Measurement of Nuclear Composition of Primary Cosmic Rays

初级宇宙射线的核组成是研究宇宙射线起源的线索信息。然而，在10^<14>eV以上的能量范围内，由于其强度较低，无法进行直接观测。观测到的初级能谱在10^<15>eV处出现了众所周知的弯曲，这种弯曲被认为与初级宇宙射线加速机制的某些变化有关。在本研究中，我们测量了玻利维亚Chacaltaya山与空气阵雨相关的空气切伦科夫光的到达时间分布。初级核成分深刻反映了相应的初级宇宙射线的纵向发展，特别是在大气上层深度，进而反映了与相应的空气阵雨相关的空气切伦科夫光的到达时间分布。在蒙特卡罗模拟的基础上，我们选择积分切伦科夫脉冲的上升时间作为表征到达时间分布的参数。观测到的上升时间根据风阵雨的大小和入射天顶以及相关风阵雨的角度进行分类。然后将分类上升时间的分布与蒙特卡罗法计算的上升时间进行了比较。初步结果表明：(1)初级宇宙射线的平均原子序数在10^<15.5>eV以下为9 ~ 15，并随着初级能量的增加而逐渐增大；(2)初级宇宙射线的主要成分是10^<15>eV的He和CNO基团，以及10^<15.5>eV以上的Ne-Fe基团，无论如何，我们的结果与以质子为主的宇宙射线起源模型是不一致的。这一结果表明，像Wolf-Rayt恒星这样的大质量恒星的超新星爆炸有利于起源。

The nuclear composition of primary cosmic rays is clue information for the investigation of the cosmic ray origin. In the energy range above 10^<14>eV,however, no direct observation can be done because of their low intensities. Observed primary energy spectra show the well known bending around 10^<15>eV,the knee of the spectrum, and this bending is thought to be related to some changes of acceleration mechanism of primary cosmic rays. In the present research, we have measured the arrival time distributions of air Cherenkov light associated with air showers at Mt, Chacaltaya in Bolivia. The primary nuclear composition reflects deeply on the longitudinal development of corresponding primary cosmic rays especially at upper atmospheric depth, and then on the arrival time distribution of air Cherenkov light associated with corresponding air showers. On the basis of a result of our Monte-Carlo simulation, we chose the rise time of integrated Cherenkov pulses as a parameter which characterizes the arrival time distribution. The observed rise times are classified according to air shower sizes and incident zenith, angles of their associated air showers. Then the distribution of classified rise times are compared with those calculated with a Monte-Carlo technique. The preliminary result on the nuclear composition at the knee region is as followings :(1) the average atomic number of primary cosmic rays is 9 to 15 below 10^<15.5>eV,and gradually increases with primary energy,(2) main component is He and CNO group at 10^<15>eV,and Ne-Fe group above 10^<15.5>eV,In anyway, our result is inconsistent with such models of cosmic ray origin that protons are dominant in the primary cosmic rays above the knee region. This result suggests that super nova explosions of massive stars such as Wolf-Rayt stars are favorable for the origin.