Calculation of nuclear reaction cross sections on excited nuclei with the coupled-channels method

Abstract

We calculate nuclear cross sections on excited nuclei in the fast neutron energy range. We partition the whole process into two contributions: the direct reaction part and the compound nuclear reactions. A coupled-channels method is used for calculating the direct transition of the nucleus from the initial excited state, which is a member of the ground-state rotational band, to the final ground and excited low-lying levels. This process is strongly affected by the channel coupling. The compound nuclear reactions on the excited state are calculated with the statistical Hauser-Feshbach model, with the transmission coefficients obtained from the coupled-channels calculation. The calculations are performed for a strongly deformed nucleus $^{169}\mathrm{Tm}$, and selected cross sections for the ground and first excited states are compared. The calculation is also made for actinides to investigate possible modification to the fission cross section when the target is excited. It is shown that both the level coupling for the entrance channel, and the different target spin, change the fission cross section.