Strong coupling effects in near-barrier heavy-ion elastic scattering

Accurate elastic scattering angular distribution data measured at bombarding energies just above the Coulomb barrier have shapes that can markedly differ from or be the same as the expected classical Fresnel scattering pattern depending on the structure of the projectile, the target or both. Examples are given such as 18O + 184W and 16O + 148,152Sm where the expected rise above Rutherford scattering due to Coulomb-nuclear interference is damped by coupling to the target excited states, and the extreme case of 11Li scattering, where coupling to the 9Li + n + n continuum leads to an elastic scattering shape that cannot be reproduced by any standard optical model parameter set. The recent availability of high quality 6He, 11Li and 11Be data provides further examples of the influence that coupling effects can have on elastic scattering. Conditions for strong projectile-target coupling effects are presented with special emphasis on the importance of the beam-target charge combination being large enough to bring about the strong coupling effects. Several measurements are proposed that can lead to further understanding of strong coupling effects by both inelastic excitation and nucleon transfer on near-barrier elastic scattering. A final note on the anomalous nature of 8B elastic scattering is presented as it possesses a more or less normal Fresnel scattering shape whereas one would a priori not expect this due to the very low breakup threshold of 8B. The special nature of 11Li is presented as it is predicted that no matter how far above the Coulomb barrier the elastic scattering is measured, its shape will not appear as Fresnel like whereas the elastic scattering of all other loosely bound nuclei studied to date should eventually do so as the incident energy is increased, making both 8B and 11Li truly "exotic".Comment: Review articl

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Multi-neutron transfer in 8^{8}He induced reactions near the Coulomb barrier

The measured inclusive $^6$He and $^4$He production cross sections of G. Marqu{\'i}nez-Dur{\'a}n {\em et al.}, Phys.\ Rev.\ C {\bf 98}, 034615 (2018) are reexamined and the conclusions concerning the relative importance of 1n and 2n transfer to the production of $^6$He arising from the interaction of a 22 MeV $^8$He beam with a $^{208}$Pb target revised. A consideration of the kinematics of the 2n-stripping reaction when compared with the measured $^6$He total energy versus angle spectrum places strict limits on the allowed excitation energy of the $^{210}$Pb residual, so constraining distorted wave Born approximation calculations that the contribution of the 2n stripping process to the inclusive $^6$He production can only be relatively small. It is therefore concluded that the dominant $^6$He production mechanism must be 1n stripping followed by decay of the $^7$He ejectile. Based on this result we present strong arguments in favor of direct, one step four-neutron (4n) stripping as the main mechanism for $^4$He production.Comment: 7 pages, 2 figure

Influence of single-neutron stripping on near-barrier ⁶He+²⁰⁸Pb and ⁸He+²⁰⁸Pb elastic scattering

The influence of single-neutron stripping on the near-barrier elastic scattering angular distributions for the 6,8He+208Pb systems is investigated through coupled reaction channels (CRC) calculations fitting recently published data to explore the differences in the absorptive potential found in the scattering of these two neutron-rich nuclei. The inclusion of the coupling reduces the elastic cross section in the Coulomb-nuclear interference region for 8He scattering, whereas for 6He its major impact is on the large-angle elastic scattering. The real and imaginary dynamic polarization potentials are obtained by inverting the CRC elastic scattering S-matrix elements. These show that the main absorptive features occur between 11 and 12 fm for both projectiles, while the attractive features are separated by about 1 fm, with their main structures occurring at 10.5 fm for 6He and 11.5 fm for 8He