Unusual near-threshold potential behavior for the weakly bound nucleus ⁹Be in elastic scattering from ²⁰⁹Bi

The cross sections for elastic scattering of the weakly bound ⁹Be on ²⁰⁹Bi around the Coulomb barrier have been measured with ∼5% absolute accuracy from 40 to 48 MeV. The potential obtained from an optical model analysis has an unusual behavior. At the strong absorption radius the imaginary (absorptive) potential is increasing (rather than decreasing) with decreasing energy, as would be consistent with a long range polarization potential arising mainly from couplings to breakup channels. The real part, on the other hand, displays a strong attractive polarization contribution with the maximum at the barrier, as would be normally expected from a polarization contribution arising from strong couplings. The inelastic multiplet in ²⁰⁹Bi of collective nature around 2.6 MeV, originating from the coupling [²⁰⁸Pb(3^-)⊗h_9/2^-]_J^π, was seen only at 48 MeV. The total multiplet cross section is well reproduced by coupled channel calculations with the potential obtained from the optical analysis and the experimental B(E3) strengths of the ²⁰⁹Bi multiplet levels.