Measurement of Ay( theta ) for n+208Pb from 6 to 10 MeV and the neutron-nucleus interaction over the energy range from bound states at -17 MeV up to scattering at 40 MeV.

Abstract

High-accuracy measurements of ${\mathit{A}}_{\mathit{y}}$(\ensuremath{\theta}) data for elastic scattering and inelastic scattering to the first excited state for n${+}^{208}$Pb have been performed at 6, 7, 8, 9, and 10 MeV. In addition, \ensuremath{\sigma}(\ensuremath{\theta}) was measured at 8 MeV. These data provide an important subset for the growing database for the n${+}^{208}$Pb system from bound-state energies to energies above 40 MeV, the limit of the range of interest here. This database has been interpreted via several approaches. First, a conventional Woods-Saxon spherical optical was used to obtain three potential representations for the energy range from 4 to 40 MeV: ``best fits'' at each energy, constant-geometry global fit with linear energy dependences for the potential strengths for the range 4.0--40 MeV, and an extension of the latter model to allow a linear energy dependence on the radii and diffuseness. A preference for a complex spin-orbit interaction was observed in all cases. Second, the dispersion relation was introduced into the spherical optical model to obtain a more ``realistic'' representation. In our approach, the strength and shape of the real potential was modified by calculating the dispersion-relation contributions that originate from the presence of the surface and volume imaginary terms. Two potentials were developed, one based only on the scattering data (from 4.0 to 40 MeV) and another based additionally on single-particle and single-hole information down to a binding energy of 17 MeV. In addition, the \ensuremath{\sigma}(\ensuremath{\theta}) and ${\mathit{A}}_{\mathit{y}}$(\ensuremath{\theta}) measurements were compared to earlier conventional and dispersion-relation models. One of the latter of these included an l dependence in the absorptive surface term, and we applied this model in the 6- to 10-MeV region to describe all the \ensuremath{\sigma}(\ensuremath{\theta}) and the new ${\mathit{A}}_{\mathit{y}}$(\ensuremath{\theta}). A reasonably good overall description was obtained by all the models; however, only the l-dependent model came close to giving a detailed agreement to the data around 7 MeV, a region where some abnormal angular dependences occur in the data.