I. Introductory remarks and present status II. Laboratory experiments and astrophysicsIII. Future options

scenariosstatus and challengesnew developments

neutron reactions in astrophysics – status and perspectives

neutron capture scenarios

big bang

stellar He burning s process in TP-AGB and in massive stars

explosive nucleosynthesis p and r processes

neutron capture accounts for 75% of the stable isotopes, but only for about 0.005% of the total post BB abundances

Maxwellian averaged cross sections required

measure (En) by time of flight, 0.3 < En < 500 keV, determine average for stellar spectrum correct for SEF high accuracy, wide energy range

produce thermal spectrum in laboratory, measure stellar average directly by activation correct for SEF very high sensitivity

prompt -rays + TOF-method

detection of neutron capture events

* Moxon-Rae ~1% * PH-weighting ~20% * Ge, NaI < 1%

single ´s

all ´s * 4BaF2

~100%

(n,):

activation in quasi-stellar spectrum most sensitive * small cross sections, 1014 atoms! selective * natural samples or low enrichment

compilation of stellar (n,) cross sections

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

Bao & Käppeler 1987

Bao et al. 2000

Beer, Voss & Winters 1992

collect experimental data, renormalize, calculate MACS, recommend

based on educated choices by experienced experimentalist

complement by theory (SEF)

current update by Dillmann & Plag: KADONIShttp://nuclear-astrophysics.fzk.de/kadonis/

status of stellar (n,) cross sections

s process: = 1-3% p and r process: ~ 5%

what do we have?

nuclear input must be good enough that it doesn‘t punch through to calculated abundances!

what do we need?

beware:

discrepancies often larger thanuncertainties!!!

activation in quasi-stellar spectrum

- neutron source 7Li(p,n)7Be

- neutron flux 197Au(n,)198Au

- 15C detected via 5.3 MeV line

(t1/2=2.45

s)

most (n,) of unstable nuclei measured this way: 14C(n,)15C

neutron cone

Au/14C/Au

lithium

proton beam

half-life limits 0.1 s < t1/2 < 10 yr with -spec

no limit with AMS!

sample properties >1014 atoms impurities acceptable

activation in quasi-stellar spectrum

possible neutron sources:

7Li(p, n)7Be kT=25 keV 2·109 neutrons/s, 100

A

3H(p, n)3He 52 keV 1·108 “ “

18O(p, n)18F 5 keV 2·105 “ “higher beam currents needed for - activations at low energies - long-lived product nuclei - studies of double neutron captures

higher beam currents require new target technology!

complete info: (En) via TOF method

& folding with stellar spectrum larger samples *

limited sensitivity optimal efficiency

higher flux

limited selectivity enriched samples **

* not desirable and even excluded for unstable samples

** mandatory

>90% up to 10 MeV casc > 98%

E/E = 6% at 6 MeV clear signatures

t = 500 ps good TOF resolution

optimal efficiency : 4 BaF2 array

sample

Pb neutron target

p-beam

collimated n-beam

now also at Los Alamos and CERN

FZK

high neutron fluxes :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

still higher fluxes in future

J-PARC spallation source similar features than LANSCE, but 50 times more flux

LANSCE improved by factor of 10 – 20 by upgrade of LAMPF

n_TOF @CERN improved by factor of 100 by shorter flight path

Low energy proton accelerators with beam currents of up to 200 mA (Soreq Nucl. Research Center, Univ. of Frankfurt/M)

unstable samples: now and then

r and p process(n,) cross sections for a varietyof selected unstable isotopes(r : 60Fe, 106Ru, 126Sn, 182Hf...(p : 91,92Nb, 97,98Tc...)

for direct use in reaction networks

to derive rates of inverse

reactions

to test and assist statistical

models

63Ni79Se81Kr85Kr147Nd147Pm148Pm151Sm154Eu155Eu153Gd160Tb163Ho170Tm171Tm 179Ta185W204Tl

branch point status

s processfuture

+ 59Fe, 125Sn, 181Hf….

summary

• numerous remaining quests for s process (branchings, grains, massive stars) and many more for explosive nucleosynthesis

• present facilities and detectors suited for most stable isotopes

• new approaches required for radioactive samples

• spallation sources, new low energy accelerators, and RIB facilities promising, both for stellar and explosive nucleosynthesis

important for quantitative picture of galactic chemical evolution