Perspectives of Nuclear Physics in Europe - European Science ...

4.3 Nuclear Structure and Dynamics
Box 2. Changing magic numbers
The monopole part of the effective nucleon-nucleon
force induces drastic changes in the single particle
energies, resulting in changes in the magic numbers
in nuclei far from stability. The strong p-n interaction
between spin-orbit partners (right figure) is essential
for these changes.
It results in a spin ½ ground state in 31 Mg, dominated
by neutron excitations across a reduced N=20 gap,
evidenced by the magnetic moment measured at
ISOLDE.
It leads to a disappearing N=28 shell gap, seen in
2 + energies in neutron-rich Si isotopes measured at
GANIL.
j > = l +1/2
proton
j < = l −1/2
neutron
n
p
p
tensor
force
(π+ρ exchange)
n
31
Mg
In the future, transfer reactions will provide a powerful
tool to map out single particle energies throughout the
nuclear chart at HIE-ISOLDE and SPIRAL2.
neutrons would be particularly stable, but experiments
found that this nucleus, 28 O, is not even bound. On the
other hand, measurements have revealed that 24 O is doubly
magic indicating that new magic numbers develop
far from stability.
Future research programmes with RIBs will use various
reactions to investigate the effects of the isospin
dependence of the mean field and of the long- and shortrange
correlations in exotic nuclei. An example of a future
programme is the single particle occupancy near the
‘normal’ neutron shell closures from N=8 to N=126 as a
function of Z, which can be investigated through transfer
reactions. This requires the widest range of nuclear
beams with energies of ~ 10 MeV/u. HIE-ISOLDE and
the upgraded SPIRAL facility will provide a wide range
of ISOL beams, while intense beams of neutron-rich
fission-fragments will be accelerated using SPIRAL2 and
SPES. There will also be possibilities of such studies at
the EXL facility at FAIR.
New experimental set ups tailored to measure simultaneously
and with increased efficiency particles and
gamma-rays will largely increase the sensitivity and resolution
of such studies. Radioactive (triton) and polarized
targets will complete the array of equipment available
to the experimentalists.
The next decade should see the mapping of single
particle structures throughout the nuclear chart with
a precision approaching that known for stable nuclei
today.
Proton-neutron symmetric nuclear matter
and the proton drip-line
Isospin symmetry is a property of the strong interaction.
In observables such as energies and masses this symmetry
cannot manifest fully due to the presence of the
electromagnetic interaction, which by its nature does
not have this symmetry.
112 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010