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，我们测量了与空气阵雨有关的空气Chrenkov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。1996年6月至10月，每两周在新月前后进行一次观测。观测到的上升时间根据风簇的大小和相关风簇的入射天顶角进行分类。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。经过两年多的观测，初步得出了膝部核组成的结果：(1)原宇宙射线的平均原子序数为9 ~ 15，(2)质子和铁的上限分数分别为25%和50%，(3)（He+CNO+Ne-S）的下限分数为40%。无论如何，我们的结果与这样的宇宙射线起源模型不一致，即质子在膝盖以上区域的主要成分中占主导地位。

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 Chrenkov light associated with air showers at Mt.Chacaltaya in Blivia. 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. Observations are made for every two weeks around new moons from June to October in 1996. 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 result will be obtained after more two year observations, but 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,(2) the upper limit fraction of proton ant that of iron is 25 and 50%, respectively,(3) the lower limit fraction of (He+CNO+Ne-S) is 40%.In anyway, our result is inconsistent with such models of cosmic ray origin that proton is dominant in the primary composition above the knee region.