exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Nuclar ‘’BUBBLE’’ structure in 34 Si
IPNO Participation: M. Grasso, E. Khan, N. Van Giai
Collaboration : CEA DAM DIF F-91297 Arpajon, France, GANIL, France, Florida State University,
USA, Zagreb University, Croatia
Les noyaux avec des structures à bulles sont caractérises par des densités très creusées au centre.
Des prédictions sur leur existence sont effectuées avec trois modèles théoriques : le modèle en couches et
les champs moyens relativiste et non relativiste. Un accord de tous les modèles indique le noyau 34 Si comme
possible candidat pour une structure à bulle, avec une déplétion de la densité centrale protonique de
40%.
The bubble structure characterizes atomic nuclei
having a central density depletion..
This phenomenon has been discussed for many
decades, starting from the work of Wilson in the
40s [1] for the study of low-energy excitations as
oscillations of bubbles up to the first microscopic
calculations of Campi and Sprung in the 70s [2].
More recently, bubbles have been discussed in
superheavy and hyperheavy nuclei [3].
The perspective of producing more exotic nuclei
with the new generation of RIB facilities has led to
a revived interest in the subject.
The s orbits have the only radial distributions
peaked in the interior of the nucleus, their wave
function extending further to the surface according
to their number of nodes. Orbits with increasing
angular momenta are more localized at the nuclear
surface. Therefore, the non filling of s orbits is expected
to provoke a central depletion of the nucleon
density.
A suitable region of the chart of nuclides to search
for a proton bubble is found for the N=20 isotones.
Between Z=20 and Z=16 the s1/2 orbit is separated
by about 6.5 MeV from the lower d5/2 orbit
and 2.5 MeV from the upper d3/2 orbit, forming two
subshell closures at Z=14 and Z=16, respectively
[4]. In addition, the N=20 shell closure is rigid
enough for these nuclei to hamper significant coupling
to collective states. Following the sequential
filling of the proton orbits, 34 Si should not contain
any proton in the 2s1/2 orbit, whereas 36 S should
contain 2 protons there. This would lead to an important
change in the proton density distribution
between the nuclei 36 S and 34 Si and make the 34 Si
a bubble nucleus.
The work in Ref. [5] aimed at determining, using
various theoretical approaches (shell model, relativistic
and non relativistic mean field), whether the
34 Si could actually be considered as good proton
bubble nucleus.
As an illustration, the HF proton density profiles,
obtained with the Skyrme parametrization SLy4
and calculated in 34 Si and 36 S (where the s state is
occupied), are shown in Fig. 1.
The depletion fraction F of the density is ~ 38\%,
where:
F
Fig 1. Proton densities of 34 Si and 36 S
max
max
central
Similar results have been found with all the other
models.
These results indicate that 34 Si is a good candidate
for a bubble density profile.
The measurement of the charge density in this nucleus
should be undertaken, for instance by electron
scattering in a exotic beam collider such as
EXL in FAIR and RIBF in Riken. The study of 34 Si,
either by high energy proton scattering to study the
matter density profile, or by direct reactions to yield
the spectroscopic factors and the low-energy excitation
spectrum is presently possible. For the
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