Study of Ultra-high Energy Cosmic Rays at Tibetan Highlands

从1989年开始，中国科学家在西藏羊八井（海拔4300米，90.5^ E和30.1^ N）开始了一项新的空气淋浴实验，旨在寻找天体伽玛射线点源。该阵列由49个用于快速计时（FT）的闪烁计数器组成，每个计数器的面积为0.5 m^2，并放置在间距为15 m的网格模式中。另外16个密度计数器围绕FT计数器阵列，以选择其核心位于阵列内的淋浴。本实验采用32位微机控制的TKO和CAMAC系统。数据记录在容量约2gb的8mm录像带上。配备稳定度<54 μ s/月的Rb时钟，精确接收风淋点到达时间。这项工作从1990年6月完全开始，现在以大约20赫兹的高触发率连续进行，以在低于100 TeV的能量区域寻找伽玛点源。目前，只有我们的阵列可以有效地探测到更多的低能量伽马射线，因为观测点位于高海拔地区。用实验数据对该阵列的性能进行了检验，并与蒙特卡罗模拟结果进行了比较。结果表明，空气淋点的到达方向可以用约0.5^ 100 TeV的角分辨率来确定。此外，100 TeV伽马射线的能量分辨率估计约为30%。可探测到的阵雨能量估计约为20 TeV，而伽马射线阵雨的非偏置能量约为40 TeV。因此，我们的实验被认为在目前世界上工作的许多空气淋室阵列中具有独特的特点，能够在几个10 TeV的能量区域观测伽玛射线点源，而地面上的镜子和阵列实验都无法实现。我们还需要至少一年多的时间来操作这个阵列来检验Cyg。X-3和其他候选点源确实稳定地发射高能伽马射线。第一个结果将在今年秋天公布。少

A new air shower experiment, aiming to search for celestial gamma ray point sources, has been started from 1989 with Chinese scientists at Yangbajing (4,300m a. s. l., 90.5^ﾟE and 30.1^ﾟN) in Tibet. The present array consists of 49 scintillation counters for fast timing (FT), each of which has an area of 0.5 m^2 and is placed in a grid pattern with a spacing of 15 m. Other 16 density counters surround the FT counter array to select the showers whose core fall inside the array. The TKO and CAMAC system, controlled by a 32 bit micro-computer, is employed in this experiment. The data are recorded on a 8 mm video tape of about 2 GB capacity. A Rb clock with a stability of <54 mus/month is equipped to receive the arrival time of air showers precisely. The operation, atarted fully from June of 1990, is now successively continued with a high trigger rate of about 20 Hz to search for gammaray point sources in the energy region lower than 100 TeV. For the time being, only our array can detect s … More uch rather low energy gamma-rays effectively, since the observation site is located at high altitude.The performance of the present array was examined by using the experimental data, comparing those with a Monte Carlo simulation. This shows that the arrival direction of air showers can be determined with an angular resolution of about 0.5^ﾟ at 100 TeV. Also the energy resolution is estimated to be about 30% for a 100 TeV gamma ray. The detectable shower energy is estimated to be about 20 TeV, and the non-bias energy corresponds to about 40 TeV for gamma ray showers. Our experiment is therefore considered to have a distinctive feature among a lot of air shower arrays working now in the world, giving a capability of observing gamma ray point sources in the several 10 TeV energy region where both of mirror and array experiment on the ground are inaccessible.We still need to operate this array at least for more than one year to examine whether Cyg. X-3 and other point source candidates really emit high energy gamma rays steadily or not. The first result will be given in this fall. Less