High-energy electron-positron photoproduction cross section close to the end of the spectrum

We consider the cross section of electron-positron pair production by a high-energy photon in a strong Coulomb field close to the end of the electron or positron spectrum. We show that the cross section essentially differs from the result obtained in the Born approximation as well as from the result which takes into account the Coulomb corrections under the assumption that both electron and positron are ultrarelativistic. The cross section of bremsstrahlung in a strong Coulomb field by a high-energy electron is also obtained in the region where the final electron is not ultrarelativistic.