Fission through quasi-molecular shapes and fragmentation. Chapter 15

The decay path which leads an initial spherical nucleus through compact and creviced shapes with almost spherical ends and to two or more spherical tangent fragments which later go away has been investigated within a generalized liquid drop model taking into account both the proximity energy, an accurate sharp radius, the decay asymmetry and the temperature effects. The original value of the surface coefficient has been maintained. Within this approach of the nuclear deformation energy, this exit channel through quasi-molecular shapes is compatible with most of the experimental data : symmetric fission barrier heights in the whole mass range, asymmetric fission barrier heights of In, Tb and Mo, Businaro-Gallone point, double-humped barriers of actinides, fragment kinetic energies, partial half-lives of radioactive nuclei emitting heavy clusters and critical momenta for light and medium nuclei. The rotational hyperdeformed states recently observed might also come up and survive in this fusion-like fission valley. The rotation as well as the thermal excitation favour the symmetric splitting. In this peculiar decay path, the fission barrier is a scission barrier hindering the rupture of the neck between the nascent fragments. The nature of the saddle-point is unusual, it corresponds to separated fragments maintained in metastable equilibrium by the balance between the attractive proximity forces and the repulsive Coulomb forces. Analytical formulas given the fission barrier characteristics in this deformation path will be provided in a close future. The fragmentation barriers for emission in a plane or in the whole space as well the deformation energies of toroids and bubbles have been determined. For very heavy systems, minima lying below the barriers of plane fragmentation exist in the toroidal deformation valley allowing some stability and relaxation of the excited toroidal system before its decay in several fragments emitted in a plane due to the surface tension and proximity forces