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

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the s process: messages from stellar He burning

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The s process messages from stellar he burning

the s process: messages from stellar He burning

  • astrophysical concepts

  • cross sections and abundances

  • problems and prospects


The s process messages from stellar he burning

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

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

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

weak s process

A<90

  • reliable abundances through advanceds-process models

  • data needs: (n,g) cross sections, b-decay rates


Low mass agb stars the main s component

low mass AGB stars – the main s component


R process abundances

Nr = N - Ns

log ABUNDANCE

r- ABUNDANCE

observed

scaled solar system

MASS NUMBER

ATOMIC NUMBER

r-process abundances


Main component the branching at 151 sm

p process

Gd

154

152

157

155

156

150

Eu

Sm

151

153

151

s process

152

154

r process

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


Weak component the bottle neck example of 62 ni n g

weakcomponent: the bottle neck example of 62Ni(n,g)

sN ≠ const.

s-process efficiency

determined by single

cross sections


Maxwellian averaged cross sections required

Maxwellian averaged cross sections required

  • measures(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


The s process messages from stellar he burning

(n,g) cross sections: status and challenges

even-even nuclei

sstar/slab

  • neutron magic nuclei

  • unstable branch point isotopes

  • A < 120


Open problems

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

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


Possible solutions

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


The s process messages from stellar he burning

PS213

n_TOF Collaboration

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


The s process messages from stellar he burning

n

10 times higher sensitivity

enables measurements of mg samples

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


A step further ncap

TOF measurements on unstable

samples of 1015 atoms (<1 mg) and

half-lives of t1/2> 10 d possible

n-beam

p-beam

Pb

samples can be made with future

RIB facilities such as GSI

neutron target

sample

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


Summary

important for quantitative picture of stellar

s process and galactic chemical evolution

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


Abundances beyond fe ashes of stellar burning

Neutrons

Fusion

BB

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


The s process messages from stellar he burning

sources of abundance information


Element abundances in the solar system meteoritic versus photospheric data

element abundances in the solar system - meteoritic versus photospheric data


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