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Figure 5. Gamma-ray spectrum of the Pygmy Dipole Resonance (PDR) in 68 Ni, measured with the RISING array at GSI (left) and a compilation
of integrated low-lying dipole strength in stable and neutron-rich nuclei from S-DALINAC and GSI, respectively, shown as a function of
asymmetry.
Coulomb projectile break-up at a few hundred MeV/u
are used for these studies. Detailed complementary
studies of this new excitation mode known as the Pigmy
Dipole Resonance (PDR), can be done in stable nuclei
by employing electromagnetic and hadronic probes. The
right part of Figure 5 shows a compilation of observed
low-lying dipole strength as a function of neutron-proton
asymmetry α.
Counts/MeV
It is important to understand the nature of the low- and
high-energy components of the dipole strength in general.
Therefore, studies of its evolution as a function of
larger neutron-proton asymmetry, deformation and temperature
are called for. A very interesting aspect is the
relation of the dipole strength to the density-dependence
of the symmetry energy and the neutron-skin thickness.
The equation of state (EOS) for asymmetric nuclear matter
is particularly important for the understanding and
description of neutron-star properties.
The measurement of giant resonances of other
multipolarities as well as magnetic excitation modes in
unstable nuclei require new experimental techniques,
such as those based on light-ion scattering in inverse
kinematics. A pilot measurement for the giant monopole
excitation, providing information on the nuclear compressibility,
was made for the unstable 56 Ni nucleus
(Figure 6) using inverse kinematics and the active target
MAYA (Figure 12). The key instrumentation to enable
major experimental steps towards the understanding
Figure 6. Giant quadrupole resonance (16.5 MeV) and giant
monopole resonance (19.5 MeV) in 56 Ni induced by the 56 Ni(d,d’)
reaction at 50 MeV/u in the MAYA active target at GANIL .
of giant resonances in exotic nuclei will be available at
FAIR with R 3 B and the storage-ring experiments EXL
and ELISe employing hadron and electron scattering,
respectively.
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 117