  The effect of screening by atomic electrons on the behavior of electron and
positron wave functions in the continuous spectrum at small distances is
studied. It is shown that these asymptotics are affected not only by the static
potential of atomic electrons, but also by the polarization potential, as well
as by the exchange interaction, which is essential for nonrelativistic
electrons. A simple analytical expression is obtained for the photoproduction
cross section of $e^+e^-$ pair in an atomic field near the threshold. The
spectrum and angular distribution of the produced particles are considered. It
is shown that screening significantly affects the cross section in the
near-threshold region.
