  The experimental gamma-families in which events called "halos" were detected
using X-ray emulsion chambers (XRECs) of the PAMIR experiment are described.
The halo nature is explained within the standard model of nuclear interactions.
Based on events with halo, the fraction of p+He in primary cosmic rays (PCR)
was estimated. It is shown that the PCR mass composition remains mixed in the
range E0 = 1-100 PeV.
