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 $\text{Xmax}$ obtained with the Yakutsk EAS array at fixed energies of $5\times {10}^{17}$, $1\times {10}^{18}$ and $5\times {10}^{18}$ 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 $\text{Xmax}$ is selected.