Energy dependence of the optical-model potential for fast-neutron scattering from bismuth

Neutron differential-elastic-scattering cross sections of bismuth were measured at approx. =0.5 MeV intervals from approx. =4.5 to 10.0 MeV. At each incident energy 40 or more differential values were obtained between approx. =18/sup 0/ and 160/sup 0/. These data were combined with lower-energy results previously reported from this laboratory, and others available in the literature, to provide a detailed data base extending from approx. =1.5 to 10.0 MeV. This data base was interpreted in terms of the conventional optical-statistical model and also using a model which included the surface-peaked real potential predicted by the dispersion relation. Particular attention was given to the energy dependence of the volume-integral-per-nucleon of the real potential, J/sub v/, to see if there was evidence of the Fermi surface anomaly. In the range 3.0--10.0 MeV, the present study indicates that dJ/sub v//dE is essentially constant, with a relatively large negative value of -6.0 to -9.0 fm/sup 3/, depending on the model used in the analysis. Below 3.0 MeV, there is some evidence for a decrease in the magnitude of dJ/sub v//dE. However, the effect is very small, and it is only when this trend is combined with considerations of the J/sub v/ values needed to give correct bound-state energies that evidence for the Fermi surface anomaly emerges. J/sub v/ and the geometry of the optical potentials found for /sup 209/Bi become equal to those explaining the higher-energy /sup 208/Pb data at about 10.0 MeV.