Theoretical models and computer programs for the calculation of prompt fission neutron spectra

Prompt fission neutron emission has to be understood as a superposition of different mechanisms connected with the dynamics of fission directly (scission neutron emission) or indirectly (evaporation from a fragment diversity). This paper summarizes the general requirements to be met in calculating emission probabilities of prompt fission neutrons in the framework of a complex statistical- model approach (SMA) assuming evaporation from fully accelerated fragments (predominant mechanism of post-fission neutron emission) as well as recent approaches and computer codes. It is shown that experimental multiple-differential emission probabilities of 252-Cf(sf) neutrons (specifically the energy spectrum as a nuclear standard) can be well reproduced by the use of a special SMA---the complex cascade evaporation model (CEM). Several global parametrizations of the optical potential for the calculation of the inverse cross sections of compound-nucleus formation have been tested. The application of a complex SMA to any fission reaction has to be based on the theoretical or semi-empirical description of the fragment data needed (distribution in nucleon numbers, kinetic energy, excitation energy, angular momentum; fragment angular distribution). The present status is briefly discussed. In the case of multiple-chance fission reactions, the SMA to post-fission neutron emission has to be connected with nuclear reaction theory including the calculation of the partial fission cross sections as well as of the spectra of pre-fission neutrons. The theoretical scheme for the adequate and consistent description of scattered-neutron cross sections, fission cross sections, fragment data, and post-fission neutron data is formulated and discussed