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