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neutron capture: from Fe to the actinides

neutron capture: from Fe to the actinides. I it all began not so long ago … an historical introduction II nucleosynthesis from the Big Bang until today III from Fe to the actinides by neutron capture reactions. nucleosynthesis. 4. observations. nucler physics. astro-theory.

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neutron capture: from Fe to the actinides

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  1. neutron capture: from Fe to the actinides I it all began not so longago… an historicalintroduction IInucleosynthesis from the Big Bang until today III from Fe to the actinides by neutron capture reactions

  2. nucleosynthesis 4 observations nucler physics astro-theory

  3. nuclearchartand isotope formation 286 stable isotopes, >3000 produced discovered by Thomson & Aston 1913 p-process, supernovae rp process novae, X-ray burster r-process, supernovae Fe s-process, Red Giants fusion, stellar energy BB

  4. from Fe to U: s- and r-process p-process abundance mass number supernovae (r-process) t ~ 1 ms Red Giants (s-process) t~ 1 yr s-abundance x cross section = N s = constant

  5. the r-process lb<< ln lgn~ lng abundances determined by t1/2 of waiting points

  6. waiting point approximation • reaction path defined by waiting points at Sn~2 MeV • waiting point abundances defined by:lbNr= const • final abundances modified by beta delayed neutron emission and (n,g)/(n,x) reactions • contribution to mass region 70 < A < 238

  7. nuclear physics for the r-process • explosive nucleosynthesis at ~1G K (kT~100 keV) -b-decayrates near neutron drip line - (n, g) during freeze-out exotic radioactive nuclei • r-process models uncertain - supernova and/or neutron star merger - nuclear data for reaction network main input: mostly theoretical, great challenge!

  8. physics of the s process • He burning at 100 – 300 MK (kT=10 – 30 keV) - abundances anti-correlated with (n,g) cross sections - specific abundance signatures in branchings - direct evidence via stellar spectroscopy (IR, visible, UV, X- and g-rays) and presolar grains • detailed models of stellar processes available - reliable nuclear reaction network important main input experimental (n,g) cross sections and b-decay rates

  9. Maxwellian averaged cross sections • 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

  10. MACS30 data of even-even nuclei

  11. thermal population of nuclear states in 187Os at kT = 30 keV P(gs) = 33% P(1st) = 47% P(all others) = 20% alberto.mengoni@cern.ch

  12. effect of thermally populated nuclear states a) MACS values moderate changes SEF = stellar s / measured s Rauscher, ApJ 755 (2012) L10 b) b-decay rates changes can be many orders of magnitude Takahashi &Yokoi , ADNDT 36 (1987) 375 alberto.mengoni@cern.ch

  13. relevant cases Takahashi & Yokoi, ADNDT 36 (1987) 375

  14. needed: cross sections with uncertainties between 1 and 5% for complete set of isotopes from 12C to 210Po including unstable samples MACS30 - status and requests

  15. 176Hf, 178Hf, 180Hf: MACS uncertainties 1 - 2% what about theory? exercise joined by 6 leading groups: calculate MACS of 174Hf and 182Hf prior to measurement BUT: theory indispensible for stellar corrections

  16. classical s process Clayton 1968, Principles of Stellar Evolution and Nucleosynthesis free parameter exercise: Fe seed neutron exposure 20 7 54 100 exponential distribution of neutron exposures r(t) yields simple solution -1 1 (1 + ) st

  17. sN systematics two s processes: main component in low mass AGB stars weak component in massive stars Clayton 1968 r-only nuclei s-only nuclei ~1990 sN sN MASS NUMBER MASS NUMBER

  18. the s process in AGB stars (~1987) 13C pocket final abundance patterns via 22Ne(a,n)during He shell flash kT=23 keV 13C(a,n)source operates during H-burning phase kT=8 keV

  19. Nd Pr Ce 140 142 141 148 150 s process the rare earth elements – a test lab for the s-process p process Sm 147 148 149 150 N = 82 Pm 147 145 144 146 143 r process

  20. the s process around N=82: classical vs. stellar s process new s process significantly better than classical approach (Ba, Gd!!) BUT:142Nd?

  21. the overall abundance distribution

  22. p process Xe 128 130 129 25 I 127 min Te 10h 124 125 123 126 128 130 131 132 s process r process 122 130Xe: sN-normalization of solar Xe abundance solar Xe abundance 128Xe 262.5 ±3.7 129Xe617 ±12 130Xe 132.0 ±2.1 Nʘ(Xe) = 5.39 ± 0.14

  23. p process Xe 128 130 129 25 I 127 min Te 10h 124 125 123 126 128 130 131 132 s process r process 122 branchings at A=127/128 ? b-/ EC = 0.94 at T8=1 ...... but 0.99 at T8=3 independent of stellar neutron flux!! branching?: sN test: 8% of reaction flow is bypassing 128Xe: T8 <1 ?

  24. vconv (cm/s) branching at 128I – a test for fast mixing! tconv ‹1 h

  25. experimental cross sections essential • TOF measurements complemented by calculated SEF high accuracy, wide energy range • activation in quasi-stellar spectrum corrected for SEF very high sensitivity

  26. activation technique at kT=25 keV • neutron production via7Li(p,n)7Bereaction • induced activity measured with HPGe detectors only possible when product nucleus is radioactive  not applicable for most branch-point isotopes

  27. Facility Φn (104 cm-2s-1) DE/E (‰) n_TOF EAR1 0.4 1 – 2 GELINA (30 m) 1.4 1.3 n_TOF EAR2 8 10 – 20 LANSCE (20 m) 13 8 – 26 flux and resolution: 10 - 100 keV LiLiT/SARAF 107 in activation mode

  28. sample aspects: mass & radiation activation: done: 30 - 100 ng (1014 atoms) possible at LiLiT: 300 pg time-of-flight: typically: 0.5 - 5 mgseparated isotopes record: 173Lu with40 mg(~1017atoms) @ DANCE tied @ n_TOF: 7Be(n,a) with1.4 mg(~1017atoms)

  29. information from individual sources presolar grains from stellar outflows and supernova ejecta stellar and galactic evolution, nucleosynthesis, mixing in supernovae, composition of stellar atmospheres, and processes on meteoritic parent bodies high resolution spectroscopy of stellar atmospheres identification of isotope pattern via hyperfine structures of molecular bands - the basic link between stellar spectroscopy and nucleosynthesis models

  30. presolar material survived in chondrites

  31. 1987: the puzzling Ne-E x 20000 ABUNDANCE … I‘m full of 22Ne R. Gallino MASS NUMBER

  32. Konvektive Hülle abundant production of Ne 14N+ 4He = 18F → 18O CNO-cycle: CNO → 14N 18O + 4He = 22Ne H He transport by convective envelope C/O s-process

  33. shapes and sources SiC Red Giants, s-process Zʘ, ½Zʘ C 14N/15N massive stars, SN r-process C 12C/13C

  34. s & r -process signatures

  35. neutron density fluctuates branching at 85Kr indicates a broad range of effective neutron densities in individual stars

  36. more work needed main branchings 1 – 3 Mʘstars: <1010 n/cm3 • unstableisotopes arecrucialforunderstanding • stellar scenarioswith higher neutron densities: • massive stars…………………up to 1012 n/cm3 • low-metallicity stars…………. 1015 • supernovae, r-process……… >1020

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