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 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.

Target-state dependence of cross sections for reactions on statically deformed nuclei

“…These results indicate that the compound formation cross sections shown in Fig. 3 of [1] for the ground and first excited states should be closer together in a fully converged calculation. The values for the nonadiabatic to adiabatic ratios of Fig.…”

“…We employ the definition of Edmonds [21] for the rotation matrices D I MK (ω), where ω = (α, β, γ) represents the set of Euler angles required to rotate the space-fixed axes of the deformed target into the body-fixed axes 1 . Eq.…”

“…It is well known that coupled-channels calculations are needed for these reactions, and the recent paper of Kawano et al [1] reported on such calculations where five levels of 169 Tm were included to calculate the compoundnucleus cross section for neutrons incident on both the ground and first-excited states. They also studied the target-state dependence of the 239 Pu fission cross section.…”

Isomeric ratio measurements for the radiative neutron capture 176Lu(n,γ) at DANCE

“…In this context, partial cross sections feeding the ground states or isomers are particularly crucial in certain cases of the s-process nucleosynthesis [1][2][3], or of astrophysical environments such as neutron stars or supernovae where reactions on the isomeric states can occur [4,5].…”

Neutron-inducedγ-ray production cross sections for the first excited-state transitions in20Ne and22Ne

“…The CoH 3 code included discrete level information for the first 47 levels, up to the 11.32-MeV 2 + state in 20 Ne. Because neon is a deformed nucleus (β 2 = 0.73 for 20 Ne and β 2 = 0.63 for 22 Ne), the CoH 3 code used a HauserFeshbach calculation, including transmission coefficients obtained from a coupled-channels calculation [18]. Although CoH 3 reproduced the data more accurately than TALYS, the discrepancy between the CoH 3 calculation and data is most likely due to the use of the spherical Koning-Delaroche potential in the coupled-channels calculation.…”