Influence of asymmetry and fissility on even-odd effect in fission-fragment yields

Based on a wide systematics of fragment distributions measured in thermal-neutron induced fission, the even-odd staggering in the fission-fragment element yields is investigated. The asymmetry evolution of the element yield distribution with the fissility of the fissioning nucleus is shown to be for an important part responsible for the decrease of the even-odd staggering with the fissility. The even-odd staggering close to symmetry is shown to be a small contribution to the global even-odd effect, and seems to vary little with the fissility of the nucleus. These experimental observations show that the established interpretation in which the intrinsic excitation energy at scission is accountable for the even-odd staggering amplitude has to be reconsidered.