Fluctuations of Xmax and Primary Particle Mass Composition in the Range   of Energy 5 10^{17} - 3 10^{19} ev by Yakutsk Data
  The experimental distributions of $\Xmax$ obtained with the Yakutsk EAS array at fixed energies of $5 \times10^{17}$, $1\times10^{18}$ and $5\times10^{18}$ eV are analysed. A recent version of the QGSJET model is used as a tool of our analysis. In the framework of this model, the most adequate mass composition of primary particles satisfying the experimental data on $\Xmax$ is selected. 
S.P. Knurenko, A.A. Ivanov, V.A. Kolosov, Z.E. Petrov, I.Ye. Sleptsov,   G.G. Struchkov