The cosmic ray primary composition at the knee region from lateral distributions of atmospheric C˘erenkov photons in extensive air showers

We have measured the lateral distributions of atmospheric C˘erenkov 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 C˘erenkov 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.