  Quantum Hadrodynamics in mean field approximation describes the effective
nucleon-nucleus potential (about -50 MeV deep) as resulting from a strong
repulsive vector (about 400 MeV) and a strong attractive scalar (about -450
MeV) contribution. This scalar-vector Lorentz structure implies a significant
lowering of the threshold for $p\bar{p}$ photoproduction on a nucleus by about
850 MeV as compared to the free case since charge conjugation reverses the sign
of the vector potential contribution in the equation of motion for the
$\bar{p}$ states. It also implies a certain size of the photon induced
$p\bar{p}$ pair creation cross section near threshold which is calculated for a
target nucleus $^{208}$Pb. We also indicate a measurable second signature of
the $p\bar{p}$ photoproduction process by estimating the increased cross
section for emission of charged pions as a consequence of $\bar{p}$
annihilation within the nucleus.
