MUONS WITH Eth ≥ 1 GEV AND MASS COMPOSITION IN THE ENERGY RANGE 1018 - 1020EV OBSERVED BY YAKUTSK EAS ARRAY

The ratio of the muon flux density to charged particle flux density at distances of 300 and 600 m from the shower axis (ρμ(300)/ρs(300) and ρμ(600)/ρs(600)) is measured. In addition, the energy dependence of ρμ(1000) is analysed for showers with energies above 1018 eV. A comparison between the experimental data and calculations performed with the QGSJET model is given for the cases of primary proton, iron nucleus and gamma-ray. We conclude that the showers with E0 ≥ 3 × 1018eV can be formed by light nuclei with a pronounced fraction of protons and helium nuclei. It is not excluded however that a small part of showers with energies above 1019 eV could be initiated by primary gamma-rays.