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

Counts/60keV
Shell structure of neutron-rich rich Oxygen isotopes
IPNO Participation: A. Ramus, D. Beaumel, Y. Blumenfeld, F.Maréchal, J.A. Scarpaci, N. De
Séréville, S. Franchoo, J. Guillot, F. Hammache, I. Stefan
Collaboration : University of Surrey (UK), University of Birmingham (UK); University of Liverpool
(UK); University of West Scotland (UK), STFC Daresbury (UK), VECC Kolkata (India); GANIL, Caen
(France); SPhN/IRFU/CEA, Saclay (France); LPC, Caen (France)
Nous avons étudié la structure en couches des isotopes d’oxygène riche en neutron à l’aide de réactions
de transfert d’un neutron induites par un faisceau d’ 20 O produit par le dispositif SPIRAL du GANIL, sur une
cible de CD 2 . Le dispositif expérimental combinait les détecteurs MUST2 et TIARA pour la détection des
particules, quatre détecteurs EXOGAM pour les gammas, et le spectromètre VAMOS pour l’identification
de l’éjectile lourd. Plusieurs états sous le seuil neutron sont observés dans le spectre en énergie d’excitation
de l’ 19 O produit via la réaction 20 O(d,t) 19 O. L’état excité à 1.41 MeV est peuplé par pickup d’un neutron
sur l’orbitale 2s 1/2 , en principe inoccupée. La valeur du facteur spectroscopique de cet état montre que la
taille du gap N=14 est encore réduite par rapport au cas de l’ 22 O. Un état à 3.23 MeV est identifié pour la
première fois comme étant de parité négative, Contrairement aux états observés plus bas, son énergie est
mal reproduite par les calculs de modèle en couches.
Introduction
Shell structure evolution is one of the major paradigms
of nowadays nuclear physics. While related
effects such as modification of the magic numbers
far from stability have been discussed for many
years [1], experimental evidences are accumulating
only since the advent of radioactive nuclear
beams. The mechanisms at work in the structural
changes in neutron-rich oxygen isotopes have
been underlined in [2]. The proton occupancy and
the strong monopole tensor force between the proton
d 5/2 and neutron d 3/2 orbitals are invoked to
explain the decrease of the N=20 gap in favour of
N=16 when going from the stable 30 Si to 24 O. The
present study aims to investigate the development
of the N=14 and N=16 shell gaps across oxygen
and neon isotopes using one neutron transfer reactions.
The (d,p) and (d,t) transfer reactions on
20 O and 26 Ne beams delivered by the SPIRAL facility
at GANIL have been studied. In the case of
20 O, the (d,p) reaction will provide the single particle
strength in 21 O and measure the energy of the
so-far unobserved 3/2+ state that carries the d 3/2
strength. Information concerning negative parity
states in 19 O measured in the (d,t) transfer reaction
should also help to resolve uncertainties about the
cross shell excitations in the N=16 region. In this
report, only the preliminary results obtained for the
20 O(d,t) reaction will be presented.
particles, offering a nearly 4 solid angle coverage.
For -ray detection, four germanium clovers of the
EXOGAM array were mounted in a cross geometry
around the target point. The heavy fragments were
detected using the VAMOS spectrometer set at
zero degrees with its standard detection. The excitation
energy in 19 O is deduced from the measured
triton angle and total energy using two-body kinematics.
For tritons in coincidence with 19 O fragments
in VAMOS, the resulting spectrum (Fig. 1)
shows three peaks with an overall excitation energy
resolution around 500 keV (FWHM).
20 O(d,t) 19 O
E*(MeV)
Fig.1: Excitation energy spectrum in 19 O reconstructed
from the energy and angles of tritons
detected in the MUST2 array.
Results
The secondary beam of 20 O was produced by the
ISOL method using the SPIRAL facility at GANIL at
11 MeV/u. A combined setup was used to detect
light recoil charged particles, gammas and heavy
ejectiles in coincidence. The TIARA [3] and
MUST2 [4] silicon arrays were placed around the
CD2 target to detect the recoiling light charged
Results are summarized and compared to Shell
Model calculations [5] performed with the WBP
interaction in table 1 and Fig.2 for the three observed
peaks. Considering the known spectroscopic
information on 19 O, the first peak could correspond
to a doublet composed of the groundstates
(GS) and a state at 89 keV. However, the
gamma-particle coincidences do not show any 89
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