The chemical composition of primary cosmic rays in the energy range ~1015 eV from muon energy spectrum with the Okayama muon telescope

The energy spectrum of muons associated with extensive air shower (EAS) is studied by the synchronized observations with the EAS array and the solid iron spectrometer for muons, in the primary energy range of about 50 TeV 5 PeV. We have determined muons momentum in EAS for 344 events. As the result, the median energy of spectrum lies on about 10 GeV and its FWHM is 2 to 70 GeV. And the obtained muon spectrum is compared with the simulated results. As follows from Aires air shower simulation with SIBYLL/HAS for hadronic interaction models assuming proton dominant model for primary cosmic ray compositions and iron dominant one, the muon energy spectrum below 20 GeV is well explained in case of the proton dominant composition. Above 20 GeV, the obtained spectrum indicates no significant difference between primary compositions.