Pair production with capture by energetic cosmic ray nuclei in a photon background

We investigate the ionization state of very energetic cosmic ray nuclei in photon fields, such as the cosmic microwave background in extragalactic propagation and the environment surrounding the acceleration site in astrophysical sources. We focus on the process of pair production with electron capture (PPC), where the interaction of a single photon with a nucleus produces an e± pair (similar to Bethe-Heitler process) with the subsequent capture of electron by the nucleus. This process effectively reduces the nucleus charge by one unit and counteracts the photoionization process. We show that during cosmological propagation, where the ultrahigh energy cosmic rays interact predominantly with the cosmic microwave background, ionization dominates over PPC for all the cases of practical interest. However, within the source environment and at sufficiently high energies, ionization and PPC processes can reach an equilibrium, leading to a significant fraction of dressed heavy nuclei. This provides a further limitation to the acceleration of high-Z nuclei to very high energies in environments with hot and dense photon fields. Published by the American Physical Society 2025