  In order to address the problem of the validity of the "background field
approximation", we introduce a dynamical model for a mirror described by a
massive quantum field. We then analyze the properties of the scattering of a
massless field from this dynamical mirror and compare the results with the
corresponding quantities evaluated using the original Davies Fulling model in
which the mirror is represented by a boundary condition imposed on the massless
field at its surface. We show that in certain circumstances, the recoils of the
dynamical mirror induce decoherence effects which subsist even when the mass of
the mirror is sent to infinity. In particular we study the case of the
uniformly accelerated mirror and prove that, after a certain lapse of proper
time, the decoherence effects inevitably dominate the physics of the quanta
emitted forward. Then, the vanishing of the mean flux obtained in the Davies
Fulling model is no longer found but replaced by a positive incoherent flux.
