Scattering and resonances on p-shell nuclei

We review recent results on low energy nucleon scattering from some light-mass nuclei and particularly consider resonances seen or expected in cross sections. We present results found using the multichannel algebraic scattering (MCAS) method which was developed to analyze such cross sections. The method gives both bound and resonant spectra of the compound nuclei. In 2006, using this method, we predicted nontrivial narrow resonances due to non-valence p-shell effects in the nucleus F-15, a nucleus that lies beyond the proton drip-line. Subsequently, in 2009, those predictions were verified experimentally. Research on other such mirror systems is underway. In particular, the structure of C-17 has been used to define an appropriate MCAS nuclear interaction for the n+C-16 system. Adding a Coulomb field to that interaction, an MCAS evaluation has been made for the mirror system p+Ne-16, to specify the low excitation spectrum of the yet unobserved, particle-unstable, Na-17. We also investigated how the scattering cross-section changes when the spectra of the colliding nuclei have low-excitation particle-emitting resonances. As a test case, we considered Be-8, being particle-unstable, and analyzed neutron scattering cross sections and the spectra of the compound Be-9 system. If the Be-8 excited states have fixed-energy decay widths, we find that the bound states of the compound system are affected in an unphysical way. The shapes of the target resonances must vary from the usual Lorentzian in an energy-dependent way. Resonance width functions must go smoothly to zero at the elastic threshold. Ways of achieving this condition are being explored.