at Mt.Norikura (2,870m a.s.l) using three water tanks of size 2.2mphi and 2.2m high installed at three corners of 10m triangle. A photomultiplier (PMT) of 8 inch diameter is set at the depth of 1.5m water in each water tank. Trigger was done when more than one photoelectron is detected by all of three PMTs and the arrival time difference between photoelectrons (PE) was less than 100ns. The angular resolution was found to be about 15ﾟ which was known by the comparison with data of a scintillation counter array set around the water tanks.This experimental results are compared with a simulation. The PE distribution was found to be similar to the experiment, and angular resolution to be 10ﾟ-12ﾟ which is also consistent with the experiment. By this simulation the energies of detected air showers are included in 70 GeV-800GeV with mode energy of about 200 GeV.Based on above results, we made simulation of air shower observation at Yangbajing (4,300m a.s.l, in Tibet) using a model water tank array which is consists of 49 water tanks of 10m*10m with 1.5m depth installed at 7*7 lattice points in area 190m*190m. From this simulation it was known that the detectable energy region of gamma-ray air shower is 300 GeV-1 TeV for both cases of extrapolated energy spectrum of GRB910503 and GRB930131, and that angular resolutions are 1.2ﾟ for 300 GeV and 0.6ﾟ for 1 TeV,respectively. Expected significances for one sec observation of GRBs are found to be 40sigma for the case of GRB910503, and 200sigma for GRB930131.