Fluctuations of Xmax and Primary Particle Mass Composition in the Range of Energy 5 10¹⁷ - 3 10¹⁹ 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.