  We estimate the transition rates of a uniformly accelerated Unruh-DeWitt
detector coupled to a quantum field with reflecting conditions on a boundary
plane (a "mirror"). We find that these are essentially indistinguishable from
the usual Unruh rates, viz. that the Unruh effect persists in the presence of
the mirror. This shows that the Unruh effect is not merely a consequence of the
entanglement between left and right Rindler quanta in the Minkowski vacuum.
Since in this setup the state of the field in the Rindler wedge is pure, we
argue furthermore that the relevant entropy in the Unruh effect cannot be the
von Neumann entanglement entropy. We suggest, in alternative, that it is the
Shannon entropy associated with Heisenberg uncertainty.
