Delbrck Scattering above the Pair Production Threshold: Going beyond the Born Approximation

A theoretical method to calculate Delbruck scattering amplitudes for photon energies above the electron-positron (e+e-${e}<^>{+}{e}<^>{-}$) pair production threshold is presented. The method is based on the application of the relativistic Dirac-Coulomb Green function and describes the interaction of the virtual e+e-$e<^>+e<^>-$ pair with the Coulomb field of a target to all orders in the coupling strength parameter alpha Z$\alpha Z$. To illustrate the application of the developed approach, detailed calculations have been performed for the scattering of 2.754 MeV photons off bare ions with a wide range of nuclear charge numbers. Results of these calculations clearly indicate that the higher-order terms beyond the Born approximation lead to a strong enhancement of the imaginary part of the Delbruck amplitude and have to be taken into account for the analysis and guidance of gamma-ray scattering experiments. This article presents an accurate approach for the calculation of Delbruck scattering amplitudes above the electron-positron pair production threshold that accounts for the Coulomb interaction with nucleus to all orders in alpha Z$\alpha Z$. Explicit calculations for the scattering of 2.754 MeV photons by zinc, cerium, lead, and plutonium nuclei are performed, and the effects of the higher order Coulomb corrections are investigated.image