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

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

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

low mass AGB stars – the main s component

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

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

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

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

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

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

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

high flux spallation sources PS213 n_TOF Collaboration 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

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

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

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

abundances beyond Fe– ashes of stellar burning Neutrons Fusion BB Fe H 30 000 C 10 Fe 1 Au 2 10-7 abundance s r s r mass number

sources of abundance information

element abundances in the solar system - meteoritic versus photospheric data