The chemical composition of primary cosmic rays with energies from 1015 to 1016.5 eV, so called "knee" region, is examined. We have observed the time structures of air Čerenkov light associated with air showers at Mt. Chacaltaya, Bolivia, since 1995. The distribution of a parameter that characterizes the observed time structures is compared with that calculated with a Monte Carlo technique for various chemical compositions. Then the energy dependence of the average logarithmic mass numbers 〈lnA〉 of the primary cosmic rays is determined. The present result at 1015.3 eV is almost consistent with the result of JACEE (A≃12) and shows gradual increase in 〈lnA〉 as a function of the primary energy (A≃24 at 1016 eV). Form the comparison of the observational results with several theoretical models, we conclude that the supernova explosion of massive stars is a plausible candidate for the origin of cosmic rays around the "knee" region.
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The authors would like to thank all the staffs of Institute de Investigaciones Fisicas, Universidad Mayor de San Andres, La Paz, Bolivia, for their helpful support to our experiment. This work is supported in part by a grant-in-aid for scientific research from the Ministry of Education, Science, Sports, and Culture, Japan. The authors also would like to appreciate helpful supports by the staffs of Institute for Cosmic Ray Research, University of Tokyo for the continuation of our experiment at Mount Chacaltaya.