Determination of the239Np(n,f)and240Np(n,f)cross sections using the surrogate reaction method

Abstract

The surrogate reaction method has been used to determine neutron-induced fission cross sections of the short-lived minor actinides ${}^{239}$Np and ${}^{240}$Np in the equivalent neutron energy range of 10.5--16.5 and 9.0--16.0 MeV, respectively. The ${}^{240}$Np and ${}^{242}$Pu compound nuclei are produced at similar excitation energies in ${}^{238}$U(${}^{6}$Li, $\ensuremath{\alpha}f$)${}^{240}$Np and ${}^{238}$U(${}^{6}$Li, $df$)${}^{242}$Pu transfer reactions at ${E}_{lab}=39.6$ MeV. The fission decay probabilities of ${}^{240}$Np [surrogate of ${}^{239}$Np($n,f$)] and ${}^{242}$Pu [surrogate of ${}^{241}$Pu($n,f$)] compound systems have been measured experimentally as a function of excitation energy to determine ${}^{239}$Np($n,f$) cross sections within the framework of hybrid surrogate ratio method by considering directly measured ${}^{241}$Pu($n,f$) cross sections as reference. Similarly, ${}^{238}$U(${}^{7}$Li, $\ensuremath{\alpha}f$)${}^{241}$Np and ${}^{238}$U(${}^{7}$Li, $tf$)${}^{242}$Pu transfer reactions at ${E}_{lab}=41.0$ MeV have been used to determine ${}^{240}$Np(n, f) cross sections. The present results for ${}^{239}$Np($n,f$) cross sections have been compared with recently reported ${}^{239}$Np(n, f) cross sections obtained by the surrogate ratio method using ${}^{236}$U(${}^{3}$He, $p$) and ${}^{238}$U(${}^{3}$He, $p$) reactions [2] and also have been compared with the predictions of the statistical model code empire-3.1 for the fission barriers obtained from the barrier formula and the evaluated nuclear data libraries such as JENDL-4.0 and ENDF/B-VII.1. The present experimental results for ${}^{239}$Np($n,f$) and ${}^{240}$Np($n,f$) cross sections are found to be reasonably consistent with the empire-3.1 predictions.