Abstract

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. An early indication that the projectile structure can modify the elastic scattering angular distribution was the large vector analyzing powers observed in polarised 6Li scattering. 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”.

Highlights

  • Some systems are found to have angular distributions that differ markedly from the norm in that where the Coulomb-nuclear interference peak should be there is instead a large reduction of the elastic scattering cross section compared to the Rutherford value

  • A comparison with fig. 1 will show that, despite the superficially completely different shapes of the elastic scattering angular distributions for the strongly coupled system, what we find in fig. 16 is a standard Fresnel-type scattering pattern superimposed on a strong Coulomb coupling angular distribution shape

  • We have shown in this review that strong coupling effects on near-barrier heavy ion elastic scattering have a long history, going back nearly forty years

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Summary

Introduction

Some systems are found to have angular distributions that differ markedly from the norm in that where the Coulomb-nuclear interference peak should be there is instead a large reduction of the elastic scattering cross section compared to the Rutherford value. (a) shows a normal near-barrier heavy-ion elastic scattering angular distribution, that for 95 MeV 16O incident on a 208Pb target [1], which contrasts with that in fig. The observed depletion of the elastic scattering cross section in the 18O + 184W system is due to strong coupling, in this case strong quadrupole Coulomb coupling to the first 2+ excited state of 184W [2]. Similar effects have been observed due to strong quadrupole Coulomb coupling in the projectile in the 20Ne + 208Pb system [3].

95 MeV 100 MeV
Strong coupling effects with stable beams
Systems with effects due to target coupling
Systems with effects due to projectile coupling
Strong coupling effects with polarised projectiles
Systems with effects due to projectile and target coupling
Strong coupling effects without strong couplings
Strong coupling effects with radioactive beams
General considerations
Strong coupling equivalent to a long-range DPP
The conditions under which strong coupling effects are expected
Suggested future experiments
Deformed beams on a heavy deformed target
Light weakly bound radioactive beam on a heavy target
Heavier radioactive beams on a heavy target
Findings
Discussion
Full Text
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