Composition of Cosmic Rays with E 0 ⩾ 1017 eV Based on Data from the Ground Scintillation Detectors of the Yakutsk EAS Array

The lateral distribution of cascade particles in extensive air showers from cosmic rays with energy E0 ⩾ 1017 eV has been studied at the Yakutsk array by the ground scintillation detectors over the period of continuous observations 1977–2017. The experimental data are compared with those computed with various models for the development of extensive air showers from the CORSIKA software package. The best agreement between the theory and experiment is observed for the QGSjet-01-d model. In the energy range (1−20)× 1017 eV there is a change in the cosmic-raymass composition from 〈lnA〉 ≈ 2.5 to the proton one.