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

ing particles was thus obtained by combining the
information on the beam (P xy and θ in ) and P′ xy .
Result
The excitation energy (E x ) spectrum of 12 O, produced
from TKE and θ e of recoiling tritons, is
shown in Fig. 1. A sharp peak is clearly observed
at 0 MeV, which corresponds to the ground state
of 12 O. One can see another peak at around 2
MeV, indicating an excited state of 12 O. A Gaussian
fit to the spectrum gives a peak energy of 1.8
(4) MeV.
Fig. 1: Excitation energy spectrum of 12 O.
The dashed line denotes the 10 C + 2p decay
threshold at –1.78 MeV. The red-hatched
histogram shows the spectrum gated by θ cm
= 35—45°.
Differential cross sections deduced for the observed
states are shown in Fig. 2 as a function of
the center-of-mass scattering angle (θ cm ). Vertical
bars represent statistical errors only, while the horizontal
ones denote the size of the angular bin. The
data were obtained by analyzing the individual
spectra gated by the each angular bin. An example
of the gated spectra is shown in Fig. 1 by the red
FIG. 2: Differential cross sections of the 14 O
(p,t) 12 O reaction. The experimental data are
compared to the distorted-wave calculations
for ΔL = 0 (full), 1 (dot-dashed) and 2.
hatched histogram. We performed distorted-wave
calculations with the code FRESCO [10], assuming
a two-neutron cluster transfer. Bound state form
factors for the two-neutron cluster were similar to
those of Ref. [11]. We employed global opticalmodel
potential parameters for proton [12] and triton
[13]; use of the recent GDP08 global potential
[14] in the exit channel led to qualitatively similar
results. In Fig. 2, we compare the 14 O(p,t) data with
calculations for ΔL = 0, 1 and 2. The pattern of the
ΔL = 1 calculation is clearly incompatible with either
angular distribution. While the ΔL = 0 distributions
most closely match the data for both states,
ΔL = 2 cannot be completely ruled out. We hence
determine the spin-parity of the newly-observed
state at 1.8(4) MeV to be 0 + or 2 + .
Discussion
The E x of the 12 O excited state is remarkably
smaller compared to the second 0 + and first 2 +
states of 14,16 O (E x ~ 6 MeV) with a firm shell closure
at Z = 8. In marked contrast, it is much closer
to those of the mirror partner 12 Be at E x ~ 2 MeV.
The lowering of the 2 + and 0 + states in 12 Be is attributed
to significant neutron sd-shell configurations.
The lowered excited state in 12 O thus indicates
that the proton shell closure at Z = 8 is diminishing.
This demonstrates the persistence of mirror
symmetry in the disappearance of the magic number
8 between 12 O and 12 Be. Implications for the
shell quenching mechanism were discussed in
terms of the shell model and the cluster model in
Ref. [1].
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