Abundance of primary protons at (1–3)×1015eV inferred from a simulation of extensive air showers withγ-ray families

A Monte Carlo simulation of extensive air showers (EAS’s) was done under a particle-interaction model and was compared with the EAS’s accompanied by γ-ray and hadronic families with total energy J $E_{\gamma }$,H greater than 10 TeV obtained in the Mt. Norikura experiment (2780 m above sea level). The strong correlation between the shower size $N_e$ of EAS’s with family and the primary energy, $E_0$, was obtained from the simulation as $E_0$/$N_e$ ${=(2.0\mathrm{}}_{\mathrm{-}0.7}^{+2.7\mathrm{}}$) GeV with 95% width. The flux of the EAS’s with family is sensitive to the chemical composition, especially to the fraction of protons in the primary cosmic-ray particles. The fraction of protons is inferred to be (20±6)% and (23±9)%, with one standard deviation (1σ), at energy (${1.4}_{\mathrm{-}0.5}^{+1.9\mathrm{}}$)×${10}^{15}$ eV and (${2.8}_{\mathrm{-}1.0}^{+3.8\mathrm{}}$)×${10}^{15}$ eV with 95% confidence interval, respectively, under the assumption of increasing cross section ${\sigma }_{p\mathrm{-}\mathrm{air}\mathrm{\propto }{E}^{0.055}}$ and approximate Feynman scaling in the fragmentation region. These values are in agreement with the values obtained from other mountain emulsion-chamber experiments within 2σ. The proton fraction thus obtained, around 20%, is much less than the normal abundance (∼40%) expected from direct observations at lower energies around ${10}^{12}$ eV.