Muons (>or=1 GeV) in large extensive air showers of energies between 1016.5eV and 1019.5eV observed at Akeno

Abstract

The properties of muons (>or=1 GeV) in giant air showers between 10^16.5 eV and 10^19.5 eV are measured by the Akeno 1 km², 20 km² and 100 km² air shower arrays. The lateral distribution of muons is well fitted by the formula given by Greisen (1960) with parameters beta =2.52+or-0.02 and log(R₀)=(0.58+or-0.04) (sec theta -1)+(2.39+or-0.05) between 10^16.5 eV and 10^19.0 eV within 800 m of the air shower axis, where theta is the zenith angle of the shower arrival direction. The relation between the total number of muons (N_mu ) and the total number of electrons (N_e) derived using these parameters is expressed on average as N_mu =(2:6+or-1.3)*10^{5.0+ alpha} (N_e/10⁷)^b where alpha =(1.07+or-0.13) (sec theta -1) and b=(0.77+or-0.02)-(0.17+or-0.02) (sec theta -1). The lateral distribution of muons becomes steeper than expected from extrapolation of the above formula for core distances >800 m and is expressed as rho _mu =N_mu (C'_mu /R₀²)r^-0.75 (1+r)^{- beta} (1+(R/800 m)³)^{- delta} (r=R/R₀, where R is a core distance). No deviation from this formula has been observed for sec theta 19.0 eV. The relation between the muon density (S(600)) and the charged-particle density on the ground at 600 m from the core (S(600)) is studied between 10^16.5 eV and 10^19.0 eV. A systematic change in the chemical composition of cosmic rays from a predominantly heavy to a predominantly light composition above 10^17.5 eV claimed by the Fly's Eye group has not been detected beyond the present experimental uncertainties. The present experiment suggests a much smaller rate of change of composition between 10^17.5 eV and 10^18.5 eV than that from the Fly's Eye experiment.