Chemical composition of primary cosmic rays with energies from 10(15) to 10(16.5) eV

The chemical composition of primary cosmic rays with energies from 10(15) t o 10(16.5),V, so called "knee" region, is examined. We have observed the ti me structures of air Cerenkov light associated with air showers at Mt. Chac altaya, Bolivia, since 1995. The distribution of a parameter that character izes the observed time structures is compared with that calculated with a M onte Carlo technique for various chemical compositions. Then the energy dep endence of the average logarithmic mass numbers < lnA > of the primary cosm ic rays is determined. The present result at 10(15.3) eV is almost consiste nt with the result of JACEE (A similar or equal to 12) and shows gradual in crease in < ln A > as a function of the primary energy (A similar or equal to 24 at 10(16) eV). Form the comparison of the observational results with several theoretical models, we conclude that the supernova explosion of mas sive stars is a plausible candidate for the origin of cosmic rays around th e "knee" region. (C) 2001 Elsevier Science B.V. All rights reserved.