Measurement of Nuclear Composition of Primary Cosmic Rays

点击翻译按钮获取中文摘要

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.