  The even-odd effect in fission is explained by a model based on statistical
mechanics. It reveals that the variation of the even-odd effect with the mass
of the fissioning nucleus and the increase towards asymmetric splits is due to
the important statistical weight of configurations where the light fission
fragment populates the ground state of an even-even nucleus. This implies that
entropy drives excitation energy and unpaired nucleons predominantly to the
heavy fragment. Therefore, the even-odd effect is an additional signature of
the recently discovered energy-sorting process in fission.
