Nuclear Data and Measurements Series PDF Download

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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 greater than or equal to40 differential values were obtained, distributed between 18° and 160°. The measured 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 a model inclusive of 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 to 10.0 MeV the present data indicate that dJ/sub v//dE is essentially constant, with a relatively large negative value of -6.0 to -9.0 fm3, depending on the model used in the analysis. Below 3.0 MeV, there is some evidence for a decrease in the magnitude of dJ/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 2°9Bi become equal to those needed to explain the high-energy 2°8Pb data at about 10.0 MeV. Since dJ/sub v//dE for the latter is smaller in magnitude than for 2°9Bi, a change in dJ/sub v//dE is clearly indicated near 10.0 MeV. This may effect the extrapolation of higher-energy and charged-particle potentials into the lower-energy neutron domain. 47 refs., 9 figs.