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

我们研究了利用水切伦科夫探测器探测小型空气簇雨的方法，目的是探测100 GeV到1 TeV能量区域的伽马射线暴。试验在norikura山（海拔2870米）进行，在10米三角形的三个角上安装了三个2.2mphi、2.2m高的水箱。每个水箱1.5m水深处设置直径为8英寸的光电倍增管（PMT）。当三个pmt均检测到一个以上的光电子且光电子之间的到达时间差（PE）小于100ns时触发。通过与设置在水箱周围的闪烁计数器阵列的数据比较，发现其角分辨率约为15‰。实验结果与仿真结果进行了比较。PE分布与实验结果相似，角分辨率为10 12，与实验结果一致。通过模拟，探测到的空气簇射能量在70gev - 800gev之间，模态能量约为200gev。基于以上结果，在西藏杨八井（海拔4300m）采用由49个10m*10m、1.5m深的水箱组成的模型阵列，在面积190m*190m的7*7格点上进行风淋观测模拟。通过模拟可知，GRB910503和GRB930131两种外推能谱的伽玛射线空气簇的可探测能量区域为300 GeV-1 TeV，角分辨率分别为1.2 1 TeV和0.6 1 TeV。GRB910503和GRB930131每秒观测的预期显著性分别为40sigma和200sigma。

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.