Influence of a medium on pair photoproduction and bremsstrahlung
  The creation of electron-positron pair by a photon and the bremsstrahlung of an electron in a medium are considered in high-energy region, where influence of the multiple scattering on the processes (the Landau-Pomeranchuk-Migdal (LPM) effect) becomes essential. The pair photoproduction probability is calculated with an accuracy up to the "next to leading logarithm". The integral characteristics: the radiation length and the total probability of pair photoproduction are analyzed under influence of the LPM effect, and the asymptotic expansions of these characteristics are derived. 
V. N. Baier and V. M. Katkov