Neutron differential-elastic-scattering cross sections of bismuth were measured at ≊0.5 MeV intervals from ≊4.5 to 10.0 MeV. At each incident energy 40 or more differential values were obtained between ≊18° and 160°. 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 ≊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, , to see if there was evidence of the Fermi surface anomaly. In the range 3.0–10.0 MeV, the present study indicates that /dE is essentially constant, with a relatively large negative value of -6.0 to -9.0 , depending on the model used in the analysis. Below 3.0 MeV, there is some evidence for a decrease in the magnitude of /dE. However, the effect is very small, and it is only when this trend is combined with considerations of the values needed to give correct bound-state energies that evidence for the Fermi surface anomaly emerges. and the geometry of the optical potentials found for become equal to those explaining the higher-energy data at about 10.0 MeV. Since /dE for the latter is smaller in magnitude than that for , a change in /dE is clearly indicated near 10.0 MeV.
- Received 22 May 1987
- Published in the issue dated October 1987
© 1987 The American Physical Society