  The origin of the prominent 'knee' in the cosmic ray energy spectrum at an
energy of several PeV is still uncertain. A recent mechanism has shown promise,
however; this involves particles from a very young pulsar interacting with the
radiation field from a very young supernova remnant. The ensuing nuclear
reaction of the particles with the photons by way of e+e- production then
causes the characteristic knee. In an earlier paper we argued that the
mechanism would imply only one source of a very rare type - if it were to
explain the spectral shape. Here we examine the mechanism in more detail and
conclude that for even a single source to work its characteristics would need
to be so unusual that the mechanism would not be possible for any known type of
pulsar-supernova combination.
