Multimodal analysis of prompt neutron spectra for 238Pu(sf), 240Pu(sf), 242Pu(sf) and 239Pu(nth,f)

The prompt neutron spectra for Pu-238(sf), Pu-240(sf), Pu-242(sf) and Pu-239(n(th),f) were calculated with the modified Madland-Nix model with consideration to the multimodal nature of the fission process. The spectra of neutrons for each mode (Standard-I, -II, -III) were calculated independently and the total spectra were synthesized. The partial spectra for the three modes were found to be considerably different from each other. Different mode branching ratios resulted in different total spectra for the three even-mass spontaneously fissioning isotopes. Comparison of spectra for Pu-240(sf) and Pu-239(n(th),f), for which the fissioning nucleus is the same, revealed that the total spectra were harder for Pu-239(n(th),f) due to the higher excitation energy of the fissioning nucleus. The calculated total spectra well represented the experimental data. (C) 2000 Elsevier Science B.V. All rights reserved.