Lawrence Livermore National Laboratory
Nicholas ScielzoPhysics Division, Physical and Life Sciences
LLNL-PRES-408002
Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA 94551
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344
Using surrogate nuclear reactions to determine (n,f) and (n,) cross sections
August 8, 2009
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Surrogate Nuclear Reactions Approach
The Surrogate Nuclear Reactions approach is an indirect method for determining cross sections of compound-nuclear reactions
Used when direct measurements are not possible because of beam and/or target limitations – create compound nucleus through reaction of light-ion beam on a (more) stable isotope
Can be used in regular or inverse kinematics
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Surrogate nuclear reaction method using inelastic scattering
“Desired” reaction
n
153Gd 154Gd
“Surrogate” reaction
,,,,,
JEGJEFEP CN
J
CN
p
154Gd 154Gd
p
Hauser-Feshbach theory describes the “desired” reaction as a product of entrance channels (
CN – can be calculated reliably) and exit-channel branching ratios (G
CN – can’t be calculated reliably)
Alternative (“surrogate”) reaction forms the same compound-nucleus and determines G
CN
We measure this ratio
EN
EN
pp
pp
,
,
,,,,,
JEGJEE CN
J
CN
t1/2=240 days stable
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Approximation simplifies technique above ~MeV
“Desired” reaction
n
153Gd 154Gd
“Surrogate” reaction
EGEP CN
p
154Gd 154Gd
p
Hauser-Feshbach theory describes the “desired” reaction as a product of entrance channels (
CN – can be calculated reliably) and exit-channel branching ratios (G
CN – can’t be calculated reliably)
Alternative (“surrogate”) reaction forms the same compound-nucleus and determines G
CN
EN
EN
pp
pp
,
, EGEE CNCN
t1/2=240 days stable
Weisskopf-Ewing Approximation: branching ratios GCN are independent of spin and parity
when many decay channels are open
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Gamma Ray Detectors
Up to 4×1000 µm E detectors
-electron & fission fragment shield
p, d,He, , 18O beamScatteredparticle
140 µm or 500 µm E detector
Fission Fragments
140 µm fission detector
Silicon Telescope Array for Reaction Studies (STARS)Livermore Berkeley Array for Collaborative Experiments (LIBERACE)
Particle solid angle: 20%
-ray photopeak @ 1 MeV: 1%
Fission fragment solid angle: 2 × 20%
En determined from scattered particle energy:
nexn SEE
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Surrogate (n,f) measurements
Surrogate reactions approach has successfully determined (n,f) cross sections in actinides
237U(n,f) from STARS/LBNL Data and Younes et al.,
0
0.5
1
1.5
2
2.5
0 2.5 5 7.5 10 12.5 15 17.5 20
Neutron Energy (MeV)
Cro
ss S
ecti
on (
bar
ns)
237U(n,f) - STARS/LBNL 12/04Younes et al.,JENDL 33ENDF-B7 (2000)
237Np(n, f)
Cro
ss s
ect
ion (
barn
)
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
This work Ref. 15 ENDF/B-VII.0 JENDL 3.3
237Np(n, f) / 235U(n, f)
Neutron Energy (MeV)
10 11 12 13 14 15 16 17 18 19 20
Ratio
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
This work and ENDF/B-VII.0Ref. 16
0
0.5
1
1.5
2
2.5
7 12 17 22
Excitation Energy (MeV)
Fiss
ion R
ati
o
237U(n,f)/235U(n,f) from 238U(,f)/236U(,f)233U(n,f)/235U(n,f) from 234U(,f)/236U(,f)
237Np(n,f) from 238U(3He,tf)
S.R. Lesher et al., Phys. Rev. C 79, 044609 (2009).
J.T. Burke et al., Phys. Rev. C 79, 054604 (2006).
M.S. Basunia et al., Nucl. Instrum. Meth. B, in press (2009).
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Surrogate (n,) measurements
Extract most-likely J distribution from comparison of data and calculations…
…and use this information to move beyond Weisskopf-Ewing approximation to extract reliable (n,) results
The measured -ray yields compared to calculated yields for different spin distributions(error bars not shown).
Compound-nuclear J distribution is important…
Sn
Probability of -ray emission for 156Gd(p,p’)
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Requirements
Experiments benefit from:
up to nano-Amp beams (regular or inverse kinematics)
light-ion reactions
efficient particle detectors with excellent PID and energy resolution
high-efficiency -ray detector arrays
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Collaborators
Lawrence Livermore National LaboratoryL.A. Bernstein, D.L. Bleuel, J.T. Burke, F. Dietrich, J. Escher, S.R. Lesher,
E.B. Norman, N.D. Scielzo, S. Sheets, I. Thompson, M. Wiedeking
U.C. Berkeley and Lawrence Berkeley National LaboratoryM.S. Basunia, R.M. Clark, P. Fallon, J. Gibelin, R. Hatarik, B. Lyles, M.A. McMahan, L. Moretto, E.B. Norman, L. Phair, S.G. Prussin, E.
Rodriguez-Vieitez
University of RichmondJ.M. Allmond, C. Beausang
Rutgers UniversityJ.A. Cizewski, R. Hatarik, P.D. O’Malley and T. Swan
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