Spontaneous decay of a two-level system close to a perfectly reflecting sphere

Within a first-order time-dependent perturbation approach, we compute the spontaneous decay rate of a two-level system placed in the vicinity of a perfectly reflecting spherical surface. We consider a model system on which the emitter is represented by a two-level monopole coupled to a Hermitian massless scalar field. Using the method of images, we determine the appropriate Green's function evaluated in the world line of the atom. The change in the spontaneous decay rate results from the interaction of the atom with its image. We provide a detailed analysis of the dependence of the decay rate on the sphere's radius, the atom's location, and the emitted radiation frequency. Both exterior and interior problems are discussed.