Average mass composition of primary cosmic rays in the superhigh energy region by Yakutsk complex EAS array data

Abstract

The characteristics relating to the lateral and longitudinal development of EAS in the energy region of 10^15-10^19eV have been analyzed in the framework of the QGSJET model and of mass composition of primary cosmic rays. It is found that at E(0) >= 5*10^15eV the mean mass composition of primary cosmic rays begins to vary as indicated by a rise of <ln A> with increasing energy. The maximum value of <ln A> is observed at E(0) ~ (5-50)*10^16eV. It is confirmed by data of many compact EAS arrays and does not contradict an anomalous diffusion model of cosmic ray propagation in our Galaxy. In the superhigh energy region (>=10^18eV) the value <ln A> begins to decrease, i.e. the mass composition becomes lighter and consists of protons and nuclei of He and C. It does not contradict our earlier estimations for the mass composition and points to a growing role of the metagalactic component of cosmic rays in the superhigh energy region.