Point-by-Point model calculation of the prompt neutron multiplicity distributionν(A)forU238(n,f)at incident neutron energies ranging from 1 MeV to 80 MeV

Abstract

Prompt neutron multiplicity distributions \ensuremath{\nu}($A$) are generally required for prompt emission correction of double energy ($2E$) measurements of fission fragments in order to determine pre-neutron fragment properties. The lack of experimental \ensuremath{\nu}($A$) data especially at higher incident neutron energy imposes the use of prompt emission models to predict \ensuremath{\nu}($A$). At incident neutron energies ($\mathit{En}$) where multiple fission chances are involved, the Point-by-Point (PbP) model of prompt emission is able to provide the individual \ensuremath{\nu}($A$) of the compound nuclei of the main and secondary nucleus chains that are undergoing fission at a given $\mathit{En}$. The total \ensuremath{\nu}($A$) are obtained by averaging these individual \ensuremath{\nu}($A$) over the fission chance probabilities (expressed as total and partial fission cross-section ratios). An indirect validation of the total \ensuremath{\nu}($A$) results is proposed. At high $\mathit{En}$ (above 70 MeV) the PbP results of individual \ensuremath{\nu}($A$) of the first few nuclei of the main and secondary nucleus chains exhibit an almost linear increase. This shape is explained by the damping of shell effects entering the superfluid expression of the level-density parameters. They tend to approach the asymptotic values for a great part of the fragments. This fact leads to a smooth and almost linear increase of fragment excitation energy with the fragment mass number that is reflected in a smooth and almost linear behavior of individual \ensuremath{\nu}($A$). The comparison of the present results with those of the GEF code reveals different shapes of \ensuremath{\nu}($A$) as well as different total average neutron multiplicity as a function of the $\mathit{En}$. At high $\mathit{En}$ the PbP calculations definitely reflect the influence of the almost linear shape of individual \ensuremath{\nu}($A$) of the first few nuclei of the U and Pa chains. The differences between the total \ensuremath{\nu}($A$) obtained by averaging the PbP results of individual \ensuremath{\nu}($A$) over fission cross-section ratios of different evaluations are insignificant.