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The s-process in massive stars

Michael Heil. The s-process in massive stars. FZK: F. Käppeler, E. Uberseder Turin: R. Gallino, M. Pignatari. Outline: Introduction The weak s- process in massive stars Nuclear data needs for the (weak) s-process Results of (n, g ) cross section measurements (activation method) Outlook

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The s-process in massive stars

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  1. Michael Heil The s-process in massive stars FZK: F. Käppeler, E. Uberseder Turin: R. Gallino, M. Pignatari Outline: • Introduction • The weak s- process in massive stars • Nuclear data needs for the (weak) s-process • Results of (n,g) cross section measurements (activation method) • Outlook • Conclusions International School of Nuclear Physics, Erice, September 2006

  2. Nucleosynthesis of the elements One of the main goals of Nuclear Astrophysics is to explain how and where the chemical elements were produced. solar abundance distribution stellar burning NSE r-process weak main s-process p-process weak r ? LEPP ? ABUNDANCE np-process ? Sneden et al., Ap. J. 591 (2003) 936 MASSNUMBER Michael Heil International School of Nuclear Physics, Erice, September 2006

  3. The main s-process Nuclear physics data (mainly neutron capture cross sections) can be determined experimentally!!! Success of the main s-process in TP-AGB stars of 1-3M⊙ r-residuals method: Arlandini et al. ApJ 525 (1999) 886 Michael Heil International School of Nuclear Physics, Erice, September 2006

  4. weak s-process • The weak component: Nucleosynthesis of A < 90 • Stellar site massive stars M>8M⊙ • core He-burning shell C-burning • Temperature 3-3.5·108 K ~1·109 K • kT=25 keV kT=90 keV • Neutron density 106 cm-3 1011-1012 cm-3 • Neutron source22Ne(a,n) 22Ne(a,n) unburned in core He-burning 13C(a,n)16O13C via 12C(p,g)13N(b+)13C • 17O(a,n)20Ne via 16O(n,g)17O • 12C(12C,α)20Ne, 12C(12C,p)23Na • Neutron exposure in the C Shell comparable with Core He-burning. • Material from core He-burning is reprocessed during shell c-burning. • The final weak s component is an superposition of two different s-components • Important: weak s goes together with r-process Michael Heil International School of Nuclear Physics, Erice, September 2006

  5. Nuclear data needs s-process abundances are determined mainly by Maxwellian averaged neutron capture cross sections for thermal energies of kT=25 – 90 keV. But also: Stellar enhancement factors stellar b-decay rates • Problems: • small cross sections • resonance dominated • contributions from direct capture 62Ni(n,g)63Ni • propagation effects previous:12.5 mb new: 28.4 mb • Methods for (n,g) measurements: • TOF • Activation Michael Heil International School of Nuclear Physics, Erice, September 2006

  6. Activation technique at kT=25 keV • Neutron production via 7Li(p,n) reaction at a proton energy of 1991 keV. Induced activity can be measured after irradiation with HPGe detectors. HPGe detector • Only possible when product nucleus is radioactive • High sensitivity -> small sample masses or small cross sections • Use of natural samples possible, no enriched sample necessary • Direct capture component included Lead shield Michael Heil International School of Nuclear Physics, Erice, September 2006

  7. Results - neutron capture cross sections Michael Heil International School of Nuclear Physics, Erice, September 2006

  8. Comparison Activation - TOF Mass Number • “old” TOF measurements seem to overestimate the cross sections of light nuclei. • Larger uncertainties then quoted. Michael Heil International School of Nuclear Physics, Erice, September 2006

  9. Background due to elastic scattering • Old measurements possibly suffer from underestimation of background from scattered neutrons. PM C6D6 neutrons Michael Heil International School of Nuclear Physics, Erice, September 2006

  10. Results – weak s-process abundances Effect for a 25 M⊙ star end of He core burning end of carbon shell burning effect of new 64Ni cross section previous: 10.7 mbarn now: 7.2 ± 0.4 Stellar model calculations performed by Marco Pignatari Michael Heil International School of Nuclear Physics, Erice, September 2006

  11. Neutron poisons Most important neutron poisons are: 16O(n,g), 12C(n,g), 23Na(n,g), 25Mg(n,g)… 16O(n,g)17O * 0.9 Measurement of 23Na(n,g)24Na with activation method Michael Heil International School of Nuclear Physics, Erice, September 2006

  12. Outlook • Disadvantage of activation method: • limited to few stellar temperatures • kT=5 keV: 18O(p,n)18F • kT=25 keV: 7Li(p,n)7Be • kT=52 keV: 3H(p,n)3He For carbon shell burning at kT=90 keV extrapolation is necessary. Solution: High resolution ToF measurements Planned at n_TOF in 2007: Fe, Ni, Zn, and 63Ni Michael Heil International School of Nuclear Physics, Erice, September 2006

  13. Conclusions • Many neutron capture cross sections for light and medium mass nuclei are not known with sufficient accuracy. • We have measured the MACS of several light and medium mass nuclei. • Old TOF measurements seem to systematically overestimate the cross sections. • Neutron capture cross sections of all involved isotopes are necessary because of propagation effect. • Neutron capture cross sections of neutron poisons are also important. • High precision TOF measurements are needed to cover full energy range of weak s-process. Michael Heil International School of Nuclear Physics, Erice, September 2006

  14. The neutron generator at Frankfurt Design by Prof. Ratzinger, Prof. Schempp, and P. C. Chau Proton source max. 200 mA ~120 keV/u ~40 ns ~250 kHz RFQ ~175 MHz ~2 MeV/u Chopper Dipol-Magnet 7Li-Target 7Li(p,n)7Be Cavity ~175 MHz ± 0.3 MeV Solenoids Sample g-ray Detector 150 kV Terminal Neutron beam Michael Heil International School of Nuclear Physics, Erice, September 2006

  15. Comparison with other neutron sources The Frankfurt neutron source will provide the highest neutron flux in the astrophysically relevant keV region (1 – 500 keV) worldwide. *Integrated flux between 1 keV and 100 keV Michael Heil International School of Nuclear Physics, Erice, September 2006

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