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

Fanny Farget

GANIL, Caen, France

During the fission process, the atomic nucleus deforms and elongates up to the two fragments inception and their final separation at the scission deformation. The evolution of the nucleus energy with deformation defines a potential energy landscape in the multi-dimensional deformation space. It is determined by the macroscopic properties of the nucleus, and is also strongly influenced by the single-particle structure of the nucleons, which modifies the macroscopic energy minima. The fission fragment distribution is a direct consequence of the deformation path the nucleus has encountered, and therefore is the most genuine experimental observation of the potential energy landscape of the deforming nucleus.  Very asymmetric fusion-fission reactions at energy close to the Coulomb barrier produce pure conditions of the compound nucleus formation, where processes such as quasi-fission, pre-equilibrium emission and incomplete fusion are negligible. In the same time, the excitation energy is sufficient to reduce significantly structural effects, and mostly macroscopic part of the potential is responsible for the formation of the fission fragments. We use inverse kinematics combined with the use of a spectrometer to select and identify the fission fragments produced in ²³⁸U+¹²C at bombarding energy close to and above the Coulomb barrier. For the first time, the isotopic yields are measured over the complete atomic-number distribution, between Z=30 and Z=63. This gives access to the symmetry-energy component of the potential, responsible in part for the charge polarisation, which was hardly accessible up to now. In the experimental set-up, it is possible to tag transfer-induced reactions, which lead to low-energy fission where the shell structure of the nucleons shows a strong influence on the fission fragment distribution.