  Photomeson production is the main energy loss for relativistic nucleons in
dense radiation fields like the cosmic microwave background and the radiation
fields in Gamma Ray Bursts (GRB) and jets of Active Galactic Nuclei (AGN). In
this paper we study photomeson production in typical GRB and AGN jet radiation
fields by using the recently developed Monte Carlo event generator SOPHIA (see
these proceedings). We discuss processes that are relevant for the physics of
cosmic ray acceleration and the production of neutrinos and gamma rays. We
compare our results with widely used approximations, and find significant
deviations, particularly for GRBs. The photoproduction of antibaryons as a so
far not considered effect in astrophysics is briefly discussed.
