Extensive air showers (EAS’s) accompanied by families of high-energy cascade showers were observed at Mt. Norikura (738 g ${\mathrm{cm}}^{\mathrm{-}2}$). 99 families of γ-ray- and hadron-origin showers with total energies J$E_{\gamma }$,H≥10 TeV were obtained. The success rate of the combination between families and EAS’s reaches to almost 90% (87 events). The families are associated with young EAS’s, with mean age parameter s∼0.7, whose sizes distribute widely over three orders of magnitude up to ${10}^8$. The size spectrum of the family-associated EAS’s coincides with the general EAS’s in the size region above 5×${10}^6$ but the former drops rapidly from the latter below this critical size. From the absolute intensity of J$E_{\gamma }$,H spectrum the proton fraction in the primary cosmic rays is deduced to be (14±5)%, with an error of one standard deviation, in the primary energies (5×${10}^{14}$)–${10}^{16}$ eV, in comparison with a Monte Carlo simulation assuming an adequate interaction model. This agrees with the result obtained by the work with other mountain data and is also compatible with the result inferred from the size spectrum gap between the family-associated EAS’s and the general EAS’s in the region below the critical size.

• Received 12 October 1988

DOI:https://doi.org/10.1103/PhysRevD.39.1267