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 ($\rhom(300)/\rhos(300)$ and $\rhom(600)/\rhos(600)$) is measured. In addition, the energy dependence of $\rhom(1000)$ 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 $\E\ge3\times10^{18}$ 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. 
S.P. Knurenko, V.A. Kolosov, I.T. Makarov, I.Ye. Sleptsov, V.R.   Sleptsova, G.G. Struchkov