Cross section of high-energy photon splitting in the electric fields of   heavy atoms
  Various differential cross sections of high-energy photon splitting in the electric fields of heavy atoms are calculated exactly in the parameter \al. The consideration is based on the quasiclassical approach applicable for small angles between all photon momenta. The expressions obtained are valid for arbitrary transverse momenta of final photons . The detailed investigation of the process is performed taking into account the effect of screening . The exact cross section turns out to be noticeably smaller than the result obtained in the Born approximation. 
R.N. Lee, A.I. Milstein, V.M. Strakhovenko