FLUCTUATIONS OF Xmax AND PRIMARY PARTICLE MASS COMPOSITION IN THE RANGE OF ENERGY 5 × 1017 - 3 × 1019EV BY YAKUTSK DATA

The experimental distributions of Xmax obtained with the Yakutsk EAS array at fixed energies of 5 × 1017, 1 × 1018 and 5 × 1018eV 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.