The mass composition of cosmic rays above 1017 eV

A new analysis has been made of the constituent masses of primary cosmic-ray nuclei, using data from the Fly's Eye experiment (Bird et al 1993) and a new air shower analysis by one of the authors (Wibig 1998). It is concluded that below 1018 eV the relative masses are approximately as expected from galactic sources with simple galactic diffusion loss. At higher energies there is increasingly an extragalactic component, a not unusual conclusion, but we argue that it is characterized by a significant fraction of heavy nuclei with probably a variety of masses. The implication is that perhaps as many as 50% of the extragalactic nuclei start their lives as heavy nuclei (probably iron) and fragment on the radiation fields in the universe in the manner described by Tkaczyk et al (1975), Puget et al (1976) and later authors. In this regard, our results are more in accord with those from Akeno (Hayashida et al 1995), who found a much smaller rate of change of composition between 1017.5 - 1018.5 eV than that found by the Fly's Eye experiment. This idea militates against the view, increasingly advanced at present, that the ultra-high energy particles are protons, derived from the decay of exotic massive particles. (author)