Photodisintegration of Iron Nuclei on Background Photons: Threshold Lorentz Factors and Survival Distances

An iron nucleus crossing intergalactic space is dismantled, nucleon by nucleon, when background photons are blueshifted in its rest frame past the giant-dipole threshold. Setting that threshold at 10 MeV gives a required Lorentz factor of 7.886e+09 against a mean microwave photon, corresponding to a total iron energy of 4.144e+20 eV; against a Wien-tail photon the energy falls to 1.642e+20 eV, and against an optical background photon to 2.627e+17 eV. With a resonance interaction length of about 3 megaparsecs, the survival probability of an intact iron nucleus is 3.567e-02 over 10 megaparsecs and 5.778e-08 over 50 megaparsecs. Iron observed at Earth at the highest energies must therefore be local, or must be the surviving core of a still heavier or still more energetic parent, and the composition arriving from any given distance is a processed composition. The density bound does not alter the photon backgrounds or the nuclear thresholds and plays no part in the result. Simulated and computed; not a measurement.