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scenarios status and challenges new developments

This article discusses the current status, challenges, and future developments in neutron reactions in astrophysics, including laboratory experiments, big bang nucleosynthesis, stellar processes, and explosive nucleosynthesis. It explores neutron capture scenarios, cross-section measurements, and the need for accurate nuclear input. The text provides insights into experimental techniques, data compilation, and the importance of understanding unstable isotopes for galactic chemical evolution.

jamieconner
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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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