Experimental approaches to s-process branchings

René ReifarthLos Alamos National Laboratory

Astrophysics and Nuclear StructureInternational Workshop XXXIV on

Gross Properties of Nuclei and Nuclear ExcitationsHirschegg, Kleinwalsertal, Austria, January 15 - 21, 2006

s-process path – branched

A A+1s-process path

~ t1/2-1(T)

~ capturens-process path

nA

A TFN

N

, 1

Presolar grains: Left-overs from stellar events

(E. Zinner, WUSTL)

What’s needed?

• Reaction rates

• Half-lives

50 100 150 200Energy keV2

4

6

8

10

12Neutrons

kTT

E

kT

E

kT

E E E

E E

v

v

e d

e d

2 0

0

DANCE @ LANL

Classical s-process

93Zr95Zr

Modern s-process models

(AGB stars)

new n-facility sample production

93Zr

DANCE @ LANL

Classical s-process

Connection between theory and experiment

(n,γ) @ radioctive isotopes

TOF experiments

sensitivity to background very high sensitivity (~ µg)

only average neutron energies only if A+1X is “reasonable“ radioactive

neutronssample

prompt-rays Ge Ge2.

1. neutrons

sample

-rays

PROMPT -detection(4 - scintillators)

Activation technique - DELAYED

( AX(n,)A+1X()A+1Y )

Activation-Method

Determination of neutron flux via 197Au(n,)198Au

Neutron source: 7Li(p,n)7Be

AX(n,)A+1Xreaction detected via

A+1X(-) A+1Y decay

lithium

copper

proton beamneutron cone

AX

Au

Experiment vs. previous estimates

<> = 22.6 + 2.4 mb M

AC

S (

mb)

kT (keV)

LANSCE @ LANL

• 160 BaF2 crystals• 4 different shapes• Ri=17 cm, Ra=32 cm• 7 cm 6LiH inside • 90 % • casc 98 %

-Detector:

• spallation source• thermal .. 500 keV• 20 m flight path• 3 105 n/s/cm2/decade

neutrons:

collimatedneutronsbeam

34 cm

»Nuclear Astrophysics with Neutron Facilities and LANL and RIA«

Detector for Advanced Neutron Capture Experiments

samplet1/2 > 100 dm ~ 1 mg

151Sm combining to decoupled branching regions

148Sm 149 150

1472.6 aPm 149

50 h

146Nd 14710 d 148

1485.4 d

Eu

Gd

BranchPoint

stable s-only

15193 a 152

15213 a

152 154

1511549 a

155

153155

4.8 a

1561530.7 a

This year

Received funding

Sm 150

Eu

Gd

152

15213 a

152 154

1511549 a

155

1531554.8 a

155

15193 a

1530.7 a

A specific example: branchpoint 154Eu

Branching ratio: f= / ( + n)

-decay rate: = (ln 2) / t1/2

n-capture rate: n = n vT Nn

f(154Eu) ~ 154Gd/152Gd

0.5 mg of 151Sm (t1/2 = 100 yr) with DANCE

Esum (MeV)

Q = 8.2 MeVMultiplicity & Energy cuts allow optimization of the signal/background ratio

0.5 mg of 151Sm(n,) – TOF, t1/2 = 100 yr

Neutron Energy (eV)

Neu

tro

n C

aptu

re C

ross

Sec

tio

n (

bar

n)

What could be done in the future• Optimize neutron source

– Increased source brightness• Improved sample production

– FAIR, RIA, ISOLDE

• NCAP•Short flight path•7Li(p,n)•intense proton source

• n-TOF2•20 m flight path•20 GeV protons, spallation

• DANCE upgrade•10 m flight path•0.8 MeV protons, spallation

Summary

• (n,) data on radioactive isotopes are extremely important for modern astrophysics

• s-process branching analysis allows knowledge about stellar convections

• DANCE contributes in the half live time range above a few hundred days

• some important isotopes can be measured now, more will have to wait for future facilities

• 152Eu, 154Eu, 153Gd is planned and funded