We consider the production of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ by ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ photodisintegration initiated by nonthermal energy releases during early cosmic epochs. We find that this process cannot be the predominant source of primordial ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ since it would result in anomalously high ${}_{}{}^3{}_{}{}^{}\mathrm{He}$/D ratios in conflict with standard chemical evolution assumptions. We apply this fact to constrain topological defect models of highest energy cosmic ray (HECR) production. Such models have been proposed as possible sources of ultrahigh energy particles and γ rays with energies above ${10}^{20}$ eV. The constraints on these models derived from ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ photodisintegration are compared to corresponding limits from spectral distortions of the cosmic microwave background radiation and from the observed diffuse γ-ray background. It is shown that for reasonable primary particle injection spectra superconducting cosmic strings, unlike ordinary strings or annihilating monopoles, cannot produce the HECR flux at the present epoch without violating at least the ${}_{}{}^4{}_{}{}^{}\mathrm{photodisintegration}$ bound. The constraint from the diffuse γ-ray background rules out the dominant production of HECR by the decay of grand unification particles in models with cosmological evolution assuming standard fragmentation functions. Constraints on massive black hole induced photodisintegration are also discussed. © 1995 The American Physical Society.

• Received 11 September 1995

DOI:https://doi.org/10.1103/PhysRevD.52.6682