Cassini-oval Description of the Energy Balance at Scission During 235U(nth, f)

For a detailed description of the excitation energy of the fission fragments, that is used to evaporate neutrons and emit γ-rays, realistic nuclear shapes at scission are necessary. It is shown that the nuclear shapes around the scission point, along the main fission mode, are well described by Cassini ovals with only two parameters: α (elongation) and α1 (mass asymmetry). This shape parametrization is used in an attempt to solve, in the case of the low-energy fission of 236U, the puzzle of the energy balance at scission. The deformation energy liberated during the neck rupture (αi=0.985 → αf =1.001)and the extra def deformation energy of the fragments immediately after scission are calculated as a function of eachdef fragment mass A. This allows us to estimate the excitation energy for each fragment pair as well as its partition among the light and the heavy fragment and compare with data extracted from measured prompt neutron and γ-ray multiplicities.The Coulomb repulsion of the fission fragments immediately after scission is also calculated as a function of their mass ratio and compared with experimental total fragment kinetic energies.