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

Abstract

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 D 0 . 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: Rh 103, Ag 107, Ag 109, In 115, I 127, and As 75. 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 ± = V 0[1 + iξ + δ 2 {−1 ± (2l + 1)}] which are found to fit the p-wave strength functions best are V 0 = 41 Mev, ξ = 0.02, δ = 0.085, and R = 1.45 A 1 3 fermis.