Screening corrections in electron pair production by intermediato and low-energy photons

The screening correction to the total cross section for coherent pair production is calculated for photon energies ranging almost from threshold to 45MeV. The correction is given as the sum of the (negative) Born screening term and a contribution which is obtained by simply shifting the point-Coulomb positron energy spectrum in energy by the amount V(sub el)(0), V(sub el)(r) being the potential caused by the atomic electrons. This contribution is positive and relatively large in the threshold region but decreases fast with increasing photon energy. Results are given for Z<= 92, in terms of tables and simple analytic fits to the numerically calculated results. (Auth.)