Precise measurement of near-barrier He 8 + Pb 208 elastic scattering: Comparison with He 6

Dramatic differences in the elastic scattering of the neutron rich nuclei He6 and He8 are found when new high quality data for the He8+Pb208 system are compared with previously published He6+Pb208 data at the same laboratory frame incident energy. The new He8 data are of the same level of detail as for stable beams. When comparing them with those previously obtained for He6+Pb208 at the same energy, it is possible to determine from the data alone that He6 has a much longer range absorption than He8. However, both nuclei show significant absorption beyond their strong absorption radii. While it has been known for a long time that elastic scattering at energies around the barrier only determines the optical potential over a small distance in radial space, typically ±0.5 fm or so, both the He6 and the He8 imaginary potentials obtained from various optical model fits to these data are the same over a much wider range of ±1.5 fm.