Reaction mechanisms and staggering in collisions

The reactions S32+Ni58 and S32+Ni64 are studied at 14.5 A MeV. After a selection of the collision mechanism, we show that important even–odd effects are present in the isotopic fragment distributions when the excitation energy is small. Close to the multifragmentation threshold this staggering appears hidden by the rapid variation of the production yields with the fragment size. Once this effect is accounted for, the staggering appears to be a universal feature of fragment production, slightly enhanced when the emission source is neutron poor. A closer look at the behavior of the production yields as a function of the neutron excess N−ZN−Z, reveals that odd–even effects cannot be explained by pairing effects in the nuclear masses alone, but depend in a more complex way on the de-excitation chain.