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Frank Strieder and Gianluca Imbriani Ruhr-Universität Bochum

13 C( ,n) 16 O – The experimental state of the art and open problems. Frank Strieder and Gianluca Imbriani Ruhr-Universität Bochum Workshop “starting-up the LUNA-MV collaboration” Laboratori Nazionali del Gran Sasso, Assergi (AQ) 07.02.2013. Element abundance in the solar system.

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Frank Strieder and Gianluca Imbriani Ruhr-Universität Bochum

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  1. 13C(,n)16O – The experimental state of the art and open problems Frank Strieder and Gianluca Imbriani Ruhr-Universität Bochum Workshop “starting-up the LUNA-MV collaboration” Laboratori Nazionali del Gran Sasso, Assergi (AQ) 07.02.2013

  2. Element abundance in the solar system a single process cannot explain the global observable of heavy element production, i.e. the solar abundance pattern for trans-iron elements. Approximately half of the observed abundance can be attributed to the so-called s-process, where the neutron capture flow proceeds at a slow rate compared to competing -decay rates due to a small neutron density, ~ 107 neutron/cm3, in the relevant environment. The remaining contribution of heavy elements is produced in the r-process at a rapid neutron capture rate (neutron density ~ 1022 cm-3)

  3. WEAK: Massive stars 22Ne(a,n)25Mg T> 109 K nn=106-1011 cm-3 MAIN: low mass AGB (1-3 M⊙) 13C(a,n)16O (dominant) T= 80 - 150  106 K nn=106-107 cm-3 22Ne(a,n)25Mg (marginal) T= 250 - 400  106 K nn=109-1010 cm-3 Stellar environments for s-process Special cases: Proton Ingestions (CEMPs and Sakurai’s objects) 13C(a,n)16O with nn=1015 cm-3

  4. s-Proess in low-mass AGB (1-3 M⊙) During the AGB phase, alternatively H and He-shells switch on and off several times. Then, the convective H-envelope can penetrate the He-burning inter-shell, dredging-up the produced elements during the s-process.

  5. s-Proess in low-mass AGB (1-3 M⊙) Convective Envelope (H rich) (Straniero et al. 1995) Variable protons profile 12C(p,g,b)13C 13C(a,n)16O

  6. Astrophysical relevant energy

  7. Experimental Approaches – 13C(a,n)16O • two approaches have been applied in the past • Neutron detector with 3He countersneutrons are slowed down in moderator matrix and captured by 3He(n,p)T in counter tubes Mainz approach (adapted by Stuttgart group) Drotleff et al. (13C(,n)16O), Jaeger et al. (22Ne(,n)25Mg) • later Harissopulos et al. (13C(,n)16O – higher energies) • moderator sphere surrounded by BaF2 -detector array • Karlsruhe BaF2 array for (n,) capture studies neutrons captured in Cd loaded moderator mainly by 113Cd(n,)114Cd  Heil et al.

  8. Experimental Approaches – 13C(a,n)16O Neutron detector with 3He counters

  9. Experimental Approaches – 13C(a,n)16O Neutron detector with 3He counters Efficiency Spectrum

  10. Experimental Approaches – 13C(a,n)16O moderator sphere surrounded by BaF2 -detector array neutron are converted into -rays with high efficiency  high efficiency determination with BaF2 array total efficiency lower than in the case of the 3He n-detector

  11. Experimental Results – 13C(a,n)16O Heil et al., 2008

  12. Experimental Results – 13C(a,n)16O • open issues for raw data: • large statistical uncertainties at low energies • scattering in absolute normalization • systematic (normalization) uncertainty largely unknown Heil et al. do not quote common systematic uncertainty of the data set (only non-statistical)  treatment in global analysis of Heil et al, questionable (not trivial anyway) • final „global“ data set by scaling of previous data – systematic uncertainty not considered consistently • electron screening: at lowest energy of Drotleff < 7% (in adiabatic limit) note: overestimated in Drotleff et al.

  13. Experimental Results – 13C(a,n)16O possible influence of subthreshold resonance only most recent low energy data sets: Heil and Drotleff analysis with various approaches, e.g. R-matrix and, in particular, with different values for subthreshold resonance at – 0.3 keV

  14. Experimental Results – 13C(a,n)16O some conclusion: - low energy data points do not constrain the fit with respect to the subthreshold resonance- quoted uncertainties are large probably also a mixture of systematic and statistical uncertainties, e.g. in Drotleff scattering of data is unexpectely small with respect to error bars- most data point below E = 350 keV are higher than the fit curves, i.e. Heil R-matrix curve indication for beam induced background? update with view from indirect methods see following talk from M. LaCognata systematic uncertainties and normalization remain an open issue

  15. Formation of the 13C pocket transition zone between convective envelope and radiative core M=2M⊙Z=Z⊙ • Maximum envelope • penetration (TDU) • 12C(p,)13N(β-)13C 13C(p,)14N • 22Ne(p,)23Na • The 3 pockets fully developed • 13C(a,n)16O and s-processstart

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