  We generalize the Unruh-DeWitt detector model to second quantization. We
illustrate this model by applying it to an excited particle in a superposition
of relativistic velocities. We calculate, to first order, how its decay depends
on whether its superposition of velocities is coherent or incoherent. Further,
we generalize the framework of quantum reference frames to allow
transformations to the rest frames of second-quantized Unruh DeWitt detectors.
As an application, we show how to transform into the rest frame of a decaying
particle that, in the laboratory frame, is in a linear superposition of
relativistically differing velocities.
