  The characteristics of extensive air showers that are sensitive to the mass
composition of primary cosmic radiation are recorded in the EAS core. These
characteristics include the parameters of the phenomenon of "halo". Studies
have shown that the bulk of the events recorded in XRECs are produced by
protons and He nuclei, which allows the estimation of the fraction of light
nuclei in PCR. The events under analysis were obtained in the XREC PAMIR
experiment, and the properties of these events were studied at distances
extending up to about 10 cm from the EAS axis with a resolution of
approximately 30 micro m. The investigation of the parameters of events in EAS
cores by the halo method permitted analyzing the mass composition of PCR at
E0=10 PeV and assessing the fraction of light nuclei in PCR, which depend only
slightly on the model of EAS propagation through the atmosphere. From an
analysis of gamma-ray families featuring a halo and multicenter halos obtained
in the PAMIR XREC, it was found that the fraction of the p+He component of PCR
was about 40%.

High Energy Astrophysical Phenomena