Neutron capture cross sections in the kev region. Part II. Spin-orbit coupling and the optical model

An intensive study of average neutron capture cross sections of nuclei with 75 ≦ A ≦ 130 has been carried out in the neutron energy region 3 to 200 kev, using the activation method. Assuming s-wave parameters within the limits of error specified by low-energy total cross sections determinations, the data has been analyzed for corresponding p-wave parameters: the neutron strength function Γ_n⁽¹⁾ D and the γ-ray strength function Γ_γ D₀. It is found that the s- and p-wave γ-ray strength functions are the same within the rather large limits of experimental error for the odd A targets: Rh¹⁰³, Ag¹⁰⁷, Ag¹⁰⁹, In¹¹⁵, I¹²⁷, and As⁷⁵. The even-even isotopes of Pd, and Mo are more complicated in this respect. The p-wave neutron strength functions Γ_n⁽¹⁾ D derived from the analysis of these capture cross sections are found to be in agreement with Γ_n⁽¹⁾ D as obtained from the analysis of neutron total cross sections. These strength functions exhibit a broad, double-peaked, distribution around A = 100 instead of the single peak predicted by the simple optical model. It is shown that the experimental results may be explained by the addition of a small spin-orbit part to the optical potential. The parameters of the square well optical potential V^± = V₀[1 + iξ + δ 2{-1 ± (2l + 1)}] which are found to fit the p-wave strength functions best are V₀ = 41 Mev, ξ = 0.02, δ = 0.085, and R = 1.45 A ^{1 3} fermis.