Measurement of Nuclear Composition of Primary Cosmic rays

原始宇宙线的核成分是研究宇宙线起源的线索信息。然而，在10^ eV以上的能量范围内<14>，由于其强度低，无法直接观察。观测到的一次能谱在10^ eV附近出现了弯曲<15>，这被认为与一次宇宙线加速机制的某些变化有关。在目前的研究中，我们已经测量了空气Chrenkov光的到达时间分布与空气阵雨在山。Chacaltaya在Blivia。初级核成分深刻地反映了相应的初级宇宙线的纵向发展，特别是在高层大气深度，然后在相应的空气簇射相关的空气切伦科夫光的到达时间分布。在蒙特-卡罗模拟结果的基础上，我们选择了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。从1996年6月到10月，每两周对新月进行一次观测。观测到的上升时间根据空气簇射的大小和入射天顶角进行分类。然后将分类后的上升时间分布与用蒙特-卡罗方法计算的上升时间分布进行了比较。这一结果还需要两年多的观测才能得出，但膝区的核组成初步结果是：（1）初级宇宙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%，这与膝区以上初级宇宙线中质子占优势的起源模型是不一致的。

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.