Comparison of the energy dependence of near-barrier He6 and He8+Pb208 optical potentials

Abstract

Optical model analyses of recent $^{8}\mathrm{He}+^{208}\mathrm{Pb}$ elastic scattering data for incident energies of 16 and 22 MeV, together with existing data for $^{6}\mathrm{He}+^{208}\mathrm{Pb}$ at the same energies, reveal a significant difference in the behavior of the imaginary part of the optical potential for the two isotopes as the incident energy is reduced below the Coulomb barrier. While the strength of the $^{6}\mathrm{He}$ imaginary potential reduces as the incident energy is reduced below the barrier, as is the case for most heavy ions, that for $^{8}\mathrm{He}$ remains constant within uncertainties and may even increase. Similar behavior was observed for the $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}+^{208}\mathrm{Pb}$ optical potentials [Nucl. Phys. A 571, 326 (1994)], with the strength of the $^{6}\mathrm{Li}$ imaginary potential appearing to increase as the incident energy is reduced below the Coulomb barrier while that for $^{7}\mathrm{Li}$ reduces. However, the explanation for this difference in behavior put forward in that work, viz. the increased importance of breakup for $^{6}\mathrm{Li}$ as the incident energy is reduced towards the Coulomb barrier, later supported explicitly by calculations [Phys. Lett. B 427, 1 (1998)], does not hold for the He isotopes since it is the $^{8}\mathrm{He}$ optical potential that behaves in a similar fashion to that of $^{6}\mathrm{Li}$ in spite of $^{8}\mathrm{He}$ having the higher breakup threshold of the two He isotopes. We conclude that the difference is due to the influence of $3n$ and $4n$ stripping on the $^{8}\mathrm{He}$ scattering, reactions which cannot take place for $^{6}\mathrm{He}$ projectiles.