Photodisintegration of Ultrahigh-Energy Cosmic Rays by the Universal Radiation Field

We have carried out a detailed calculation to determine the effectiveness of the universal radiation field in disintegrating ultrahigh-energy cosmic-ray nuclei. We conclude that this process cannot lead to a pure-proton interpretation of the air-shower observations above ${10}^8$ GeV. Photodisintegration by the universal radiation is unimportant on time scales of ${10}^{10}$ yr or less for helium nuclei below ${10}^{10}$ GeV and for iron nuclei below ${10}^{11}$-GeV total energy. Photodisintegration by dilute starlight photons in intergalactic space is unimportant on time scales of ${10}^{10}$ yr or less at all energies.