Isotopic fission fragment distributions as a deep probe to fusion-fission dynamics

摘要

During the fission process, the nucleus deforms and elongates up to the twofragments inception and their final separation at scission deformation. Theevolution of the nucleus energy with deformation is determined by themacroscopic properties of the nucleus, and is also strongly influenced by thesingle-particle structure of the nucleus. The fission fragment distribution isa direct consequence of the deformation path the nucleus has encountered, andtherefore is the most genuine experimental observation of the potential energylandscape of the deforming nucleus. Very asymmetric fusion-fission reactions atenergy close to the Coulomb barrier, produce well-defined conditions of thecompound nucleus formation, where processes such as quasi-fission,pre-equilibrium emission and incomplete fusion are negligible. In the sametime, the excitation energy is sufficient to reduce significantly structuraleffects, and mostly the macroscopic part of the potential is responsible forthe formation of the fission fragments. We use inverse kinematics combined withspectrometers to select and identify the fission fragments produced in$^{238}$U+$^{12}$C at a bombarding energy close to and well-above the Coulombbarrier. For the first time, the isotopic yields are measured over the completeatomic-number distribution, between Z=30 and Z=63. The experimental set-up alsoallows to identify transfer-induced reactions, which lead to low-energy fissionwhere the nuclear shell structure shows a strong influence on thefission-fragment distributions. The resulting set of data gives the possibilityto observe the fission fragment properties over a wide range of excitationenergy, and they reveal the vanishing of the shell effects in the potentialenergy of the fissioning nucleus, as well as the influence of fission dynamics.