  We evaluate dissipative effects for a system consisting of a massive Dirac
field confined between two walls, one of them oscillating, in 1+1 dimensions.
In the model that we consider, a dimensionless parameter characterizing each
wall is tuned so that bag-boundary conditions are attained for a particular
value. We present explicit results for the probability of creating a fermion
pair out of the vacuum, and relate the total vacuum decay probability to the
imaginary part of the effective action.
