States insensitive to the Unruh effect in multilevel detectors

We give a general treatment of the spontaneous excitation rates and the non-relativistic Lamb shift of constantly accelerated multilevel atoms as a model for multilevel detectors. Using a covariant formulation of the dipole coupling between the atom and the electromagnetic field we show that new Raman-like transitions can be induced by the acceleration. Under certain conditions these transitions can lead to stable ground and excited states that are not affected by the noninertial motion. The magnitude of the Unruh effect is not altered by multilevel effects. Both the spontaneous excitation rates and the Lamb shift are not within the range of measurability.