Interactions of UHE cosmic ray nuclei with radiation during acceleration: consequences for the spectrum and composition

In this paper, we study the diffusive shock acceleration of cosmic-ray protons and nuclei, taking all the relevant interaction processes with photon backgrounds into account. We investigate how the competition between protons and nuclei is modified by such acceleration parameters as the acceleration rate, its rigidity dependence, the photon density, and the confinement capability of the sources. We find that protons are likely to be accelerated to higher energies than nuclei in the case of interaction-limited acceleration processes, whereas nuclei are accelerated to higher energies than protons for confinement-limited acceleration. Finally, we discuss our results in the context of possible astrophysical accelerators and in the light of recent cosmic-ray data.