  We present a principal component analysis of galaxy spectra from the CNOC
sample of rich X-ray luminous clusters at 0.18 < z < 0.55. Composite radial
distributions of different stellar populations show strong gradients as a
function of cluster-centric redshift. The composite population is dominated by
evolved populations in the core, and gradually changes to one which is similar
to coeval field galaxies at radii greater than the virial radius. We do not see
evidence in the clusters for an excess of star formation over that seen in the
coeval field. Within this redshift range, significant evolution in the gradient
shape is seen, with higher redshift clusters showing steeper gradients. This
results in larger numbers of younger galaxies seen towards the inner regions of
the clusters-- in effect, a restatement of the Butcher-Oemler effect.
Luminosity density profiles are consistent with a scenario where this
phenomenon is due to a decline over time in the infall rate of field galaxies
into clusters. Depending on how long galaxies reside in clusters before their
star formation rates are diminished, this suggests an epoch for maximal infall
into clusters at z > 0.7. We also discuss alternative scenarios for the
evolution of cluster populations.
