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4.3 Nuclear Structure and Dynamics
The influence of nucleon transfer on the fusion process
continues to be a matter of debate, especially for
reactions involving neutron-rich exotic beams. Fusion
cross-sections for stable nuclei well below the Coulomb
barrier for several medium-heavy systems are found to
be much smaller than standard predictions. This “fusion
hindrance”, particularly for deep sub-barrier fusion can
be used to address aspects of nuclear compressibility
and viscosity and to give an insight into adiabatic or sudden
change in nature of the ion-ion potential between
colliding nuclei. It also has a large impact on calculated
nucleosynthesis rates.
Indeed, the future experimental advances with RIBs
are expected to be remarkable as indicated by the experiments
with 4 He and 8 He beams, where comparable
sensitivity was achieved even though the 8 He beam was
six orders of magnitude less intense.
The arsenal of both stable and unstable beams combined
with advances in detection techniques will also
play an essential role for these advances.
Direct and deep-inelastic reactions
The recent implementation of large acceptance spectrometers
in conjunction with efficient gamma-ray
detector arrays has enabled studies of the mechanism of
producing nuclei by transferring nucleons with and without
an exchange of a large amount of energy in heavy-ion
collisions. The relative role of single-particle and pairtransfer
modes can be probed in such reactions. The
population of excited states in the final nuclei provides
information on the contribution of surface vibrations
(bosons), single particles (fermions) and their coupling.
This, in conjunction with the theoretical progress in treating
quasi-elastic and deep-inelastic processes on the
same footing, has given a new impetus to the field.
Measurements of transfer reactions at energies well
below the barrier will enable probing of particle-particle
correlations with possible signatures of the nuclear
Josephson effect. Reactions involving next generation
radioactive beams are of particular interest, because
for nuclei far from stability pair correlations should play
an important role and more microscopic form-factors
are required.
Deep-inelastic transfer reactions can also be used
as a tool to produce neutron-rich nuclei at high angular
momentum. In this way, evidence for rigid triaxiality at
low spin in 48 Ar was found.
The advent of high-intensity RIBs will open path ways
towards the discovery of new phenomena at high spin
in neutron-proton asymmetric nuclei.
Fission process
Despite that the fission process has been known for
nearly 70 years, the dynamics of nuclear fission remains
one of the most interesting processes of the collective
flow of nuclear matter. It involves an unsolved nuclear
many-body problem, where the nuclear viscosity affects
the large amplitude of collective motion leading to scission.
In addition, fission provides the means to extend
studies of nuclear structure under non-equilibrium
dynamical conditions.
Recent data have revealed unexpected results:
(i) Contrary to expectations, the position of the fission
channels was found to be almost constant in Z.
(ii) Observed fine structures in the properties of fission
fragments are still unexplained. They provide an
opportunity to investigate the influence of super-fluidity
and the onset of dissipation, beyond the traditional
models of pairing.
In addition, experimental inputs for the open problem
of the damping of collective nuclear excitations into more
complicated excitations rely not only on spectroscopy
in the continuum but also on the systematics of fission
probabilities of spherical and deformed nuclei as a function
of excitation energy.
Promising perspectives are also foreseen for the
detailed study of the fission fragments, so far restricted
to the light fragments, from thermal-neutron induced
fission. This is now possible for all A and Z using inversekinematics
reactions and high-resolution magnetic
spectrometers. The future project ELISe at the FAIR
storage rings, that aims at investigating the fission of
heavy-secondary beams induced by virtual photons will
be a powerful tool in this direction.
Quasi-free scattering
One-nucleon knock-out reactions using radioactive
beams at energies > 50 MeV/u constitute a quantitative
spectroscopic tool for valence nucleons. Recently
they have been used to study the removal of deeply
bound nucleons in neutron-proton asymmetric nuclei.
The striking observation of a strong quenching of the
single-particle strength as a function of asymmetry of
the neutron and proton separation energies presents
a challenge to any theoretical understanding. An isospin
dependent effect of the short-range correlations
is predicted to be much smaller than the observed
one. In stable nuclei, the reduction of single-particle
spectroscopic factors has been established mainly
from electron-induced and proton-induced quasi-free
knockout reactions.
Recent experiments at GSI have demonstrated
the feasibility of proton-induced knockout reactions
120 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010