Simultaneous observation of γ-families and their accompanying air showers was performed at Yangbajing (4300 m above sea level) in Tibet with the emulsion chambers and burst detectors which were linked with the Tibet-II air-shower array. For three years operation, we observed 177 γ-families (∑Eγ > 20 TeV) accompanying air showers with the size Ne > 2 × 105. Using this data and the simulation code CORSIKA, we obtained the primary proton spectrum in the energy range of 1015-1016 eV. The slope of the observed spectrum is steeper than that extrapolated smoothly from the lower energy region below 1014 eV, irrespective of the interaction models assumed in the simulation. The uncertainty of the absolute flux of protons due to the interaction model dependence involved in the simulation codes is at most 30%, which is much weaker than results of other air-shower experiments. Our experiment shows low intensity of light components (proton and helium) independent of the interaction models suggesting that the main component responsible for making the knee structure of the all-particle spectrum is composed of nuclei heavier than helium. This is the first measurement of the differential energy spectra of primary protons by selecting them event by event at the knee energy region.
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