Mass composition of cosmic rays above 0.1 EeV by the Yakutsk array data

The paper presents the results of the longitudinal development of extensive air showers (X$_{max}$) of ultra-high energies and mass composition of cosmic rays. The measurements of X$_{max}$ are based on data from observations of the Cherenkov radiation at the Yakutsk array for the period 1974-2014. The cascade curves of individual showers and the depth of maximum X$_{max}$ were reconstructed over the energy range 10$^{16}$-5.7$\cdot$10$^{19}$ eV. It is shown that the displacement rate of the parameter dX$_{max}$ / dE in the atmosphere is nonlinear and depends on the energy. Such a feature indicates a change in mass composition, which is confirmed by fluctuations of X$_{max}$ in this energy region. The composition of cosmic rays was determined by interpolation using the QGSJetII-04 model.