Slow neutron captures in stars

weak and main s process • abundances determined by (n, ) cross sections

(n, ) reactions • experiment and theory• variety of types: 187Os, 22Ne, 60Fe• Maxwellian averages – laboratory data and thermal

corrections

F. Käppeler, KIT Karlsruhe

Dust in Eurogenesis environments, Perugia, Nov 11-14, 2012

0 50 100 150 200MASS NUMBER

10-2

10-1

100

101

102

103

104

105

106

107

108

109

1010

AB

UN

DA

NC

E

(Si =

106 )

the observed abundances – the observed abundances – ashes of stellar burning and of SNashes of stellar burning and of SN

Neutrons FusionBB

Fe

sr

neutrons produce 75% of the stable isotopes

s process

mass number

ab

un

dan

ce

r process22Ne

60Fe

187Os

from Fe to U: from Fe to U: ss- and - and rr-process-process

p-Region

Red Giants(s-process)

supernova

e(r-p

rocess)

Massenzahl

H

äufi

gkei

t

Maxwellian averaged cross sectionsMaxwellian averaged cross sections

measure (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

accurate experimental cross section data essential

status of stellar (n,) cross sections

s process: = 1-3%

what do we have?

nuclear input must be good enough that uncertainties don‘t dominate calculated abundances!

what do we need?

beware:

- discrepancies often larger than uncertainties!

- experimental data often incomplete

-theory needed for thermal effects

what about theorywhat about theory?? 176Hf, 178Hf, 180Hf:

MACS uncertainties

1 - 2%

exercise joined by 6 leading groups:

calculate MACS of174Hf and 182Hf

prior to measurement

BUT: theory indispensible for stellar corrections

prompt -rays + TOF-method

measurement of neutron capture data

* Moxon-Rae ~ 1% * PH-weighting ~ 20% * Ge, NaI < 1%

single ´s

all cascade ´s * 4BaF2 ~100%

(n,):

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 burning13C(,n)

kT~8 keV

T~90 MK

nn= 107-108 cm-3

He flash22Ne(,n)

kT~25 keV

T~250 MK

nn= 1010-1011 cm-3

abundances anti-correlated with cross sections: Ns = const detailed models for realistic description of stellar evolution

reaction flow in equilibrium

Case 1: Case 1: 187187Os (n,Os (n,))

W W 182 26.3

Re

Os

Re 183 71 d

Os 184 0.02

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

Re 184 38 d

Re 185 37.4

Re 186 90.64 h

Re 187 62.6

42.3x109 a

Re 188 16.98 h

Re 189 24.3 h

Re 190 3.1 m

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

Now4.5 Gyr?BANG!

galax

ies

solar

syste

m

s-only

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

Os (n, ) cross sections measured at n_TOF/CERN

(n,) cross sections

02.042.0)187(

)186(

thermal population of nuclear states

in 187Os at kT = 30 keV:

P(gs) = 33%P(1st) = 47%P(all others) = 20%

stellar enhancement factorSEF = / exp

m

kTEm

kTEk

k m

k

eJ

eJEP /

/

)12(

)12()(

186Os 187Os 188Os

stellar 187Os(n,) cross section: SEF

JJ J

lslsn

lslsn

Jls

lsn

Jn

nn WTTT

TTg

kE ,

,,',

,,

2, )(

Hauser-Feshbach statistical model:

• neutron transmission coefficients, Tn :from OMP calculations

• -ray transmission coefficients, T:from GDR (experimental parameters)

• nuclear level densities:fixed at the neutron binding from <D>exp

all these parameters can be derived all these parameters can be derived and fixed from the analysis of and fixed from the analysis of experimental data at low-energyexperimental data at low-energy

kTkT ‹‹σσ187187››lablab ‹ ‹σσ187187››calccalc ‹ ‹σσ187187››** ff187 187 FFσσ

(keV) (mbarn) (mbarn) (mbarn) (keV) (mbarn) (mbarn) (mbarn)

