Study of High Energy Cosmic Rays in the Tibetan Highlands

1995年，中国与日本合作在西藏羊八井（海拔4300）运行的风淋阵进行了扩建（探测器数量从65台增加到221台），我们正在寻找10 TeV的天体伽玛射线点源，并研究超高能区的一次宇宙射线。这是一个独特的风淋阵，能够以足够的精度探测10 TeV的宇宙射线，我们已经获得了许多重要的科学成果。利用实验获得的数据，我们得到了4 × 10^{14} ~ 2 × 10^{16} eV能量区域的一次能谱，在2 × 10^{15} eV附近呈现出逐渐变陡的趋势。这一结果将为高能宇宙射线加速的机理提供强有力的约束。我们还研究了1990 ~ 1993年宇宙射线对太阳阴影的年时间变化（对应于太阳活动周期的接近最大值或衰退期），发现太阳阴影的运动与太阳活动的相位之间存在很强的相关性。最终，我们的观测将获得关于太阳和行星际磁场的新信息。1996年，我们又在旧阵列中心附近建造了高密度风淋阵（探测器间距为7.5m）和由乳化液室和爆发计数器组成的探测器综合体，总面积为80 m²。前者是寻找超TeV伽玛射线源，后者是观测膝盖附近的主质子谱，将持续到1999年夏天。由于数据正在分析中，初步结果将在今年年底左右得出。

In 1995, our air shower array, which has been operating at Yangbajing (4300 above sea level in Tibet as a collaboration experiment between Japan and China, was enlarged (the number of detectors increased from 65 to 221), and we are searching for 10 TeV celestial gamma-ray point sources and also studying the primary cosmic rays in ultra-high energy region. This is a unique air shower array capable of detecting 10 TeV cosmic rays with sufficient accuracy and we have already obtained a lot of important scientific results. Using the data obtained from this experiment, we obtained the primary energy spectrum in the energy region from 4x10^{14} to 2x10^{16} eV,showing a gradual steepening at around 2x10^{15} eV.This result will give a strong constraint to the mechanism of high energy cosmic ray acceleration. We also examined a yearly time variation of the Sun's shadow by cosmic rays in the period during 1990 through 1993 (corresponding to near maximum or declining phase of the solar activity cycle, and found that there exists a strong correlation between the movement of the sun's shadow and the phase of the solar activity. Eventually, our observation will obtain new information about the solar and interplanetary magnetic fields.In 1996, we also constructed a high density air shower array (detector separation is 7.5m) and a detector complex of emulsion chambers and burst counters with the total area of 80 m^2 near the center of the old array. The former is to search for supra TeV gammma ray sources and the latter is to observe the primary proton spectrum around the knee region and will be continued until summer of 1999. A preliminary results will be obtained around the end of this year since the data are now under analysis.