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

Study of the astrophysical key reaction
12C(
, ) 16 O through the transfer reaction 12 C( 7 Li,t) 16
16 O
IPNO Participation: F. Hammache, N. Oulebsir, L. Audouin, D. Beaumel, S. Fortier, M. G. Pellegriti,
P. Roussel
Collaboration: IPNO, Université de Béjaïa, CSNSM, ULB-Bruxelles, GANIL, GSI
La réaction de capture radiative 12 C( , ) 16 O joue un rôle crucial dans la combustion stellaire. En effet, dans
les étoiles géantes rouges, cette réaction intervient juste après le processus triple alpha et le rapport des
taux de production de ces deux réactions détermine directement le rapport d’abondance 12 C/ 16 O dans les
étoiles en phase finale de combustion de l’hélium. Ce rapport influence aussi les stades ultérieurs d’évolution
stellaire des étoiles massives. Toutefois, malgré de nombreuses études expérimentales, la section efficace
de cette réaction reste encore très incertaine aux énergies stellaires. L’extrapolation des données obtenues
à plus hautes énergies est rendue particulièrement difficile par la superposition de diverses contributions
dont certaines sont mal connues, c’est le cas de celles des résonances sous le seuil à 6. 92 et 7.12
MeV de l’ 16 O dont nous avons déterminé les largeurs a au moyen de la réaction de transfert 12 C( 7 Li,t) 16 O.
Introduction
. 12 C( ) 16 O reaction plays a crucial role in stellar
burning. Indeed, in stars like red giants, where the
stellar core is in helium burning phase, 12 C( ) 16 O
reaction follows the production of 12 C by triple alpha-process.
The ratio of the yields of these two
reactions determines directly the 12 C to 16 O abundance
ratio in stars at the end of helium burning
phase. This ratio has important consequences for
the nucleosynthesis of elements heavier than carbon
which are almost exclusively produced in this
kind of stars. It has also an influence on the subsequent
stellar evolution of the massive stars [1,2]. It
somehow determines the final fate of the stars
(blackhole, white dwarfs,…). The rate of the triple
alpha process is well determined (10-15% uncertainty),
but it is not the case of 12 C( ) 16 O reaction
which has an uncertainty of about 41% despite the
various experiments which studied it these last 4
decades.
12 C( ) 16 O reaction occurs at temperature around
0.2 GK which corresponds to the Gamow peak of
300 keV. At this energy, the cross section is expected
to be 10 -8 nbarn which excludes any direct
measurement with the available techniques.
Though direct measurements have been performed
at rather low energy down to 0.9 MeV
(CM), the R-matrix extrapolation to stellar energy is
quite complicated. Indeed, the -capture cross
section at 300 keV which corresponds to the excitation
energy region of 16 O around 7.46 MeV, is
expected to be dominated by the E1 and E2 transitions
through the low energy tail of the large resonance
à 9.6 MeV state and the high energy tails of
the two-subthreshold resonances at 7.12 (1 - ) and
6.92 (2 + ) MeV states of 16 O which interfere to enhance
the alpha-capture cross section. These two
subthreshold states make the extrapolation complicated
because their contribution to 12 C( ) 16 O
cross section at 300 keV are not very well known
especially for the 2 + state since their reduced -
width and so their spectroscopic-factors are
spread over a large range of values [3]. Moreover,
in the R-matrix calculation, one has to take into
account also the contribution of the non-resonant
direct capture and all possible interference effects
between the different resonances [4].
In view of the importance of 12 C( ) 16 O, we addressed
the problem concerning the values of the
reduced -width of the two sub-threshold states
by performing a new determination of these quantities
through 12 C( 7 Li,t) 16 O transfer reaction measurements
at two incident energies and a detailed
DWBA analysis of the data.
Experiment description
The experiment was performed using a 7 Li 3+ beam
provided by the Orsay Tandem-ALTO facility. A
self-supporting 12 C targets was used. The reaction
products were analyzed with an Enge Split-pole
magnetic spectrometer and detected at the focal
plane by a 50 cm long position-sensitive gas
chamber and a E proportional gas-counter. The
particle identification was made unambiguously
using E versus position measurements. The tritons
were detected at angles ranging from 0 to 31
degrees corresponding to angles up to 44 degrees
in the center of mass frame.
Results
The experimental 12 C( 7 Li,t) 16 O differential cross
sections measured for the 6.05, 6.13, 6.92 and
7.12 at the two incident energies of 34 and 28
MeV, are displayed in Fig.1a and Fig.1b, respectively.
The error bars assigned to our measured
cross sections includes the uncertainties on the
peak yield, the number of target atoms, the solid
angle and the integrated charge.
Finite-range DWBA calculations, using the
FRESCO code were performed. For the triton
channel, the optical potential parameters used
13