We have measured the lateral distributions of atmospheric Čerenkov photons associated with extensive air showers to study the chemical composition of the primary cosmic rays in the energy range from 1014.5 to 1016 eV, so called knee region. The atmospheric Čerenkov photon detectors were installed in the EAS array at Mt. Chacaltaya. The measured lateral distributions were fitted with an empirical formula with a parameter which is sensitive to mass numbers of primary cosmic rays. A three species model (proton, carbon, and iron nuclei) is used for the chemical composition study of the present experiment. To determine the mixing ratio of the three species, the distributions of the parameter for the observed events were examined to reproduce those for the simulated ones calculated with the CORSIKA code using the QGSJET hadronic interaction model. From the obtained mixing ratio of the three species, we determined the mean logarithmic mass numbers of primary cosmic rays, 〈ln A〉, in the knee region. The present result shows that the values of 〈ln A〉 are higher than the logarithmic mass number for carbon and the proton component is no more dominant at the knee.
|Número de páginas||8|
|Estado||Publicada - jul. 2008|
Nota bibliográficaFunding Information:
We would like to thank all the staffs of Instituto de Investigaciones Fisicas, Universidad Mayor de San Andres for their daily support to our experiment. We also wish to acknowledge helpful support by the staffs of Institute for Cosmic Ray Research, University of Tokyo. From the Ministry of Education, Culture, Sports, Science and Technology of Japan, this work was supported by a grant-in-aid for Scientific Research, and HT was supported by the 21st Century COE Program at Tokyo Tech “Nanometer-Scale Quantum Physics”.