Cross sections of $^{ 120}$Sn($\alpha$,$\alpha$)$^{ 120}$Sn elastic scattering have been extracted from the $\alpha$ particle beam contamination of a recent $^{ 120}$Sn($^6$He,$^6$He)$^{ 120}$Sn experiment. Both reactions are analyzed using systematic double folding potentials in the real part and smoothly varying Woods-Saxon potentials in the imaginary part. The potential extracted from the $^{ 120}$Sn($^6$He,$^6$He)$^{ 120}$Sn data may be used as the basis for the construction of a simple global $^6$He optical potential. The comparison of the $^6$He and $\alpha$ data shows that the halo nature of the $^6$He nucleus leads to a clear signature in the reflexion coefficients $\eta_L$: the relevant angular momenta $L$ with $\eta_L \gg 0$ and $\eta_L \ll 1$ are shifted to larger $L$ with a broader distribution. This signature is not present in the $\alpha$ scattering data and can thus be used as a new criterion for the definition of a halo nucleus.
Categories: nucl-th nucl-ex