  Doppler backscattering of optical laser photons on a "flying mirror" of
relativistic electrons promises to yield coherent photons with MeV-range
energies. We compare the nuclear interaction of such a laser pulse with the
standard atom-laser interaction. The mean-field description of atoms must be
replaced by a rate equation and the classical field strength, far too faint in
nuclei, by the dipole transition rate. Significant nuclear excitation occurs
for photon numbers much smaller than typical for atoms. That drastically
reduces the requirements on the experimental realization of a "flying mirror".
