Primary-cosmic-ray protons above 1015 eV derived from the observation of superhigh-energy halo events

The primary-cosmic-ray proton flux >1015 eV is derived through analyses on superhigh-energy halo events observed by the Mt. Fuji and Mt. Kanbala emulsion-chamber experiments. Interpretations are made for the origin of halo events based on Monte Carlo simulation, which clarifies the sensitivity of the halo events to the primary-composition and nuclear-interaction models. It is shown that the intensity of the halo events is most sensitive to the primary proton flux and the feature of the fragmentation region of the multiple production. The proton flux >1015 eV is estimated as nearly 3 times less than the extrapolated low-energy measurement by Ryan et al. suggesting the heavy-nuclei dominance above the knee energy of the total spectrum.