New study of the astrophysical reaction 13C,n16O by transfer reaction 13C7Li,t17O Maria Grazia Pelle

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New study of the astrophysical reaction
13C(?,n)16O by transfer reaction 13C(7Li,t)17O
Maria Grazia Pellegriti, IPN-Orsay, France
?Astrophysical Motivation ?Status of 13C(a,n)16O
cross section measurement ?Spectroscopic factor
approach ?Experiment at the Orsay Tandem-ALTO
facilities ?Data analysis angular
distributions ?DWBA calculations
results ?Conclusion
23rd Meeting between Astrophysicists and
Nuclear Physicists Brussels, 11 December 2006
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 Astrophysical Motivation
s-process nucleosynthesis (n,?) half of the
heavy elements in the Univers
90ltAlt209 ? low-mass AGB stars 1-3 M?
(Temperatures?108K) 13C(a,n)16O is considered
as the main neutron source for the s-process
13C in synthesized in the He inter-shell of the
star by the CNO cycle 12C(p,g)13N(b)13C
formation of the so-called 13C pocket on the
top of the inter-shell region


production of the neutrons through the burning
reaction 13C(a,n)16O
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 State of the art of the 13C(a,n)16O reaction 
R-matrix (Sa0.7)?A R-matrix (Sa0.0)?B ?
Drotleff et al.
? The experimental data points seem to show an
increasing of the S-factor toward zero
energy. This would come from a large contribution
of the sub-threshold state at 6.356 MeV in
17O!!!
? Theoretical values ?S?0.3-0.7 Enough neutrons
to trigger the s-process nucleosynthesis
? Experimental determination of Sa ?Kubono
et al. 13C(6Li,d)17O(DWBA) S?(exp)?0.011!!!
Keeley et al. reanalysis of Kubono data
S?(exp)0.4
factor 40 ?!?!?!
?Johnson et al. ANC study of the 6Li(13C,d)17O
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 Spectroscopic factor and transfer reactions 
The experimental principle consists in measuring
the normalization factor between the
experimental differential cross section of
a-transfer reaction and its DWBA calculation.
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 Measure of the 13C(7Li,t)17O transfer
reaction
17O level scheme
17O
Advantages of the 13C(7Li,t)17O with respect to
13C(6Li,d)17O
?Different reaction ? independent determination
of the Sa factor ?less momentum mismatch (L?1
in 7Li) ? larger cross section
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Experimental Setup
?E
tritons
pos

• ?E7Li 28 MeV
• Transfer angular distribution measurements
  0-32
• Elastic scattering angular distribution
  measurements
• E7Li 34 MeV
• Transfer angular distribution measurements
  0-32
• Elastic scattering data _at_ 34 MeV from Schumacher
  et al. NPA 212 (1973) 573

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Procedure _at_ each angle

• 13C(7Li,t)17O measurement with 13C enriched
  targets
• 12C(7Li,t)16O measurement with the pure 12C
  target for calibration
• background estimation (12C
  impurities 12C build-up)

12C(7Li,t)16O
10.35 MeV
7.12 6.92 MeV
6.12 6.05 MeV
13C(7Li,t)17O
10.35 MeV
7.38 MeV 17O
Background due to 16O levels _at_ 6.917 7.12
MeV!!!
6.356 MeV
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13C Targets Analysis with 7Li elastic
scattering measurements _at_ 28 MeV
12C target
13C(84) target
7Li
7Li
7Li
7Li

• ?Thickness determination from energy loss
  measurement of 241Am ?s
• ?Number of 12C atoms in 13C targets with respect
  to the number of 12C atoms in 12C target
• Number of 11B atomes in 13C targets with respect
  to 13C number of atoms in 13C target.
• Cook et al. NPA466 (1987)168 (7Li11B _at_34 MeV)
  elastic calculations _at_ 28 MeV

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7Li on 13C 12C targets _at_ ?lab6, E34MeV

• Fixed parameters
• P1, P2 (calibration), ? A1/A2
• ? width of ?6.356 MeV state
• Free parameters A3, N

o Data ____ FIT ____ 16O (6.92 7.12
MeV) ____ 17O (6.356 MeV) ____ 15N
(10.45,10.53,10.70,10.80 MeV)
A3
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6.356 MeV transfer angular distributions
Einc34 MeV
Main source of errors Counts ? FIT Thickness of
the target Solid angle
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 What inputs do we need for DWBA calculations? 
(Distorted Wave Born Approximation)
XA? Yb with Aab

• Optical potential parameters of entrance channel
• ?Elastic scattering measurements ?
  13C(7Li,7Li)13C
• Schumacher et al.(Pot III)NPA 212 (1973) 573
• Optical potential parameters of exit channel
• ? Elastic scattering measurements ? 17O(t,t)17O
• Garrett et al. (Pot I-f7/2shell) NPA 212 (1973)
  600
• Interaction potential

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Comparison between our data and the FRESCO
calculation ?no need to add Compound nucleus
34 MeV
28 MeV
28 MeV
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Finite range DWBA calculations (FRESCO)
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• Finite range DWBA calculations (FRESCO)

6.356 MeV (2s1/2)
_at_ 7Li incident energy of 28 MeV
?
S?0.32
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Conclusion

• ?Extraction of angular distributions of 6.356 MeV
  
• _at_ 28 MeV and 34 MeV
• ?DWBA calculations (FRESCO) and ? spectroscopic
  factor determination
• S?0.45?0.13
• ?
• In good agreement with Keeley deduced values
• ? No need to add compound nucleus mechanism in
  the transfer angular distribution analysis
• to describe our state of interest
• Next
• ?Cross section calculation (R matrix)
• ?Reaction rate calculations

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Collaborators
M. G. Pellegriti, F.Hammache, P. Roussel, L.
Audouin, D. Beaumel, S. Fortier (IPN-Orsay) J.
Kiener, A. Lefebvre-Schuhl, V. Tatischeff
(CSNSM-Orsay) L. Gaudefroy (GANIL-Caen) M.
Stanoiu (GSI-Darmstadt)