Double Pair Production by Ultra High Energy Cosmic Ray Photons
  With use of CompHEP package we've made the detailed estimate of the influence of double e+e- pair production by photons (DPP) on the propagation of ultra high energy electromagnetic cascade. We show that in the models in which cosmic ray photons energy reaches few thousand EeV refined DPP analysis may lead to substantial difference in predicted photon spectrum compared to previous rough estimates. 
S.V. Demidov, O.E. Kalashev