7Be versus 7Li: The influence of breakup threshold on the dynamic polarization potential

Previous continuum-discretized coupled-channel (CDCC) calculations comparing 6Li+208Pb with 7Li+208Pb suggested that the observed difference in the dynamic polarization potentials for these systems at bombarding energies close to the Coulomb barrier was due to the ∼1 MeV difference between the α+d and α+t breakup thresholds for 6Li and 7Li rather than differences in their ground state structures. In this work we compare CDCC calculations for 7Be+208Pb with similar calculations for 7Li+208Pb to further test whether the breakup threshold is the controlling factor in determining the near-barrier behavior of the dynamic polarization potential. We conclude that this is indeed the case and that 7Be, with its lower breakup threshold, will behave like 6Li rather than 7Li, in that it will not exhibit a threshold anomaly. Furthermore, we predict a clear experimental signature for this difference in behavior: the lack of a pronounced Coulomb rainbow in the 7Be elastic scattering angular distributions. It is proposed that elastic scattering measurements of 7Be+208Pb be carried out at two energies near the Coulomb barrier to confirm this conclusion