Changes in the chemical composition of primary cosmic radiation at ultrahigh energies

The signals from particles of extensive air showers in the energy region of 1017–1020 eV in both the surface and underground scintillation detectors of the Yakutsk array are calculated using the CORSIKA 6.616 and GEANT4 software packages and compared with experimental data. It is shown that a transition from a heavy primary composition to proton primaries at energies (1–2.6) × 1018 eV and from primary protons again to heavy primaries at energies above 1.3 × 1019 eV might be observed.