Photodisintegration of Ultrahigh Energy Cosmic Rays: A New Determination

We present the results of a new calculation of the photodisintegration of ultrahigh energy cosmic ray (UHCR) nuclei in intergalactic space. The critical interactions for energy loss and photodisintegration of UHCR nuclei occur with photons of the 2.73K cosmic background radiation (CBR) and with photons of the infrared background radiation (IBR). We have reexamined this problem making use of a new determination of the IBR based on empirical data, primarily from IRAS galaxies, consistent with direct measurements and upper limits from TeV gamma- ray observations. We have also improved the calculation by including the specific threshold energies for the various photodisintegration interactions in our Monte Carlo calculation. With the new smaller IBR flux, the steepness of the Wien side of the now relatively more important CBR makes their inclusion essential for more accurate results. Our results indicate a significant increase in the propagation time of UHCR nuclei of a given energy over previous results. We discuss the possible significance of this for UHCR origin theory.