Slow neutron captures in stars F. Käppeler, KIT Karlsruhe weak and main s process abundances determined by (n, g) cross sections (n, g) reactions experiment and theory variety of types: 187Os, 22Ne, 60Fe Maxwellian averages – laboratory data and thermal corrections Dust in Eurogenesis environments, Perugia, Nov 11-14, 2012

the observed abundances – ashes of stellar burning and of SN BB Fusion Neutrons s process 22Ne 60Fe 187Os r process Fe abundance s r mass number neutrons produce 75% of the stable isotopes

from Fe to U: s- and r-process p-Region Häufigkeit Massenzahl supernovae (r-process) Red Giants (s-process)

Maxwellian averaged cross sections measure s(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 accurate experimental cross section data essential

status of stellar (n,g) cross sections what do we need? s process: Ds/s = 1-3% nuclear input must be good enough that uncertainties don‘t dominate calculated abundances! beware: - discrepancies often larger than uncertainties! experimental data often incomplete theory needed for thermal effects what do we have? Point out that nuclear uncertainties must be small enough that they no longer determine the abundance uncertainties from models; only then, observations can be used to constrain models!!!

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

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

the s process in low mass stars (1-3 M) s abundances from 90Zr – 209Bi: the main component H shell burning 13C(a,n) kT~8 keV T~90 MK nn= 107-108 cm-3 He flash 22Ne(a,n) kT~25 keV T~250 MK nn= 1010-1011 cm-3 abundances anti-correlated with cross sections: sNs = const detailed models for realistic description of stellar evolution reaction flow in equilibrium

Case 1: 187Os (n,g) BANG! ? 4.5 Gyr Now solar system galaxies s-only 0.02 Os 185 94 d Os 186 1.58 Os 187 1.6 Os 188 13.3 Os 189 16.1 Os 190 26.4 Os 191 15.4 d Os 192 41.0 Re Re 183 71 d Re 184 38 d Re 185 37.4 Re 186 90.64 h Re 187 62.6 Re 188 16.98 h Re 189 24.3 h Re 190 3.1 m 42.3x109 a W W 182 26.3 W 183 14.3 W 184 30.67 W 185 75.1 d W 186 28.6 W 187 23.8 h W 188 69 d

Os (n, g) cross sections measured at n_TOF/CERN 186Os (2 g, 79 %) 187Os (2 g, 70 %) 188Os (2 g, 95 %) Al can environmental background 197Au (1.2g) flux normalization (using Ratynski and Macklin high accuracy cross section data) natPb (2 g) in-beam gamma background natC (0.5 g) neutron scattering background Neutron beam

(n,g) cross sections

thermal population of nuclear states 186Os 187Os 188Os in 187Os at kT = 30 keV: P(gs) = 33% P(1st) = 47% P(all others) = 20% stellar enhancement factor SEF = s* / sexp

stellar 187Os(n,g) cross section: SEF Hauser-Feshbach statistical model: neutron transmission coefficients, Tn : from OMP calculations g-ray transmission coefficients, Tg : from GDR (experimental parameters) nuclear level densities: fixed at the neutron binding from <D>exp SEF ± 2-3% stellar correction factor Fσ = f186 / f187 kT ‹σ187›lab ‹σ187›calc ‹σ187›* f187 Fσ (keV) (mbarn) (mbarn) (mbarn) 10 1988 2111 2324 1.10 0.91 20 1171 1193 1402 1.18 0.85 30 874 876 1059 1.21 0.86 40 715 712 877 1.23 0.89 50 614 610 766 1.26 0.93 all these parameters can be derived and fixed from the analysis of experimental data at low-energy

the s-process abundance distribution

Case 2: 22Ne(n,g) light nuclei low level densities, HF not valid neutron poisons & grains 2 TOF measurements, faint resonances at 266, 304, 422 keV activation method more sensitive (kT=25 and 52 keV) thermal cross section important

