在西藏高海拔（＞5000米）基于水切伦科夫技术利用单粒子方法探测几十GeV伽玛暴

Observations of gamma rays burst (GRB) above 10GeV are important in the research of radiation process and quantum gravitational effects of high-energy GRBs. The space-borne detector of Fermi-LAT has observed tens of GRBs in the energy range from MeV to GeV. However, only a few of photons with energy greater than 10GeV were detected due to the limitations of the space-based detector (e.g. the effective area of Fermi is about 1m^2. The energy spectrum of GRBs in highenergy range can't be accurately measured,which limits our knowledge about radiation processes of high-energy GRBs. Ground-based detectors operated at present have their limitations. Extensive air shower (EAS) arrays has its advantages such as a large effective area, high duty cycle, wide field of view, however, the thresholds of EAS arrays in operation are larger than hundreds of GeVs. Because the shower development induced by gamma rays below 50GeV is approach the end at an altitude of 4,000 meters. The elevation of the present large EAS arrays are all lower than 4300 meters, which results in the poor efficiency of EAS arrays in observation of the tens of GeV -bursts. Imaging Atmospheric Cherenkov Telescopes (IACTs) can reduce the threshold to 50GeV by creating a large optical mirror. However, the disadvantages of small limit field of view and low duty cycle restricts the detectability for gamma rays in such energy. A classical method to use is called "single particle technique" (SPT). When high energy photons from a GRB reach the atmosphere, they produce cosmic ray cascades that can be detected. The energies are not enough to produce a shower detectable at ground level (even at high altitudes). However, many photons are expected to arrive during the burst, in a short period of time. One would therefore see an increase of the background rate on all the detectors on this time scale. This technique has already been applied in INCA in Bolivia ,ARGO in Tibet and LAGO in South America . The main advantage of single particle technique using water Cherenkov is their sensitivity to photons, which represent up to 90%of the secondary particles at ground level for high energy photon initiated showers. Taking all the above considerations into account ,we plan to set up a water Cherenkov detector array, which is based on the experiments using single particles techniques for detection of GRBs in a above 5000-meter-high region located in Tibet .

伽玛射线暴（Gamma Ray Burst, GRB）是时间和方向都无法预知的来自宇宙深处的伽玛射线突然增强的现象。目前，人们对GRB的细致研究主要来自于卫星实验，然而，由于卫星实验受到有效面积的限制，随能量升高，到达地球的光子数急剧下降，虽然观测到了GeV能区的γ辐射，但也仅仅只有为数不多的几个，因此，卫星实验不能对GRB高能辐射能段做精细的测量。地面广延大气簇射实验阈能一般大于几百个GeV，因此，对GeV能区GRB高能辐射更是无能为力。因为几十GeV能区的宇宙线粒子，它的广筵大气簇射发展过程在海拔4000米左右时已接近尾声，因此，只有到更高的海拔，比如5200米左右时才能实现更有效的观测。在本项目中，我们计划在海拔5200米的地方建造80平方米的水切伦科夫(WCD)探测器阵列，利用单粒子技术，以来实现地面实验多GRB几十GeV光子的首次正观测，为大规模实验提供预言支持。

本项目计划在海拔5000多米的西藏高海拔地区建造利用单粒子技术探测GRB的水切伦科夫探测器，以期实现地面实验多GRB几十GeV光子的首次正观测。为以后在此海拔建造大型 WCD探测器阵列提供预言支持，在项目实施过程中主要取得了如下成果：1)研究了五千米海拔极端气温下的水介质防冻问题，在海拔5200米处开展了小型水探测器防冻实验，并以傅立叶定律为基础建立了单元探测器的热交换模型，用于预期水温的长期变化，进而指导采取防冻措施；2)模拟研究了基于水切伦科夫技术利用单粒子方法在五千米海拔探测GRB的灵敏度,模拟显示，我们探测器可以研究一些强暴的光变曲线变化；3）在海拔3650米的西藏大学研制了基于单粒子计数的水切伦科夫探测器样机，初步测试结果表明，该探测器性能稳定，可以探测到宇宙线，总事例率约为0.66KHz/㎡，事例率分布呈现出比较好的高斯分布特性；4）在海拔4300米的羊八井宇宙线观测站安装了一台水切伦科夫探测器，海拔5000米的西藏那曲巴乔拉垭口安装了两台水切伦科夫探测器，目前已取得半年的数据，得到了初步结果。5）为了观测GRB，我们还积极研究了超广角水透镜切伦科夫技术。该技术有大视场的特点，可以快速的巡天扫描，并且不逊于目前 IACT 技术的角分辨和能量分辨。这样不止适合研究如 GRB 和活动星系核这样远距离的河外源，还可以通过这些源的研究测量河外背景光，进而研究早期的星系演化和宇宙演化等；6）开展了利用EAS热中子探测技术研究宇宙线“膝”区物理的预研工作。搭建了由 16 台热中子探测器组成的宇宙线探测阵列，阵列的成功运行表明新型热中子探测器在高海拔地区探测宇宙线是可行的，该项工作目标是拓展 LHAASO 实验的探测手段和技术储备；7）参与了天文选址中，海拔5200m西藏阿里小孜达坂的气象数据采集工作。