Multimodal Analysis of Prompt and Delayed Neutron Emission inFission

Abstract

The multimodal model of fission, in which the nucleus is considered to follow a distinctive path on the deformation energy surface, finally reaching a definite scission point configuration, has been proved to give a good description of the mass and kineticenergy distributions of fission fragments. The Madland-Nix model of prompt fission neutron spectra (PFNS) considered the cooling of the fission fragments by neutron emission and the energy dependence of the inverse reaction, providing a basic formalism for evaluation of the PFNS. Combination of the two models led to the multimodal Madland-Nix model. With further extension to consider the shell effects on the level density and asymmetry in neutron multiplicity and nuclear temperature of the fragments, it was successfully applied to evaluation of the PFNS. On the other hand, the summation method of the delayed neutron yields (DNY) is the most fundamental approach to the estimation of the quantity. Combination of the method with the multimodal fission model led to a new approach to interpret the energy dependence of the DNY. This physics-based approach has made it possible to understand the PFNS and DNY in a manner consistent with many other facets of fission physics. Some problems about possible effects of scission neutrons, anisotropy of neutron emission in the center-of-mass system of the fragments, and neutron emission during acceleration are discussed.