Extragalactic heavy nuclei in cosmic rays

It might be thought that if the energetic particles in the cosmic radiation are of extragalactic origin then they must be protons because heavy nuclei would be completely fragmented by interactions with the radiation fields in space. Here it is argued that this is not the case and that it is, in principle, possible to choose an origin model in which the very energetic particles commence their life as heavy nuclei; these nuclei then interact with extragalactic photons and give rise to the observed primary spectrum. Such a model would relate mainly to energies above about 1018 eV where there is the well known need to postulate extragalactic origin on the grounds of the observed near-isotropy of arrival directions. Specific experimental tests of models of this type are suggested. (author)