Mass Composition of Cosmic Rays with Energies Greater than 1017 eV, According to the Data from Surface Scintillation Detectors of the Yakutsk EAS Array

The spatial distribution of cascade particles in extensive air showers from cosmic rays with energies of E0 ≥ 1017 eV is studied at the Yakutsk array using surface scintillation detectors in continuous observations from 1977 to 2017. The experimental data are compared to calculations obtained with different models of hadron interactions. The mass composition of the cosmic rays changes from 〈lnA〉 ∼ 2.5 to the mass composition of protons in the energy range of (1–20) × 1017 eV.