1. Experimental approaches to s-process branchings
René Reifarth
Los Alamos National Laboratory
Astrophysics and Nuclear Structure
International Workshop XXXIV on
Gross Properties of Nuclei and Nuclear Excitations
Hirschegg, Kleinwalsertal, Austria, January , 2006

2. s-process path – branched
~ t1/2-1(T)
s-process path
~ scapturefn

3. Presolar grains: Left-overs from stellar events
(E. Zinner, WUSTL)

4. What’s needed?
Reaction rates
Half-lives

5. Connection between theory and experiment
LANL
Classical s-process
93Zr
95Zr
Modern
s-process
models
(AGB stars)
new n-facility
sample production
93Zr
LANL
Classical s-process

6. (n,γ) @ radioctive isotopes
Activation technique - DELAYED
( AX(n,g)A+1X(b-)A+1Y )
PROMPT g-detection
(4p - scintillators)
 TOF experiments
 sensitivity to background
 very high sensitivity (~ µg)
 only average neutron energies
 only if A+1X is “reasonable“ radioactive
neutrons 1.
neutrons
sample
sample Ge 2.
prompt g-rays
g-rays

7. Activation-Method AX(n,g)A+1X reaction detected via A+1X(b-) A+1Y
decay
copper Au
proton beam
Determination of neutron flux via
197Au(n,g)198Au
neutron cone
lithium AX
Neutron source:
7Li(p,n)7Be

8. Experiment vs. previous estimates
<s> = mb
MACS (mb)
kT (keV)

9. LANL

10. Detector for Advanced Neutron Capture Experiments
spallation source
thermal keV
20 m flight path
3 105 n/s/cm2/decade
neutrons:
collimated
neutrons
beam
160 BaF2 crystals
4 different shapes
Ri=17 cm, Ra=32 cm
7 cm 6LiH inside
eg  90 %
ecasc  98 %
g-Detector:
sample
t1/2 > 100 d
m ~ 1 mg
34 cm
»Nuclear Astrophysics with Neutron Facilities and LANL and RIA«

11. 151Sm combining to decoupled branching regions
Received
funding Gd
151
93 a
152
13 a
154
9 a
155
153
4.8 a
156
0.7 a
This year Eu Sm
148
149
150 Pm
147
2.6 a
148
5.4 d
149
50 h
Branch
Point
stable
s-only Nd
146
147
10 d
148

12. 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

13. 0.5 mg of 151Sm (t1/2 = 100 yr) with DANCE
Multiplicity & Energy cuts allow optimization of the signal/background
ratio
Q = 8.2 MeV
Esum (MeV)

14. 0.5 mg of 151Sm(n,g) – TOF, t1/2 = 100 yr
Neutron Energy (eV)
Neutron Capture Cross Section (barn)

15. 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

16. Summary
(n,g) 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