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Nuclei far from Stability
The study of exotic nuclei, far from the valley of beta stability, presents
an important challenge for nuclear physics. Nuclear models have to be radically
improved in order to accommodate the wealth of new phenomena that have
been unveiled by the continuing experimental work. Prominent milestones
are: the discovery of the heaviest chemical elements; the discovery of
nuclear halos in the lightest neutronrich nuclei, immediately followed
by a widespread theoretical effort that has considerably advanced our understanding
of this phenomena; the experimental mapping of the nuclear shell structure
far from stability, reaching in some cases the neutron or proton drip lines.
Although we are still far from having a unified theoretical description
valid far from stability, important progress has been made. New clues to
the isospin dependence of the spinorbit field have been given by the relativistic
mean field calculations, and the study of the role of the protonneutron
pairing is being actively pursued in N=Z proton rich nuclei
and this may provide new insight into the properties of the mean field
of nuclei far from stability. The most detailed description of nuclear
structure is given by the shell model and new techniques such as the Shell
Model Monte Carlo Method may greatly help in dealing with the heavier nuclei.
In this context, there have been improvements in the use of interactions
obtained from nucleonnucleon potentials that fit the NN phase shifts.
There are some aspects specific to the theoretical description of drip
line nuclei, for example how to deal with resonant quasibound states.
This calls for new developments regarding the treatment of the continuum
and the correlations  particularly pairing  on the same footing, including
fewbody dynamics. Another example is the possibility of enhanced charge
symmetry breaking due to Coulomb effects in quasibound orbits. The lowenergy
behaviour of giant resonances in drip line nuclei has also raised a lot
of interest recently, be it soft dipole ``pigmy'' resonances or threshold
peaks of the dipole and quadrupole channels. Many new features of nuclear
structure have been found  or are predicted to show up  in the realm
of exotic nuclei, a few of which are singled out in the following.



Halos: Outstanding structural dripline phenomena with extreme clusterisation
into an ordinary core nucleus and a veil of halo nucleons  forming exceptionally
dilute neutron matter. The origin is only partly understood, but a prerequisite
is low angular momentum for the halo particles and fewbody dynamics such
as in Borromean nuclei characterised by pairwise constituents with no bound
states. In the limit of vanishing binding extremely large halos may occur.



Skins: For a large neutron excess, the bulk of the neutron density
is predicted to extend beyond the proton density creating a sort of ``neutron
skin''. Similar effects may appear for a large proton excess. There are
many suggestions how these skins might change nuclear properties. One could
imagine different deformations of the neutron and proton distributions
etc.; first experimental observations and theoretical work hint into this
direction. It provides the opportunity to study the behaviour of abnormal
nuclear matter with very large T_{z}. This could be helpful
for improving the reliability of calculations of the properties of neutron
stars.



Shell stabilisation: It has been verified that the heaviest elements
owe their existence to the N=162 neutron deformed shell closure.
These results strongly indicate that even heavier elements are shell stabilised



Vanishing of shell closures: Theory and experiment are now indicating
that shell closures may change far from stability. A wellknown example
is the disappearance of N=20 as a neutron magic number in the neon,
magnesium and sodium isotopes. But the physical mechanism which is responsible
for this disappearance could have other consequences for nuclei of increasing
instability such as reduction of the shell gaps thus allowing manyparticle,
manyhole excitations to become more favoured in energy, leading to shape
changes and shape coexistence



Nuclei with N=Z: When protons and neutrons occupy the same orbitals,
mutual reinforcement of the shell gaps may lead to a stabilisation of exotic
shapes in the ground state. Here also the protonneutron pairing interaction
is exposed to view at its maximum. The study of isospin symmetry breaking,
particularly for the heaviest mirror nuclei, is also of strong interest.
As for medium heavy nuclei, the N=Z line coincides with the
proton dripline, proton decay from discrete excited states is possible,
as recently observed in ^{58}Cu.



Superdeformation and hyperdeformation: Far from the line of stability,
medium heavy nuclei may exhibit large deformations, even at low excitation
energies. Thus it has been shown that ^{76}Sr , ^{80}Zr
and ^{100}Zr are amongst the most deformed nuclei known in their
ground states. Recently a sequence of superdeformed states has been observed
in ^{80}Sr and it is predicted that hyperdeformed states should
exist in nuclei in this region as well.
Next: Exploring
the nuclear landscape Up: Nuclear
Structure under Extreme Previous: Introduction
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