1. 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

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

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

4. 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

5. 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!!!

6. 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

7. 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 BaF ~100%
activation in quasi-stellar spectrum
most sensitive * small cross sections,
1014 atoms sufficient
selective * natural samples or low enrichment

8. 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= cm-3
He flash
22Ne(a,n)
kT~25 keV
T~250 MK
nn= cm-3
abundances anti-correlated with cross sections: sNs = const
detailed models for realistic description of stellar evolution
reaction flow in equilibrium

9. 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

10. 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

11. (n,g) cross sections

12. 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

13. 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›* f Fσ
(keV) (mbarn) (mbarn) (mbarn)
all these parameters can be derived and fixed from the analysis of experimental data at low-energy

14. the s-process abundance distribution

15. 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

16. 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

17. 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

18. 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 ?
MACS ±5-10%
±7-20%
s-wave
p-wave
no SEF required

19. 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= cm-3
reaction flow NOT saturated propagation waves!
weak s process complicated by
small and resonance dominated cross sections
contributions from direct capture, SEF?

20. 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

21. 60Fe in interstellar space
19 HPGe Detectors, W coded mask 3º resolution, 16ºx16º field of view Eγ= 15 keV – 8 MeV, 2.5 MeV
Diehl, NAR 50 (2006) 534
Wang et al. A&A 469 (2007) 1005
60Fe, inner Galaxy

22. 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

23. 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

24. 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

25. 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

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

27. 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)

28. 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

29. 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

30. 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