AMBIGUITIES IN THE ELASTIC-SCATTERING OF 8 MEV NEUTRONS FROM ADJACENT NUCLEI

Ratios of the cross sections for the elastic scattering of 8 MeV neutrons from adjacent nuclei are measured over the angular range approximately 20-degrees-160-degrees for the target pairs V-51/Cr, Co-59/Ni-58, Cu/Zn, Y-89/Nb-93, Y-89/Zr, Nb-93/Zr, In/Cd and Bi-209/Pb. The observed ratios vary from unity by as much as a factor of approximately 2 at some angles for the lighter target pairs. The ratios are generally consistent with a model based upon a real Woods-Saxon potential whose geometry is essentially constant between pairs, and whose strength has an (N - Z)/A (symmetry) component with a magnitude of approximately 16 MeV. For pairs with A > 100, a constant-geometry derivative Woods-Saxon imaginary potential, with a symmetry strength of approximately 15 MeV, explains the ratios. However, for the lighter nuclei, the difffuseness of the imaginary potential varies rapidly near the N = 28 and N = 50 shell closures, and for the Co-59-Ni-58 and Cu-Zn pairs, the imaginery symmetry-potential strength is about twice the 15 MeV global value. A method for approximating the spin-spin potential is given, and it is shown that this interaction makes a negligible contribution to the calculated ratios. However, channel coupling can lead to large reorientation effects which can substantially change the calculated ratios. Differences in the spin-orbit interaction between neighboring pairs can significantly affect the calculated ratios.