  Considering the contributions of the main term of the relativistic Coulombian
Green function given by Hostler to the second order of S matrix element and
taking into account only the large components of the Dirac spinor of the ground
state we obtain the imaginary part of the Rayleigh amplitudes in terms of
elementary functions. Thereby simple and high accurate formulae for the total
cross-sections for photoeffect and pair production with the electron created in
the K-shell are obtained \textit{via} the optical theorem.Comparing the
predictions given by our formulae with the full relativistic numerical
calculations of Kissel \textit{et al} [Phys. Rev. A 22, 1970 (1980)] and
Scofield [LLRL, Internal Report,1973], a good agreement is found for photon
energies above the pair production threshold up to 5 MeV for any Z elements. We
present our numerical results for the total photoeffect and pair production
cross-sections, for various photon energies for the K-shell of Ag and Pb.
