  We explore in some detail the scenario proposed to explain the observed knee
of the cosmic ray (CR) spectrum as due to the effects of photodisintegration of
the CR nuclei by interactions with optical and soft UV photons in the source
region. We show that the photon column densities needed to explain the
experimental data are significantly lower than those obtained in previous
estimations which neglected multinucleon emission in the photodisintegration
process. We also treat more accurately the photodisintegration thresholds, we
discuss the effects of photopion production processes and the neutron escape
mechanism, identifying the physical processes responsible for the qualitative
features of the results. This scenario would require the CR nuclei to traverse
column densities of $\sim 5 \times 10^{27}- 2 \times 10^{28}$ eV/cm$^2$ after
being accelerated in order to reproduce the observed knee, and predicts that
the CR composition should become lighter above $\sim 10^{16}$ eV.
