Photodisintegration of ultra-high-energy cosmic rays: A new determination

We present the results of a new calculation of the photodisintegration of u ltra-high-energy cosmic-ray (UHCR) nuclei in intergalactic space. The criti cal interactions for energy loss and photodisintegration of UHCR nuclei occ ur with photons of the 2.73 K cosmic background radiation (CBR) and with ph otons of the IR background radiation (IBR). We have reexamined this problem , making use of a new determination of the IBR based on empirical data prim arily from IRAS galaxies, which is consistent with direct measurements and upper limits from TeV gamma-ray observations. We have also improved the cal culation by including the specific threshold energies for the various photo disintegration interactions in our Monte Carlo calculation. With the new sm aller IBR flux, the steepness of the Wien side of the now relatively more i mportant CBR makes their inclusion essential for more accurate results. Our results indicate a significant increase in the propagation time of UHCR nu clei of a given energy over previous results. We discuss the possible signi ficance of this for UHCR origin theory.