The primary cosmic-ray composition in 1015 - 1016 eV inferred from gamma-ray families observed at Mt. Fuji

By an analysis of the γ-ray families observed at Mt.Fuji ( -- 3725 m a.s.l.), a suggested primary composition in 1015 - 1016 eV is derived. A Monte-Carlo simulation is used in the analysis on the ΣEγ spectrum and on the cluster feature of the family. The observed ΣEγ spectrum in 102-5 x 103 TeV suggests that the flux of primary protons in 1015 - 1016 eV should be lower by a factor of 2 - 3 than a flux extrapolated simply from lower energies. This means that the integral proton spectrum tends to be steeper than Esup(-1.7) over 1014 eV and the spectral index might be -- -2.0 between 1015 - 1016 eV. If the total flux of primary particles shows no steepening up to 1016 eV, the fraction of primary protons to all particles should be less than -- 20 % in contrast to -- 45 % in a low energy region. Such a heavy-enriched composition is inferred also by the cluster feature of γ-ray families. (author)