Ultra high energy cosmic rays (UHECR) are cosmic rays which are measured to have over 1018 eV of kinetic energy. Many of these particles exist beyond the Greisen–Zatsepin–Kuzmin limit, a theoretical limit based on the interaction of the cosmic rays above a certain energy threshold and the photons of the cosmic microwave background radiation. In essence if the particles were of too high energy they would have interacted and slowed down, but this restriction only applies over a certain distance. UHECRs are believed to be produced locally and so are not restricted by the limit. There is also a possibility that heavier nuclei may circumvent the limit also, but what particles make up UHECRs are still unknown. Despite this the mass compositions have been measured by the Pierre Auger Observatory in Argentina which is believed to show particles of higher mass than helium with an upper limit of about iron.
The ideal thing to understand cosmic rays, of all types, is to get a proper grasp on where they come from. The source propagation models that have been created are hindered in multiple ways. There is the fact that the environments in which cosmic rays are produced have been studied thoroughly; that computational information for detailed simulation is still not present; and also the interaction of radiation processes is very complicated. Now when it comes to heavier nuclei the effect of photodisintegration plays a much more significant role than in smaller nuclei. This is where a high energy gamma ray is absorbed by a nucleus which then releases an alpha particle, proton or neutron. As more and more evidence is collected to show that these UHECRs are heavy in nature, the need for the cosmic ray sources to be understood by nuclear reactions is required, especially photodisintegration as it applies when there are large nuclei and intense amounts of radiation. This paper has looked into these nuclear models and showed that they matched observed data with much greater detail than previous models that used only the basics of nuclear physics.