The present understanding of reaction processes involving light
unstable nuclei at energies around the Coulomb barrier is
reviewed. The effect of coupling to direct reaction channels on
elastic scattering and fusion is investigated, with the focus on
halo nuclei, for which such effects are expected to be most important.
With the aim of resolving possible ambiguities in the terminology
a short list of definitions for the relevant processes and
quantities is proposed. This is followed by a review of the
experimental and theoretical tools and information presently
available. The effect of breakup couplings on elastic scattering
and of transfer couplings on fusion is investigated with a series
of model calculations within the coupled-channels framework. The
experimental data on fusion are then compared to ``bare\'\'
no-coupling one-dimensional barrier penetration model
calculations employing reasonably realistic double-folded
potentials. On the basis of these model calculations and
comparisons with experimental data, conclusions are drawn from the
observation of recurring features. The total fusion cross sections
for halo nuclei show a suppression with respect to the ``bare\'\'
calculations at energies just above the barrier that is probably
due to single neutron transfer reactions. The data for total
fusion are also consistent with a possible sub-barrier
enhancement; however, this observation is not conclusive and other
couplings besides the single-neutron channels would be needed in
order to explain any actual enhancement. A characteristic feature
of halo nuclei seems to be the dominance of direct reactions over
fusion at near and sub-barrier energies; the main part of the
cross section is probably related to neutron transfers, while
calculations indicate only a modest contribution from the breakup
process. |
