Muons with E_th >= 1 Gev and Mass Composition in the Energy Range 10^{18}-10^{20} ev 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 ($\text{rhom}(300)\mathrm{/}\text{rhos}(300)$ and $\text{rhom}(600)\mathrm{/}\text{rhos}(600)$) is measured. In addition, the energy dependence of $\text{rhom}(1000)$ is analysed for showers with energies above ${10}^{18}$ 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 $\text{E}\ge 3\times {10}^{18}$ eV 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 ${10}^{19}$ eV could be initiated by primary gamma-rays.