  It has recently been suggested that neutron stars inside the shells of young
supernova remnants (SNR) are the sources of PeV cosmic rays and that the
interaction of the particles with the radiation field in the SNR causes
electron pair production, which has relevance to recent observations of 'high'
positron fluxes. Furthermore, the character of the interaction is such that the
well-known knee in the cosmic ray energy spectrum can be explained. Our
examination of the mechanism leads us to believe that the required parameters
of SN and pulses are so uncommon that the knee and positron fraction can only
be explained if a single, local and recent SN - and associated pulsar - are
concerned.
