Delbrück Scattering above the Pair Production Threshold: Going beyond the Born Approximation.

A theoretical method to calculate Delbrück 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 αZ$\alpha Z$. 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 Delbrück amplitude and have to be taken into account for the analysis and guidance of gamma‐ray scattering experiments. [ABSTRACT FROM AUTHOR]