Composition of Primary Cosmic Rays Beyond the ``Knee''from Emulsion   Chamber Observations
  We show that the simplest assumptions for the dynamics of particle production allow us to understand the fluxes of hadrons and photons at mountain altitudes as well as the structure of individual events. The analysis requires a heavy nuclear component of primary cosmic rays above the ``knee" in the spectrum with average mass number $<A> = 7.3 \pm 0.9$. 
C. G. S. Costa, F. Halzen, J. Bellandi and C. Salles