Variations of the cosmic ray composition at energy above 0.1 EeV as observed by muon detectors of Yakutsk array

The lateral distribution of muons with ~1.0xsec(theta) GeV in extensive air showers within ~10^{17}-10^{19} eV energy region obtained during different observational periods from November 1987 to June 2013 has been analyzed. Experimental data have been compared to predictions of various hadron interaction models. The best agreement is observed with QGSJETII-04. Until 1996, the mass composition of cosmic rays with energy below 2x10^{18} eV was significantly lighter than in later periods.

Muons in EAS with E>=10^17eV according to the data from Yakutsk array

The lateral distribution of muons with threshold energy e ~ 1*sec(theta) GeV have been studied in showers with energy E>=10^17 eV. The data considered in the analysis have been collected from November 2011 to June 2013. Experimental values are compared to predictions obtained with the use of CORSIKA code within the framework of various hadron interaction models. The best agreement between experiment and theory is observed for QGSJETII-04 model. At E~10^17 eV it complies to a mixed cosmic ray composition with the mean atomic number <lnA>~3.0+/-0.5. At E>= 4*10^17 eV the composition varies around the value <lnA>~0.5.