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Halos, clusters and few-body correlations
A specific challenge in light nuclei is to find the correct
degrees of freedom for weakly bound systems,
in order to describe structures such as halos, alpha
clusters and nuclear molecules. Cluster structures in
many-body states manifest themselves close to cluster
channel thresholds due to the coupling of a given
many-body state with the decay channel. Thus halos are
only particular examples of a more general clustering
phenomenon. This example illustrates why one has to
consider many-body states in the extended configuration
space including decay channels.
Since the experimental discovery of the halo structure
in the mid 80’s, detailed studies have been performed to
reach a deeper understanding of the ground-state wave
function of halo nuclei and in particular of neutron correlations
in the case of 2-neutron halo nuclei, such as 6 He
and 11 Li. In the case of the benchmark drip-line system
11 Li, the results indicate large ground-state configuration
mixing and strong correlations between the two halo
neutrons. Recently, evidence for low-lying and some of
the high-lying excited states in 6,8 He has been found.
Experiments in the next years should reveal a clear final
picture of the most studied halo nuclei and to allow the
investigation of heavier candidates for which data are
still scarce or which are presently out of reach.
The envisaged studies of these exotic structures in
the coming decade involve cluster and single-widths,
knock-out or transfer reactions, quasi-free and electron
scattering. Studies of exotic cluster-decay modes are
needed to investigate which sub-clusters are possible
far from stability and/or at high excitation energies.
It is challenging to obtain cluster correlations in abinitio
approaches such as the no-core shell model.
The Fermionic Molecular Dynamics (FMD) or Antisymmetrised
Molecular Dynamics (AMD) approaches
are powerful alternatives for studying this aspect of
nuclear structure.
Specific instrumentation for studies
of exotic light nuclei
The toolbox for studying the lightest exotic nuclei has to
be extremely diverse. Since most experiments are on the
limit of what is possible both concerning ion production
and detection, an integral approach is often necessary
where the accelerator and separation facilities are parts
of the experimental set-up. In addition to the generic
state-of-the-art detection systems for charged particles
and gamma rays, systems specific to studies of light
nuclei are active targets for low-momentum transfer
experiments as well as high-efficiency, high-granularity
neutron detectors for reaction and neutron-decay studies.
A wealth of spectroscopic information has become
available by the advancement of high-granularity detectors
for charged particles; the potential for similar studies
through neutrons is at least as large, but as of yet has
hardly been possible to address.
4.3.4 Shell Structure and the
Isospin Degree of Freedom
Changing shell structure
The modification of shell gaps far from stability raises
doubts about one of the firmest paradigms of nuclear
structure – the universality of magic numbers throughout
the nuclear chart. Nuclei are more stable and difficult
to excite at particular neutron or proton numbers, 8,
20, 28, 50… the so-called magic numbers. In recent
years, evidence has surfaced pointing to changing shell
structure with a varying number of protons and/or neutrons.
These findings furnish a stringent test for modern
nuclear structure models and have important astrophysical
implications, in particular for the understanding of the
r-process. From a theoretical point of view, the reasons
for this shell evolution are not well established and different
scenarios are under consideration; variations in
the mean field when approaching the neutron drip-line
as well as specific components (pairing, tensor interaction…)
in the residual interaction, to name a few.
Vanishing and new shell gaps
in light nuclei
These specific phenomena have a vast influence on
the predictions of nuclear properties far from stability.
A classical example is the parity inversion of the 11 Be
ground state, which indicates that the shell gap between
the p and sd shells has disappeared. In this case the
1s 1/2 orbital has become the intruder ground state,
leading to the vanishing of the N = 8 magic number. A
similar phenomenon has been observed in the resonant
nucleus 12 O, proving that such shell evolution occurs
also on the proton rich side of the valley of stability and
therefore is strongly linked to shell model n and p occupancy.
Furthermore, the magic character of the neutron
number N=20 appears to have vanished in the exotic
nucleus 32 Mg (Z=12). Such an “island of inversion” has
also been identified around N=28 (see Box 2). Recently
shell changes have been seen below Z=28 in neutronrich
Co isotopes, and appear to occur also for Ca, Ti
isotopes with N~34 and in Cu isotopes approaching
N=50. It had been widely speculated that, besides the
doubly magic 16 O (Z=N=8), the oxygen isotope with 20
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