  We investigate a two-level Unruh-DeWitt detector coupled to a massless scalar
field or its proper time derivative in $(1+1)$-dimensional Minkowski spacetime,
in a quantum state whose correlation structure across the Rindler horizon
mimics the stationary aspects of a firewall that Almheiri et al have argued to
ensue in an evaporating black hole spacetime. Within first-order perturbation
theory, we show that the detector's response on falling through the horizon is
sudden but finite. The difference from the Minkowski vacuum response is
proportional to $\omega^{-2}\ln(|\omega|)$ for the non-derivative detector and
to $\ln(|\omega|)$ for the derivative-coupling detector, both in the limit of a
large energy gap $\omega$ and in the limit of adiabatic switching. Adding to
the quantum state high Rindler temperature excitations behind the horizon
increases the detector's response proportionally to the temperature; this
situation has been suggested to model the energetic curtain proposal of
Braunstein et al. We speculate that the $(1+1)$-dimensional derivative-coupling
detector may be a good model for a non-derivative detector that crosses a
firewall in $3+1$ dimensions.
