Role of neutrons in the propagation of extremely high energy cosmic rays

Attention is given to the problem of explaining the shape of the spectrum of very energetic cosmic rays, particularly those above 1019 eV where evidence for a flattening of the spectrum is becoming stronger. It is proposed that the particles above 1019 eV are mainly extragalactic and that they originate from heavy nuclei generated in clusters of galaxies which are largely trapped but from which fragmentation neutrons are able to escape and decay to protons outside the region of confining fields. (author)