  The phenomenon of gamma-ray families featuring halos that is observed in an
experiment with x-ray emulsion chambers (XREC) in the Pamir experiment and in
other XREC experiments is explained. The experimental properties of halos are
analyzed via a comparison with the results of their simulation. It is shown
that gamma-ray families featuring halos are predominantly produced (more than
96 % of them) by protons and heliumnuclei. This makes it possible to employ the
experimental properties of halos to estimate the fraction of protons and helium
nuclei in the mass composition of primary cosmic radiation.
