Halos and rainbows: the elastic scattering of light ''exotic'' nuclei

The scattering of an ''exotic'' light nucleus with a ''halo'' is compared with that of a normal nucleus. Four, sometimes opposing, effects are identified, arising from the real and imaginary parts of (i) the extended tails of the complex optical potential due to the halo, and (ii) the complex polarization potential arising from breakup. Semiclassical expressions are derived which embody the expected differences, and compared with fully quantal optical-model calculations. The cases of 11Li and 11C scattering from 12C at E/A ∼ 60 MeV are examined. The net effect of the halo depends upon the details of the interactions, but we conclude that 11Li does not exhibit greatly enhanced refraction compared to 11C, and that its elastic scattering cross sections are similar to, and may be smaller than, those for 11C, in agreement with preliminary analyses of the measurements. (orig.)