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the s process: messages from stellar He burning

the s process: messages from stellar He burning. astrophysical concepts cross sections and abundances problems and prospects. from Fe to U: s- and r-process. p-Region. Häufigkeit. Massenzahl. supernovae (r-process). Red Giants (s-process).

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the s process: messages from stellar He burning

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  1. the s process: messages from stellar He burning • astrophysical concepts • cross sections and abundances • problems and prospects

  2. from Fe to U: s- and r-process p-Region Häufigkeit Massenzahl supernovae (r-process) Red Giants (s-process) s-abundance x cross section = N s = constant

  3. massive stars M> 10 M⊙ neutron source: 22Ne(a,n) core helium burning T ~ 2-3·108 K, nn ~1·106 cm-3 shell carbon burning T ~1·109 K, nn ~1·1011 cm-3 s-process contributions to the heavy elements thermally pulsing low mass AGB stars of 1<M/M⊙<3 neutron sources: 13C(a,n), 22Ne(a,n) T ~ 1-3·108 K, nn ~ 4 ·108 cm-3 main s process 90<A<209 s process weak s process A<90 • reliable abundances through advanceds-process models • data needs: (n,g) cross sections, b-decay rates

  4. low mass AGB stars – the main s component

  5. Nr = N - Ns log ABUNDANCE r- ABUNDANCE observed scaled solar system MASS NUMBER ATOMIC NUMBER r-process abundances

  6. p process Gd 154 152 157 155 156 150 Eu Sm 151 153 151 s process 152 154 r process main component: the branching at 151Sm ingredients: - s-only isotopes in total reaction flow and in branches - unstable branch point isotopes - sN = constant 151Sm: lab half-life of 93 yr reduced to t1/2 = 3 yr at s-process site info on s-process temperature! 151 152 154

  7. weakcomponent: the bottle neck example of 62Ni(n,g) sN ≠ const. s-process efficiency determined by single cross sections

  8. Maxwellian averaged cross sections required • measures(En) by time of flight, 0.3 < En < 300 keV, • determine average for stellar spectrum • correct for SEF • produce thermal spectrum in laboratory, • measure stellar average directly by activation • correct for SEF

  9. (n,g) cross sections: status and challenges even-even nuclei sstar/slab • neutron magic nuclei • unstable branch point isotopes • A < 120

  10. main s process:MACS for mass range 90 < A < 209, kT= 5 – 25 keV s-only isotopes, branchings (incl. unstable branch points), neutron magic bottle necks high accuracy required samples of unstable isotopes difficult to produce experimental challenges open problems weak s process: MACS for mass range A<120, kT=25 – 90 keV seed nuclei, s-only isotopes, neutron poisons small cross sections resonance dominated contributions from direct capture

  11. possible solutions higher neutron flux: spallation sources (up to 300 n/p at 20 GeV proton energy) intense low energy accelerators (Spiral 2, NCAP, …) advanced detection techniques: segmented calorimeter type detectors, new scintillators data acquisition with fast flash ADC combination with AMS sample production: RIB facilities, spallation targets

  12. PS213 n_TOF Collaboration high flux spallation sources since 1987 since 2001 0.8 proton energy (GeV) 24 20 repetition rate (Hz) 0.4 250 pulse width (ns) 5 20 flight path (m) 185 200 average proton current (mA) 2 20 neutrons per proton 760 wide neutron energy range from thermal to 250 MeV

  13. n 10 times higher sensitivity enables measurements of mg samples advanced detection techniques • high detection efficiency: ≈100% • good energy resolution • 40 BaF2 crystals • 12 pentagons & 28 hexagons • 15 cm crystal thickness • Carbon-fibre 10B-enriched capsules • full Monte Carlo simulations • all EM cascades • capture events for BG determination

  14. TOF measurements on unstable samples of 1015 atoms (<1 mg) and half-lives of t1/2> 10 d possible n-beam p-beam Pb samples can be made with future RIB facilities such as GSI neutron target sample a step further: NCAP enhancement of sensitivity in TOF measurements by low energy accelerator with 1000 times higher beam current average current 1 mA, pulse width of ~1 ns, repetition rate 250 kHz

  15. important for quantitative picture of stellar s process and galactic chemical evolution summary • numerous remaining quests for accurate (n,g) cross sections • .... s process branchings, grains, massive stars, ... • present facilities and detectors suited for stable isotopes • improved neutron sources and RIB facilities needed for • radioactive samples • ... s process and explosive nucleosynthesis ... new options by AMS

  16. Neutrons Fusion BB abundances beyond Fe– ashes of stellar burning Fe H 30 000 C 10 Fe 1 Au 2 10-7 abundance s r s r mass number

  17. sources of abundance information

  18. element abundances in the solar system - meteoritic versus photospheric data

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