ON THE ORIGIN OF THE HIGHEST ENERGY COSMIC-RAYS

We present a new estimate of the photodisintegration of ultrahigh ener gy cosmic-ray (UHCR) nuclei in intergalactic space. Critical interacti ons for energy loss and photodisintegration of UHCR nuclei occur with the infrared background radiation (IBR). We have made use of a new det ermination of the IBR based on IR data and on TeV gamma-ray observatio ns. Our results indicate that a 200 EeV nucleus which started out as F e can have propagated similar to 100 Mpc though the IBR. It is thus po ssible that the highest energy cosmic rays observed may be nuclei.