Precise measurement of near-barrier ${}^8\mathrm{He}+{}^{208}\mathrm{Pb}$ elastic scattering: Comparison with ${}^6\mathrm{He}$

Dramatic differences in the elastic scattering of the neutron rich nuclei ${}^6\mathrm{He}$ and ${}^8\mathrm{He}$ are found when new high quality data for the ${}^8\mathrm{He}+{}^{208}\mathrm{Pb}$ system are compared with previously published ${}^6\mathrm{He}+{}^{208}\mathrm{Pb}$ data at the same laboratory frame incident energy. The new ${}^8\mathrm{He}$ data are of the same level of detail as for stable beams. When comparing them with those previously obtained for ${}^6\mathrm{He}+{}^{208}\mathrm{Pb}$ at the same energy, it is possible to determine from the data alone that ${}^6\mathrm{He}$ has a much longer range absorption than ${}^8\mathrm{He}$. 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 $\pm 0.5$ fm or so, both the ${}^6\mathrm{He}$ and the ${}^8\mathrm{He}$ imaginary potentials obtained from various optical model fits to these data are the same over a much wider range of $\pm 1.5$ fm.

Figure 1

Typical calibrated $\mathrm{\Delta }E\text{−}{E}_{\mathrm{tot}}$ spectrum for the lower telescope, for pixels with a laboratory scattering angle around ${60}^{\circ }$ at $E_{\mathrm{lab}}=22$ MeV. The solid ellipsoid encloses the elastically scattered ${}^8\mathrm{He}$ while the dot-dashed and dashed curves enclose the ${}^6\mathrm{He}$ and ${}^4\mathrm{He}$ reaction fragments, respectively.

Figure 2

(a) Angular distributions of the ${}^8\mathrm{He}$ (filled circles) and ${}^6\mathrm{He}$ (open circles) [2, 3] + ${}^{208}\mathrm{Pb}$ elastic scattering at 22 MeV plotted on a linear cross section scale. (b) The data plotted as a function of the classical distance of closest approach for a Coulomb trajectory, $r_{\max }$. The solid and dashed curves denote the Woods-Saxon optical model fits of Table 1 to the ${}^8\mathrm{He}$ and ${}^6\mathrm{He}$ data, respectively. The thin solid and dashed vertical lines in (b) denote the distances at which the ${}^8\mathrm{He}$ and ${}^6\mathrm{He}+{}^{208}\mathrm{Pb}$ elastic scattering differential cross sections first deviate from the Rutherford value by more than 2%; see text.

Figure 3

Solid and dashed curves: imaginary parts of two of the best fit 22 MeV ${}^8\mathrm{He}+{}^{208}\mathrm{Pb}$ optical model potentials. Dotted and dot-dashed curves: imaginary parts of two of the best fit 22 MeV ${}^6\mathrm{He}+{}^{208}\mathrm{Pb}$ optical model potentials. The other best fit potentials are omitted for the sake of clarity but coincide equally closely over this radial range.