General Laws of Quantum and Statistical Mechanics Governing Fission

It is shown that many of the apparently complex empirical features of low-energy fission can be explained by general laws of quantum and statistical mechanics. Early studies with the two-centre shell model have shown that the shells of the separated fragments decisively influence the microscopic properties of the fissioning system already way before scission. This lead us to postulate the separability of macroscopic and microscopic features of the potential energy surface, of the friction and of the inertia on the fission path. According to this postulate, only the macroscopic properties are specific to the compound nucleus, while the microscopic properties are attributed to the nascent fragments. The fragment shells may strongly differ from the shells of the separated fragments, because the Coulomb force favours strongly elongated shapes on the fission path. Scarce experimental information on prompt-neutron yields indicates that the thermal energy available at scission is fully transferred to the heavy fragment. This is explained by the laws of statistical mechanics that lead to an energy-sorting process. The semi-empirical model GEF was developed on the basis of these ideas.