Low energy fission dynamics

In nuclear fission, a nucleus undergoes an extreme rearrangement of nuclear matter. In this process, the nucleus passes through several stages in the deformation space. If the nucleus reaches a critical point of deformation (the saddle point) its movement towards scission where it breaks up into two smaller fragments becomes irreversible. The fission dynamics is expected to show up between the last saddle point and the scission point. The outstanding problem that continues to confront low energy nuclear fission is to understand the nature of this dynamics. In this paper, we analyse different types of available fission data on odd-even effect, the total fission kinetic energy distribution and the neutron conditional covariance in order to try to understand the low energy fission dynamics. These data concern the thermal fission of the actinides from Th to Cf and indicate that the movement of the fissioning nucleus between the saddle point and the scission point is mostly collective in nature. The quasi-particle excitation, hence pair-breaking, seems to be produced during a fast neck-rupture at scission. (author)