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

were taken from ref. [5]. Concerning the 7 Li channel,
we used the optical potential parameters of
Schumacher et al. [6]. The optical potential parameters
finally selected are those giving the best
fit for all the studied transitions in the ( 7 Li,t) reaction.
The calculated angular distributions normalized to
the data are shown in Figure 1. For both incident
energies, the calculated curves agree fairly well
with all the measured angular distributions of the
different populated states.
The -spectroscopic factors were extracted from
the normalization of the finite-range DWBA curves
to the experimental data. The spectroscopic factor
for the overlap between +t and 7 Li was taken to
We find an 16
44 E2-Sfactor S(0.3 MeV)=50
23
19 keV-b with the best fits shown in
Fig 2. The results are in good agreement with the
results of Brune et al.
Fig 2: Astrophysical S factor (see text)
Fig 1: Angular distributions (see text)
be 1.0 [7].
The good agreement between the DWBA calculations
and the measured differential cross sections
of the different excited states of 16 O at the two
bombarding energies of 28 MeV and 34 MeV
respectively, gives strong evidence of the direct
nature of the ( 7 Li,t) reaction populating these levels
and confidence in our DWBA analysis. An S mean
value of 0.15 0.05-0.04 and 0.07 0.03 are deduced
for the states of interest at 6.92 MeV and
7.12 MeV of 16 O respectively. The -reduced width
2
of about 26.7 10.3keV and 7.8 2.7 keV for the
6.92 MeV and 7.12 states respectively were obtained
at the radius of 6.5 fm. The present value of
2
have been included in R-matrix fits of both 12 C
( , ) 16 O S-factors obtained by direct measurements
at high energies and the 12 C( , ) measured
phase shifts [8]. The E1 and E2 contributions were
fitted separately. The best fits were determined
through a 2 minimization.
The l=2 fitting ( see Figure 2) was performed using
a 4-level including a background state and the astrophysical
S-factors data from [9-11]. The l=1 R-
matrix fitting is still under calculation.
S(300keV)=
keV-b who deduced the
-widths of interest via a sub-coulomb ANC measurement
and with those of references [8,10]. However,
it is in disagreement with the value recommended
in NACRE compilation [12].
Conclusion
We determined the reduced -widths of the subthreshold
2 + and 1 – states of 16 O from the transfer
reaction 12 C( 7 Li,t) 16 O at two incident energies. The
obtained result for the 2 + sub-threshold resonance
was introduced in the R-matrix fitting of radiative
capture and elastic-scattering data to determine
the low-energy extrapolations of E2 S-factor. The
result confirms the value obtained in previous
works [4,8,10] and disagrees with the value
adopted in NACRE [12] and the one deduced in [9]
where the -reduced width of the 6.92 MeV was
considered as a free parameter. Calculations for
the 1 – sub-threshold state are under development.
References
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14