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

Red Giants(s-process)

supernov

ae(r-p

rocess)

s-abundance x cross section = N = constant

Massenzahl

H

äufi

gkei

t

s-process contributions to the heavy elements

s process

weak s processA<90

main s process90<A<209

thermally pulsing low mass AGB stars of 1<M/M⊙<3 neutron sources: 13C(,n), 22Ne(,n) T ~ 1-3·108 K, nn ~ 4 ·108 cm-3

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

massive stars M> 10 M⊙

neutron source: 22Ne(,n) core helium burning

T ~ 2-3·108 K, nn ~1·106 cm-3

shell carbon burningT ~1·109 K, nn ~1·1011

cm-3

low mass AGB stars – the main s component

MASS NUMBER

r- A

BU

ND

AN

CE

Nr = N - Ns

r-process abundances

ATOMIC NUMBER

log

AB

UN

DA

NC

E

observed scaled solar system

main component: the branching at 151Sm

152 154

151

Sm

153

151

152 155

Eu

Gd

p process

s process

r process

150

154 156 157

152 154

151

151Sm: lab half-life of 93 yrreduced to t1/2 = 3 yr at s-process site

info on s-processtemperature!

ingredients: - s-only isotopes in total reaction flow and in branches - unstable branch point isotopes - N = constant

weak component: the bottle neck example of 62Ni(n,)

N ≠ const.

s-process efficiencydetermined by single cross sections

Maxwellian averaged cross sections required

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

(n,) cross sections: status and challenges

20 40 60 80 100 120NEUTRON NUMBER

1

10

100

1000

MA

XW

EL

LIA

N A

VE

RA

GE

D C

RO

SS S

EC

TIO

N

(mb)

BaBa

CdCd

CeCe

CrCr

DyDy ErEr

FeFe

GdGd

GeGe

HfHf

HgHgKrKr

MoMo NdNdNiNi

OsOs

PbPb

PdPd

PtPt

RuRuSeSe

SmSm

SnSn

SrSr

TeTe

WW

XeXe

YbYbZnZn

ZrZr

even-even nuclei

neutron magic nuclei unstable branch point isotopes

A < 120

60 80 100 120 140 160 180 200MASS NUMBER

0.8

1

1.2

1.4

SEF

Rauscher 1998Rauscher 1998Holmes et al. 1976Holmes et al. 1976star/lab

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

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 (A) 2 20 neutrons per proton 760

since 1987

since 2001

wide neutron energy range from thermal to 250 MeV

advanced detection techniquesadvanced detection techniques• high detection efficiency: ≈100% • good energy resolution

n

• 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

enhancement of sensitivity in TOF measurements by low energy accelerator with 1000 times higher beam

current

a step further: NCAP

samples can be made with future

RIB facilities such as GSI

TOF measurements on unstable samples of 1015 atoms (<1 g) and half-lives of t1/2> 10 d possible

sample Pb neutron target

p-beam

n-beam

average current 1 mA, pulse width of ~1 ns, repetition rate 250 kHz

summary

• numerous remaining quests for accurate (n,) 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

important for quantitative picture of stellar s process and galactic chemical evolution

... new options by AMS

abundances beyond Fe– ashes of stellar burning

rs

Neutrons

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 )

FusionBB

H 30 000C 10Fe 1Au 2 10-7

Fe

mass number

ab

un

dan

ce

sr

sources of abundance information

element abundances in the solar system - meteoritic versus

photospheric data