The even-odd effect in fission-fragment Z yields – a new kind of nuclear clock

A model for the even-odd effect in fission based on statistical mechanics is presented. 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 eveneven nucleus. This implies that excitation energy and unpaired nucleons are predominantly transferred to the heavy fragment. Our results indicate that the time for transfer of excitation energy and nucleons is shorter than the saddle-to-scission time.