States insensitive to the Unruh effect in multi-level detectors

We give a general treatment of the spontaneous excitation rates and the non-relativistic Lamb shift of constantly accelerated multi-level atoms as a model for multi-level 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 which are not affected by the non inertial motion. The magnitude of the Unruh effect is not altered by multi-level effects. Both the spontaneous excitation rates and the Lamb shift are not within the range of measurability. PACS: 04.62.+v, 42.50.-p, 32.70.Jz