Gamma-ray families with halos: Main characteristics and possibilities of using them to estimate the p plus He fraction in the mass composition of cosmic rays at energies 1-100 PeV

Characteristics of gamma-ray families with halos (XREC, Pamir) and data of experiments with EAS are analyzed to estimate the proton and helium (p+He) fractions in the primary cosmic radiation at E-0 = 1-100 PeV. It is shown that at energies E-0 similar to 1-100 PeV the fraction of p+He remains significant, namely, the fraction of p+He is near 40% at E-0 = 10 PeV.