Possible composition of the primary particles at ultrahigh energies observed at the Yakutsk array

The composition of the primary cosmic radiation should be studied by various alternative approaches. A promising way is to exploit a dependence of the fraction of muons in an extensive air shower on the atomic number of the primary nuclei. A comparison of the fraction of muons at 600 m from the shower axis observed at the Yakutsk array in the vertical air showers at ultra-high energies with results of simulations in terms of QGSJET-II and Gheisha- 2002d models carried out with the help of the CORSIKA 6.616 and GEANT4 codes showed rather heavy composition of the primary radiation. But some errors in these models should be taken into account. At last, one has to allow for the fact that signals in the surface and underground scintillation detectors of the Yakutsk array from various particles of extensive air showers are measured in different units. All these corrections taken together show the proton composition of the primary radiation in the energy region of ~2.∙10 18 − 10 19 eV. At lower energies a composition is heavier. The change from the heavy composition to the primary protons occurs in the energy interval of 9.∙10 17 − 2.∙10 18 eV. It is not excluded that at energies above 1.1∙10 19 eV the composition may be also heavier as illustrated by a trend of data.