On the Origin of the Highest Energy Cosmic Rays

We present a new estimate of the photodisintegration of ultrahigh energy cosmic-ray (UHCR) nuclei in intergalactic space. Critical interactions for energy loss and photodisintegration of UHCR nuclei occur with the infrared background radiation (IBR). We have made use of a new determination of the IBR based on IR data and on TeV $\gamma$-ray observations. Our results indicate that a 200 EeV nucleus which started out as Fe can have propagated $\sim 100\mathrm{Mpc}$ though the IBR. It is thus possible that the highest energy cosmic rays observed may be nuclei.