22Ne(n,g) by activation quasi-stellarspectra for kT=25 keV via 7Li(p, n) 7Be 52 keV via 3H(p, n)3He 5 kev via 18O(p, n)18F 7Li(p, n)7Be: highest sensitivity small samples, small cross sections

activation measurement at kT=25 keV 22Ne + natKr t1/2 (23Ne) = 38 s high pressure gas cells (loaded with 30 to 100 bar) enriched 22Ne gas (98.87%) with natKr, 200 mg each measurement relative to 80,82,84Kr HPGe detector and pneumatic slide

no or few resonances DRC dominant - p-wave normalized to fit keV data Direct Radiative Capture (calculated) - s-wave normalized at thermal kT (keV) MACS (mb) old new 5 133 109 25 62 56 50 61 49 100 95 54 ? MACS ±5-10% ±7-20% s-wave p-wave no SEF required

the s process in massive stars s abundances from 56Fe – 89Y: the weak component He core burning 22Ne(a,n) kT~25 keV T~300 MK nn= 106 cm-3 C shell burning 22Ne(a,n) kT~90 keV T~109 K nn=1011- 1012 cm-3 reaction flow NOT saturated propagation waves! weak s process complicated by small and resonance dominated cross sections contributions from direct capture, SEF?

intermediate-mass nuclei theory very uncertain, HF approach questionable experimental data incomplete sample material unstable (t1/2 = 2.6 Myr) total sample mass 1.4 mg sample contains all stable Fe isotopes and 150 MBq of 55Fe (t1/2 = 2.7 yr) the special case of 60Fe

60Fe in interstellar space 19 HPGe Detectors, W coded mask 3º resolution, 16ºx16º field of view Eγ= 15 keV – 8 MeV, 2.5 keV@1 MeV Diehl, NAR 50 (2006) 534 Wang et al. A&A 469 (2007) 1005 60Fe, inner Galaxy

60Fe in deep-sea manganese crust growth rate via 10Be (1.5 Myr) (mm/Myr) archived period  20 Myr Knie et al. PRL 93, 2004

60Fe from s process in massive stars t1/2 = 6 min Eg = 298, 1027, 1205 keV Ig = 22, 43, 44(5)% 56Fe 91.7 57Fe 2.2 58Fe 0.28 59Fe 44.5 d 60Fe 2.6 Myr 61Fe 6.0 m world supply of 60Fe: extracted from Cu beam dump at PSI 1.3·1016 atoms (1.4 mg) on thin carbon disk (6 mm diam.) active impurities 55Fe (100 MBq), ingrowth of 60Co no experimental data for 60Fe(n, g) theoretical estimates ranging from 1 to 20 mbarn first measurements at VdG Karlsruhe and TRIGA reactor Mainz by activation

search for g-decay of 61Fe 61Co cascade transitions (298 & 1027 keV) 1205 keV single transitions 70 mm sample 118 single, 17 cascade events 1.7·1014 neutrons

cross section results ‹σ› = 5.7 ± 1.6stat ± 0.8syst mbarn Karlsruhe: kT=25 keV ‹σ› = 5.7 ± 1.6stat ± 0.8syst mbarn ± 30% Mainz: thermal σth = 203 ± 21stat ± 24syst mbarn ± 16% 2114 824 0+ 2+ 4+ 60Fe DC component small (<10%) normalization of HF calculations SEF = 1.0

how much 60Fe per Supernova? production mostly before SN-explosion Chieffi & Limongi ApJ 647, 2006

propagation waves: the example of 62Ni cross sections near Fe seed have strong effect on abundances of weak s process 62Ni(n,g)63Ni 12.5 35 22.6 mass number abundance ratio reaction flow not in equilibrium stellar (n, g) cross sections (mb): TOF 25.8 ± 3.7 (2008) 37.0 ± 3.2 (2005) 12.5 ± 4.0 (1983) 26.8 ± 5.0 (1975) activation 20.2 ± 2.1 (2009) 23.4 ± 4.6 (2008) 26.1 ± 2.6 (2005)

data for the main component Zr – Pb/Bi; kT= 8 and 23 keV measured data for stable nuclei available Hauser-Feshbach applicable in most cases thermal corrections small, can be handled if experimental information complete Problems: gaps in data base, esp. for SEF neutron poisons unstable branch point isotopes

data for the weak component Fe – Sr/Y ; kT= 26 and 90 keV experimental data incomplete Hauser-Feshbach questionable thermal corrections uncertain, esp. at kT=90 keV Problems: large gaps in data for stable nuclei SEF determination uncertain neutron poisons unstable branch point isotopes error propagation

summary stellar (n, g) cross sections need further improvement - assessment of weak s process in massive stars (TOF data) - better accuracy of many key cross sections of main s process - touching the region of unstable, neutron-rich isotopes 60Fe: example for required sensitivity in case of - small cross sections and - rare unstable samples 60Fe stands for a variety of rare & radioactive samples that can be studied with new, advanced facilities such as n_TOF-2, FRANZ, SARAF