  We consider a real massless scalar field in 3+1 dimensions satisfying a Robin
boundary condition at a nonrelativistic moving mirror. Considering vacuum as
the initial field state, we compute explicitly the number of particles created
per unit frequency and per unit solid angle, exhibiting in this way the angular
dependence of the spectral distribution. The well known cases of Dirichlet and
Neumann boundary conditions may be reobtained as particular cases from our
results. We show that the particle creation rate can be considerably reduced
(with respect to the Dirichlet and Neumann cases) for particular values of the
Robin parameter. Our results extend for 3+1 dimensions previous results found
in the literature for 1+1 dimensions. Further, we also show that this
inhibition of the dynamical Casimir effect occurs for different angles of
particle emission.
