  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.
