1. the s process: messages from stellar He burning
astrophysical concepts
cross sections and abundances
problems and prospects

2. from Fe to U: s- and r-process
p-Region
Häufigkeit
Massenzahl
supernovae
(r-process)
Red Giants
(s-process)
s-abundance x cross section = N s = constant

3. s-process contributions to the heavy elements
thermally pulsing low mass AGB stars of 1<M/M⊙<3
neutron sources: 13C(a,n), 22Ne(a,n)
T ~ 1-3·108 K, nn ~ 4 ·108 cm-3
main s process
90<A<209
s process
massive stars M> 10 M⊙
neutron source: 22Ne(a,n)
core helium burning
T ~ 2-3·108 K, nn ~1·106 cm-3
shell carbon burning
T ~1·109 K, nn ~1·1011 cm-3
weak s process
A<90
reliable abundances through advanced s-process models
data needs: (n,g) cross sections, b-decay rates

4. low mass AGB stars – the main s component

5. r-process abundances Nr = N - Ns log ABUNDANCE r- ABUNDANCE
MASS NUMBER
r- ABUNDANCE
Nr = N - Ns
ATOMIC NUMBER
log ABUNDANCE
observed
scaled solar system

6. main component: the branching at 151Sm
ingredients: s-only isotopes in total reaction flow and in branches
- unstable branch point isotopes
- sN = constant Sm
153
151
152
155 Eu Gd
p process
s process
r process
150
154
156
157
151Sm:
lab half-life of 93 yr
reduced to t1/2 = 3 yr
at s-process site
info on s-process
temperature!
151
152
154

7. weak component: the bottle neck example of 62Ni(n,g)
sN ≠ const.
s-process efficiency
determined by single
cross sections

8. Maxwellian averaged cross sections required
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

9. (n,g) cross sections: status and challenges
even-even nuclei
sstar/slab
neutron magic nuclei
unstable branch point isotopes
A < 120

10. open problems
weak s process: MACS for mass range A<120, kT=25 – 90 keV
seed nuclei, s-only isotopes, neutron poisons
small cross sections
resonance dominated
contributions from direct capture
main s process: MACS for mass range 90 < A < 209, kT= 5 – 25 keV
s-only isotopes, branchings (incl. unstable branch points),
neutron magic bottle necks
high accuracy required
samples of unstable isotopes difficult to produce
experimental challenges

11. possible solutions higher neutron flux: spallation sources
(up to 300 n/p at 20 GeV proton energy)
intense low energy accelerators
(Spiral 2, NCAP, …)
advanced detection techniques:
segmented calorimeter type detectors,
new scintillators
data acquisition with fast flash ADC
combination with AMS
sample production:
RIB facilities, spallation targets

12. high flux spallation sources
PS213
n_TOF Collaboration
since 1987
since 2001
proton energy (GeV)
repetition rate (Hz)
pulse width (ns)
flight path (m)
average proton current (mA)
neutrons per proton
wide neutron energy range from thermal to 250 MeV

13. advanced detection techniques
high detection efficiency: ≈100% n
good energy resolution
40 BaF2 crystals
12 pentagons & 28 hexagons
15 cm crystal thickness
Carbon-fibre 10B-enriched capsules
full Monte Carlo simulations
all EM cascades
capture events for BG determination
10 times higher sensitivity
enables measurements of mg samples

14. a step further: NCAP enhancement of sensitivity
in TOF measurements by
low energy accelerator with
1000 times higher beam current
sample Pb
neutron target
p-beam
n-beam
average current 1 mA, pulse width of ~1 ns, repetition rate 250 kHz
TOF measurements on unstable
samples of 1015 atoms (<1 mg) and
half-lives of t1/2> 10 d possible
10% of statistics
samples can be made with future
RIB facilities such as GSI

15. summary numerous remaining quests for accurate (n,g) cross sections
.... s process branchings, grains, massive stars, ...
present facilities and detectors suited for stable isotopes
improved neutron sources and RIB facilities needed for
radioactive samples
... s process and explosive nucleosynthesis
... new options by AMS
important for quantitative picture of stellar
s process and galactic chemical evolution

16. abundances beyond Fe– ashes of stellar burning
Neutrons
Fusion BB Fe H C Fe Au
abundance s r s r
mass number

17. sources of abundance information

18. element abundances in the solar system - meteoritic versus photospheric data