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scenarios status and challenges new developments

neutron reactions in astrophysics – status and perspectives. scenarios status and challenges new developments. I. Introductory remarks and present status II. Laboratory experiments and astrophysics III. Future options. big bang stellar He burning

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scenarios status and challenges new developments

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  1. neutron reactions in astrophysics – status and perspectives scenariosstatus and challengesnew developments I. Introductory remarks and present status II. Laboratory experiments and astrophysics III. Future options

  2. big bang stellar He burning s process in TP-AGB and in massive stars explosive nucleosynthesis p and r processes neutron capture scenarios neutron capture accounts for 75% of the stable isotopes, but only for about 0.005% of the total post BB abundances

  3. Maxwellian averaged cross sections required • measures(En) by time of flight, 0.3 < En < 500 keV, • determine average for stellar spectrum • correct for SEF • high accuracy, wide energy range • produce thermal spectrum in laboratory, • measure stellar average directly by activation • correct for SEF • very high sensitivity

  4. detection of neutron capture events prompt g-rays+ TOF-method (n,g): *Moxon-Rae eg ~1% *PH-weighting~20% *Ge, NaI< 1% singleg´s allg´s*4p BaF2 ~100% activation in quasi-stellar spectrum most sensitive * small cross sections, 1014atoms! selective * natural samples or low enrichment

  5. even-even nuclei compilation of stellar (n,g)cross sections Bao &Käppeler 1987 Beer, Voss & Winters 1992 Bao et al. 2000 • collectexperimental data, • renormalize, • calculate MACS, • recommend • based on educated choices • by experienced experimentalist • complement by theory (SEF) KADONIS current update by Dillmann & Plag: http://nuclear-astrophysics.fzk.de/kadonis/

  6. status of stellar (n,g)cross sections s process: Ds/s = 1-3% p and r process: Ds/s~ 5% what do we need? nuclear input must be good enough that it doesn‘t punch through to calculated abundances! what do we have? beware: discrepancies often larger than uncertainties!!!

  7. proton beam neutron cone lithium Au/14C/Au activation in quasi-stellar spectrum most s(n,g) of unstable nuclei measured this way: 14C(n,g)15C • neutron source7Li(p,n)7Be • neutron flux197Au(n,g)198Au • 15Cdetected via 5.3 MeV line • (t1/2=2.45 s) • half-life limits • 0.1 s <t1/2 < 10 yr with g-spec • no limit with AMS! • sample properties • >1014 atoms • impurities acceptable

  8. activation in quasi-stellar spectrum • possible neutron sources: • 7Li(p, n)7Be kT=25 keV  2·109neutrons/s, 100 mA • 3H(p, n)3He 52 keV  1·108 “ “ • 18O(p, n)18F  5 keV  2·105 “ “ higher beam currents needed for - activations at low energies - long-lived product nuclei - studies of double neutron captures higher beam currents require new target technology!

  9. complete info: s(En) via TOF method& folding with stellar spectrum larger samples * limited sensitivity optimal efficiency higher flux limited selectivity enriched samples ** * not desirable and even excluded for unstable samples ** mandatory

  10. collimated n-beam g g g g p-beam neutron target Pb sample optimal efficiency : 4pBaF2 array eg>90% up to 10 MeVecasc >98% DE/E = 6% at 6 MeV clear signatures Dt= 500 ps good TOF resolution FZK now also at Los Alamos and CERN

  11. PS213 n_TOF Collaboration high neutron fluxes :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

  12. still higher fluxes in future • J-PARC spallation source • similar features than LANSCE, but 50 times more flux • LANSCE improved by factor of 10 – 20 by upgrade of LAMPF • n_TOF @CERN improved by factor of 100 by shorter flight path • Low energy proton accelerators with beam currents of up to 200 mA • (Soreq Nucl. Research Center, Univ. of Frankfurt/M)

  13. sprocess r and p process future branch point status • (n,g) cross sections for a variety • of selected unstable isotopes • (r : 60Fe, 106Ru, 126Sn, 182Hf... • (p : 91,92Nb, 97,98Tc...) • for direct use in reaction networks • to derive rates of inverse reactions • to test and assist statistical models 63Ni 79Se 81Kr 85Kr 147Nd 147Pm 148Pm 151Sm 154Eu 155Eu 153Gd 160Tb 163Ho 170Tm 171Tm 179Ta 185W 204Tl + 59Fe, 125Sn, 181Hf…. unstable samples: now and then

  14. important for quantitative picture of galactic chemical evolution summary • numerous remaining quests for s process (branchings, grains, massive stars) • and many more for explosive nucleosynthesis • present facilities and detectors suited for most stable isotopes • new approaches required for radioactive samples • spallation sources, new low energy accelerators, and RIB facilities • promising, both for stellar and explosive nucleosynthesis

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