The mass composition of cosmic rays above 10(17) eV

A new analysis has been made of the constituent masses of primary cosmic-ra y nuclei, using data from the Fly's Eye experiment (Bird el al 1993) and a new air shower analysis by one of the authors (Wibig 1998). It is concluded that below 10(18) eV the relative masses are approximately as expected fro m 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 n uclei 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 er al (1976) and later aut hors. 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 composit ion between 10(17.5)-10(18.5) eV than that found by the Fly's Eye experimen t. This idea militates against the view, increasingly advanced at present, tha t the ultra-high energy particles are protons, derived from the decay of ex otic massive particles.