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 overlineΓn(1)/D and the γ-ray strength function overlineΓγ/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 overlineΓn(1)/D derived from the analysis of these capture cross sections are found to be in agreement with overlineΓn(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 V0 = 41 Mev, ξ = 0.02, δ = 0.085, and R = 1.45 A 1/3 fermis.
- Publication:
-
Annals of Physics
- Pub Date:
- August 1960
- DOI:
- 10.1016/0003-4916(60)90118-4
- Bibcode:
- 1960AnPhy..10..477W