Mass composition of cosmic rays with energy above 10**17 eV according to   surface detectors of the Yakutsk EAS array
  We discuss the lateral distribution of charged particles in extensive air showers with energy above $10^{17}$ eV measured by surface scintillation detectors of Yakutsk EAS array. The analysis covers the data obtained during the period from 1977 to 2013. Experimental values are compared to theoretical predictions obtained with the use of CORSIKA code within frameworks of different hadron interaction models. The best agreement between theory and experiment is observed for QGSJet01 and QGSJet-II-04 models. A change in the cosmic ray mass composition towards proton is observed in the energy range $(1-20) \times 10^{17}$ eV. 
A.V. Glushkov, A. Sabourov
astro-ph.HE