10 1988 2111 2324 10 1988 2111 2324 1.101.10 0.91 0.91

20 1171 1193 1402 20 1171 1193 1402 1.181.18 0.85 0.85

30 874 876 1059 30 874 876 1059 1.211.21 0.86 0.86

40 715 712 877 40 715 712 877 1.231.23 0.89 0.89

50 614 610 766 50 614 610 766 1.26 1.26 0.93 0.93

stellar correction factor Fσ = f186 / f187

SEF ± 2-3%

the s-process abundance distributionthe s-process abundance distribution

Case 2: 22Ne(n,)

2 TOF measurements, faint resonances at 266, 304, 422 keV activation method more sensitive (kT=25 and 52 keV) thermal cross section important

light nuclei low level densities, HF not valid neutron poisons & grains

22Ne(n,) by activation

7Li(p, n)7Be: highest sensitivity small samples, small cross sections

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

activation measurement at kT=25 keV

22Ne+

natKr

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

t1/2 (23Ne) = 38 s

no or few resonances DRC dominant

Direct Radiative Capture (calculated) - s-wave normalized at thermal

- p-wave normalized to fit keV data

kT (keV) MACS (b)old new

5 133 109 25 62 56 50 61 49100 95 54

±7-20%

MACS ±5-10%

no SEF required

s-wave?

p-wave

the s process in massive stars s abundances from 56Fe – 89Y: the weak component

He core burning22Ne(,n)

kT~25 keV

T~300 MK

nn= 106 cm-3

reaction flow NOT saturated propagation waves!

C shell burning22Ne(,n)

kT~90 keV

T~109 K

nn=1011- 1012 cm-3

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 g

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) 534Wang 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

0,0E+00

5,0E-16

1,0E-15

1,5E-15

2,0E-15

2,5E-15

3,0E-15

0 2 4 6 8 10 12 14

age [Myr]

60

Fe/F

e_

me

as

ure

d

Knie et al. PRL 93, 2004

60Fe from s process in massive stars

no experimental data for 60Fe(n, )theoretical estimates ranging from 1 to 20 mbarn

58Fe 0.28

59Fe 44.5 d

60Fe2.6 Myr

57Fe 2.2

56Fe 91.7

61Fe 6.0 m

first measurements at VdG Karlsruhe and TRIGA reactor Mainz by activation

t1/2 = 6 minE = 298, 1027, 1205 keVI = 22, 43, 44(5)%

world supply of 60Fe:

extracted from Cu beam dump at PSI

1.3·1016 atoms (1.4 g) on thin carbon disk (6 mm diam.)

active impurities 55Fe (100 MBq), ingrowth of 60Co

search for -decay of 61Fe

61Co

118 single, 17 cascade events

1.7·1014 neutrons

1205 keV

single transitions

70 mm

sample

cascade transitions (298 & 1027 keV)

cross section results

Karlsruhe: kT=25 keV

‹σ› = 5.7 ± 1.6stat ± 0.8syst mbarn

Mainz: thermal

σth = 203 ± 21stat ± 24syst mbarn

• DC component small (<10%)• normalization of HF calculations• SEF = 1.0

± 30%

± 16%

2114

824

0 0+

2+

4+

60Fe

how much 60Fe per Supernova?

Chieffi & Limongi ApJ 647, 2006

production mostly before SN-explosion

propagation waves: the example of 62Ni

stellar (n, ) cross sections (mb):

TOF25.8 ± 3.7 (2008)37.0 ± 3.2 (2005)12.5 ± 4.0 (1983)26.8 ± 5.0 (1975)

activation20.2 ± 2.1 (2009)23.4 ± 4.6 (2008)26.1 ± 2.6 (2005)

cross sections near Fe seed have strong effect on abundances of weak s process

62Ni(n,)63Ni

12.5

35

22.6

mass number

abun

danc

e ra

tio

reaction flow not in equilibrium

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 componentdata 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 nucleiSEF determination uncertain neutron poisons unstable branch point isotopes error propagation

60Fe: example for required sensitivity in case of - small cross sections and

- rare unstable samples

summary

60Fe stands for a variety of rare & radioactive samples that can be studied with new, advanced facilities such as

n_TOF-2, FRANZ, SARAF

stellar (n, ) 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