Detection of High Energy Neutrino and Gamma Ray

利用1000m^2的原型探测器，研究了在月球表面安装大型水切伦科夫探测器探测天体物理来源高能中微子的可行性。这些数据是在1992年年初取得的。(1)触发率：高能中微子的信号是向上的μ子，是中微子与探测器下岩石的带电电流相互作用产生的。据估计，这种通量是宇宙射线的10^<-10>数量级。这个巨大的背景被切伦科夫光的方向性所抑制。20英寸光电管放置在水面以下10m处，间隔5m。触发要求有4个以上的光电管被击中，时间窗为30ns。即使使用这种简单的触发器，也可以获得10^5的背景抑制，产生约4Hz的触发率。(2)事件的组成：利用探测器上部与探测器主体光学分离的脉冲高度，分析了事件的组成。分析表明，近一半的触发事件是由气淋引起的，气淋在探测器上部沉积了较大的脉冲高度。其余的事件是由于具有大天顶角的μ子引起的。(3)最终的背景抑制能力：利用1992-1993年购买的附加光电管的宇宙射线介子，对20"光电管进行了校准。同时使用时序和脉冲高度信息的分析，产生约2 × 10^<10>的背景抑制。我们认为，本研究的最初动机，即大型水切伦科夫探测器在地表运行的可行性，已经得到了证明。

Feasibility of a large water Cherenkov detector on the surface for high energy neutrinos of astrophysical origin was examined, using a 1000m^2 prototype detector. The data was taken in early part of 1992.(1) Trigger rate :The signal for high energy neutrino is upward-going muon, which is created by the charged current interaction of neutrino with rock under the detector. This flux is estimated to be an order of 10^<-10> of that of cosmic rays. This huge background has been suppressed by utilizing the directionality of the Cherenkov light.20" phototube has been placed 10m below the water surface, with 5m separation. The trigger requires more than 4 phototube has been hit with 30ns coincidence time window. Even with this simple trigger can attain a background suppression of the order of 10^5, giving about 4Hz trigger rate.(2) The composition of the events :The composition of the events has been analyzed, using the pulse height in the upper part of the detector which is optically separated from the main part of the detector. The analysis showed about half of the triggering events are due to air shower which deposit large pulse height in the upper part of the detector. The rest of the events are due to muons which have large zenith angle.(3) The final background rejection capability :The 20" phototube has been calibrated, using cosmic ray muons by the additional phototube which has been purchased in 1992-1993. An analysis, using both timing and pulse height information, yields about 2x10^<10> suppression of the backgrounds.We believe that the original motivation of this study, i.e.feasibility of operation of a large water Cherenkov detector on the surface, has been proved.