  A uniformly accelerated detector in an inertial vacuum undergoes an
unavoidable dissipation, and the final steady-state becomes thermal. However,
to attain such a mixed state, there is no bound for the acceleration of the
single atomic detector. Here we show that the scenario is entirely different
for two atoms with the same energy levels. There exists a critical limit of the
acceleration for two atomic detectors, below which the purity of a particular
initial state can be preserved. We observe that the generator of the
dissipative dynamics (Lindbladian) is invariant under a weak symmetry
transformation at this limit. Hence one of the eigenstates of the symmetry
operator is unchanged during the evolution. This kind of state is called a
quantum dark state, which is essentially a decoherence-free subspace. As a
consequence, the system becomes localized, and it can skip the Unruh
thermalization. Beyond the critical limit, the symmetry is explicitly broken.
Therefore our results suggest that the system goes through a first-order
dissipative phase transition from a localized to a thermal phase.
