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

Abstract

Dramatic differences in the elastic scattering of the neutron rich nuclei 6He and 8He are found when new high quality data for the 8He+208Pb system are compared with previously published 6He+208Pb data at the same laboratory frame incident energy. The new 8He data are of the same level of detail as for stable beams. When comparing them with those previously obtained for 6He+208Pb at the same energy, it is possible to determine from the data alone that 6He has a much longer range absorption than 8He. 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 6He and the 8He imaginary potentials obtained from various optical model fits to these data are the same over a much wider range of ±1.5 fm.