the s process: messages from stellar He burning

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the s process messages from stellar He burning

• astrophysical concepts
• cross sections and abundances
• problems and prospects

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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
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s-process contributions
to the heavy elements
thermally pulsing low mass AGB stars of
1ltM/M?lt3 neutron sources 13C(a,n),
22Ne(a,n) T 1-3108 K, nn 4 108 cm-3
main s process 90ltAlt209
s process
weak s process Alt90

• reliable abundances through advanced s-process
  models
• data needs (n,g) cross sections, b-decay rates

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low mass AGB stars the main s component
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r-process abundances
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main component the branching at 151Sm
ingredients - s-only isotopes in total
reaction flow and in branches
- unstable branch point isotopes
- sN constant
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
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weak component the bottle neck example of
62Ni(n,g)
sN ? const. s-process efficiency determined
by single cross sections
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Maxwellian averaged cross sections required

• measure s(En) by time of flight, 0.3 lt En lt 300
  keV,
• determine average for stellar
  spectrum
• correct for SEF
• produce thermal spectrum in laboratory,
• measure stellar average directly
  by activation
• correct for SEF

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(n,g) cross sections status and challenges

• neutron magic nuclei
• unstable branch point isotopes
• A lt 120

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open problems
weak s process MACS for mass range Alt120, kT25
90 keV seed nuclei,
s-only isotopes, neutron poisons
small cross sections resonance
dominated contributions from direct capture
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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
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high flux spallation sources

since 1987
since 2001
0.8 proton energy
(GeV) 24 20
repetition rate (Hz) 0.4
250 pulse width (ns)
5 20
flight path (m)
185 200 average proton
current (mA) 2 20
neutrons per proton 760
wide neutron energy range from thermal to 250 MeV
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advanced detection techniques

• high detection efficiency 100

• 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

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a step further NCAP
enhancement of sensitivity in TOF
measurements by low energy accelerator
with 1000 times higher beam current

average current 1 mA, pulse width of 1 ns,
repetition rate 250 kHz
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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
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abundances beyond Fe ashes of stellar burning
Fe
H 30 000 C 10 Fe 1 Au 2 10-7
abundance
s
r
s
r
mass number
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sources of abundance information
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element abundances in the solar system -
meteoritic versus photospheric data