Improved formula for prompt fission neutron spectrum

Madland-Nix model for the calculation of prompt fission neutron spectrum was
solved numerically for the determination of prompt fission spectrum of several fissile
nuclei. The model is based on the standard nuclear evaporation theory and considers
the effect of fission fragment motion and the distribution of residual nuclear
temperature on the prompt fission neutrons spectrum. The results of the model are
fitted into simple analytical formula which is easy to be applied in both shielding and
reactor core calculations. These results are compared with the empirical formula and
other theoretical ones. Good agreements were well found.