Measured Coulomb corrections to the pair production cross sections of Ta and Bi

Abstract

The cross sections for production of electron pairs on Ta and Bi nuclei by photons of energies between 3 and 30 MeV were deduced from total cross sections for photon absorption. Comparison with pair cross section values calculated in the plane-wave approximation and corrected for screening gave experimental values of the further correction required to account for Coulomb distortion. Above 15 MeV the experimental values of the Coulomb correction differ from values calculated using a semi-empirical recipe due to Maximon and Gimm, but agree fairly well with a recipe due to Øverbø up to 20 MeV, where they begin to differ. Either of these Coulomb correction formulas combined with screening corrections calculated using relativistic atomic form factors allows the pair cross section of Ta or Bi to be calculated accurately enough to reproduce the total photon cross section to about 1%. Neither formula can be relied upon to determine an unknown photonuclear cross section to useful accuracy from a measured total cross section.