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The experimental evidence for the exotic shape
changes at higher T, is so far scarce. More favourable
conditions are expected to be met in exotic, moderately
neutron-rich nuclei. The main tool for studying these
phenomena is the GDR as its strength function is sensitive
to the deformation of the system.
Shape coexistence, phase transitions
and dynamical symmetries
The concept of energetic gaps occurring in the nuclear
shell structure can also be extended to non-spherical
equilibrium shapes. For particular neutron or proton
numbers several shell gaps occur, which can lead to
different shapes in the same nucleus. In some cases
(e.g. in the neutron-deficient Z ~ 82 nuclei) spherical and
deformed shapes even compete for the nuclear ground
state. In recent years, much progress has been made in
the understanding of such shape coexistence by measuring
ground-state properties and by probing low-lying
nuclear excitations of proton-rich and neutron-rich nuclei
in tagging- and Coulomb excitation experiments.
The future programme using the new and more intense
heavy RIBs will shed light on the phenomena related to
nuclear shapes and dynamical symmetries. The occurrence
of new exotic shapes such as proton-neutron
triaxiality or tetrahedral deformation and new dynamical
symmetries associated with the critical points characterizing
the quantum phase transitions will be elucidated in
particular by measuring their electromagnetic transition
properties.
The two-fluid character of the nuclear quantum system
is manifested in near-spherical nuclei as low-lying
quadrupole collective isovector valence-shell excitations
and in deformed nuclei as scissors-mode like excitations.
Systematic information extended to unstable
nuclei is needed to understand the evolution of these
mixed-symmetry states and the underlying microscopic
mechanisms.
Instrumentation
All these studies will benefit from advanced instrumentation,
as provided by high-sensitivity gamma-ray
spectrometers (AGATA), arrays for high-energy gammaray
detection (PARIS, CALIFA), advanced devices for
charged particle detection (GASPARD, HYDE, FAZIA)
and high-energy neutrons (NeuLAND at R 3 B) as well
as new spectrometers for isotopic identification of the
nuclei of interest. The implementation of storage-ring
methods at FAIR will enable measurements of hadronic
scattering at low momentum transfer as well as electron
scattering off exotic nuclei.
4.3.6 Reaction Dynamics
Nuclear reactions are tools with which the nuclear phase
space of temperature, angular momentum and isospin
can be investigated. Identification of key variables controlling
the dynamics of complex colliding nuclei is still
a challenge and is also called for in nuclear structure
studies and astrophysics. Tremendous progress has
been made in explaining reaction observables for rather
simple few-body collisions at low energies, starting from
EFT. A systematic comparison between results of different
scattering approaches, with the same structure and
dynamical inputs, with accurate data will provide valuable
insights. Reactions to be investigated range from a
gentle rearrangement of individual nucleons to a massive
rearrangement of nucleons in deep inelastic or fission
processes up to the multi- fragmentation regime.
Fusion reactions
Nuclear fusion provides information on basic aspects of
a quantum many-body system of fermions under various
external forces, including quantum tunnelling with its
implication on the modelling of stellar environments. It
is also an essential tool to extend the periodic table of
the elements. The dynamics of collisions are extremely
sensitive to the plasticity of the intrinsic and evolving
structure and to the interplay of many open and virtual
channels, whose amplitudes can be tuned by varying the
beam energy and projectile-target combinations.
σ F [mb]
10 3 4 He+
197 Au
6 He+
197 Au
8 197 He+ Au
10 2
10 1
10 0
15 20 25
10 -1
E CM [MeV]
Figure 7. Measured fusion cross section as a function of the
center-of-mass energy for the 4,6,8 He + 197 Au systems. The dotted
line shows the one-dimensional barrier penetration calculation
for the 4 He + 197 Au system.
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 119