Experimental approaches to sprocess branchings

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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 15 - 21, 2006
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s-process path branched
A
A1
scapturefn
s-process path
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Presolar grains Left-overs from stellar events
(E. Zinner, WUSTL)
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Whats needed?

• Reaction rates
• Half-lives

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Connection between theory and experiment
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(n,?) _at_ radioctive isotopes
Activation technique - DELAYED (
AX(n,g)A1X(b-)A1Y )
PROMPT g-detection (4p - scintillators)
? TOF experiments ? sensitivity to background
? very high sensitivity ( µg) ? only average
neutron energies ? only if A1X is reasonable
radioactive
sample
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Activation-Method
AX(n,g)A1X reaction detected via A1X(b-) A1Y
decay
Au
copper
proton beam
Determination of neutron flux via 197Au(n,g)198Au
neutron cone
lithium
AX
Neutron source 7Li(p,n)7Be
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Experiment vs. previous estimates
ltsgt 22.6 2.4 mb
MACS (mb)
kT (keV)
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LANSCE _at_ LANL
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Detector for Advanced Neutron Capture Experiments
collimated neutrons beam
34 cm
Nuclear Astrophysics with Neutron Facilities and
LANL and RIA
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151Sm combining to decoupled branching regions
Gd
Eu
148
150
149
Sm
148 5.4 d
147 2.6 a
149 50 h
Pm
Branch Point
stable
s-only
146
147 10 d
148
Nd
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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
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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)
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0.5 mg of 151Sm(n,g) TOF, t1/2 100 yr
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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

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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