  The characteristics of the odd-even effect in fission-fragment Z
distributions are compared to a model based on statistical mechanics. Special
care is taken for using a consistent description for the influence of pairing
correlations on the nuclear level density. The variation of the odd-even effect
with the mass of the fissioning nucleus and with fission asymmetry is explained
by the important statistical weight of configurations where the light nascent
fission fragment populates the lowest energy state of an even-even nucleus.
This implies that entropy drives excitation energy and unpaired nucleons
predominantly to the heavy fragment. Therefore, within our model, the odd-even
effect appears as an additional signature of the recently discovered
energy-sorting process in nuclear fission.
