precision measurements of the (n,2n ) reaction cross sections on actinides

Precision Measurements of the (n,2n) Reaction Cross Sections on Actinides

TUNL and Duke Univ.

Goal: Improve partial cross section data of actinide nuclei for

the Stockpile Stewardship Program

Emphasis : The 239Pu(n,2n) cross section

Technique: High-resolution -ray spectroscopy measurements:

in beam with pulsed and monoenergetic neutron beam

off-line with DC neutron beam

Modeling: Hauser-Feshbach theory (GNASH, EMPIRE)

TUNL and In-beam Measurements

TUNL and Duke Univ.

DENIS Shielded Neutron Source Area

D(d,n) neutron source

n = 3x104 n /(cm2 sec)

Tunable from 4 – 18 MeV

Pulsed at 2.5 MHz

Neutron production

Id =2A

7.8 atmbeam

pick-off

Deuterium gas cell

TUNL and In-beam Measurements

TUNL and Duke Univ.

DENIS Shielded Neutron Source Area

Neutron Collimator and Beam Profile

TUNL and Duke Univ.

Shielded Neutron Source Area

Scanning conditions

step: 0.3 cm in and direction

nd

75 cm

Neutron profile scan

d=215 cm d=427 cm

NNSA Setup at TUNL

TUNL and Duke Univ.

4 Clovers + BGO

2 Planars + BGO

10 keV < E < 10 MeV

200 < lab < 1600

array = 1.4%@E = 1.33 MeV

Excellent tool for precision neutron induced cross section measurements in the fast energy region (4 En 18 MeV)

- coincidence measurements

Angular distribution measurements

Lifetimes (by Doppler method)

Capabilities

n-beam

Neutron Induced Cross Section on 235U

TUNL and Duke Univ.

En = 10 MeV, Beam time = 87 h

234U

First Results from 238U(n,n)238U at TUNL

TUNL and Duke Univ.

Egamma Transition TUNL OakRidge1) LANL2)

(keV) (mb) (mb) (mb)

635.2 1- --> 2+ 83 (4) 76(7) 67(2)

687.0 3- --> 2+ 86 (4) 80(7) 106(2)

885.5 (1-) --> 2+ 63 (4) 56(7) 79(2)

905.5 2- --> 2+ 34 (3) 33(5) 40(2)

957.3 3+ --> 4+ 50 (3) 45(5) 65(2)

1060.3 2+ 0+ 95 (4)

A. Hutcheson et al. NIMB (2007) in press.

Partial Cross Sections at En = 6 MeV

93(11) 116(3)

1) D.K. Olsen, et al., ORNL report TM-6832 1979.

2) N. Fotiades et al., PRC 69 (2004) 024601.

Motivation for the 241Am(n,2n) Reaction Cross Section Measurements

TUNL and Duke Univ.

Improved 241Am(n,2n) cross sections are needed for:

Nuclear forensics ΔA(240Am/241Am) Analogous to P (238Pu/239Pu) Sensitive to high-energy neutrons Advance Fuel Cycle / new fast reactors Important in transmutation studies

Goal: 5-10% measurement from threshold (6.7 MeV) to 14.5 MeV

241Am(n,2n)240Am Measurements

TUNL and Duke Univ.

(n,2n) data for 241Am around

14 MeV

Energy dependence near 14 MeV

No data below ~13 MeV

Different evaluation


TUNL and Duke Univ.

(n,2n) data for 241Am around

14 MeV

Energy dependence near 14 MeV

No data below ~13 MeV

Different evaluations

The recent data shows very high

(n,2n) CS at En 11MeV

Phys. Rev. C 73, 067601 (2006)

Need to extend the 241Am(n,2n)240Am cross section to lower energies

Activation Measurements at TUNL

TUNL and Duke Univ.

Irradiation setup

n = 1x108 n /(cm2 sec)

~ 1mg 241Am targets

3 monitor foils (Al, Ni, Au)

n-flux monitoring: 3 neutron detectors (00, 100)

Activation parameters

DENIS Neutron TOF area

Preparation room

Activation Measurements at TUNL

TUNL and Duke Univ.

Highly enriched 241Am

HPGe detectors

Genie 2000 spec. system

High Res. Gamma Spec.

Low-background counting system


TUNL and Activation Measurements

TUNL and Duke Univ.

Irradiation setupLow-background counting system



TUNL and Duke Univ.

Activation technique: count 240Am activity after irradiation of 241Am

trick: attenuate the low-energy -rays (59.5 keV) coming from 241Am target using 3 mm Pb

Irradiated target with mono-energetic neutrons produced by D(d,n) reaction TUNL

En= 7.6 - 14.5 MeV / n ~ 107–108 n/cm2-s

~1 mg/cm2 241Am target / 1 cm dia. sandwich targets on Pt backing

Measure neutron fluence using a set of witness foils mounted before & after target

197Au(n,2n)196Au (t1/2 = 6.2 d), T= 8.1 MeV

27Al(n,)24Na (t1/2 = 15 hr), T= 3.2 MeV

58Ni(n,p)58Co (t1/2 = 71 d), T= 0 MeV

58Ni(n,2n)57Ni (t1/2 = 36 hr), T= 12.4 MeV

240Am

240Pu 0+; 0.0 keV2+; 42.94+; 141.8

3+; 1030.6, T1/2 = 1.32 ns

99.6(11)%

987.8 keV

888.8

25.1(4)%

73.2(10)%

High-Resolution Gamma Ray Spectra

TUNL and Duke Univ.

4

44

4

Plastic ring Foil 8 Foil 9 Plastic ring

0.5 mil Pt 0.5 mil Pt

519 g/cm2

241Am513 g/cm2

241Am

Fabrication of 241Am target (LANL and LLNL)

3 stippled targets (Evelyn Bond/LANL)

6 electroplated targets after additional

chemical purification (Mark Stoyer/LLNL)

~ 2% statistical uncertainties

Improved -spectra from the electroplated targets

Need for nuclear radiochemists

First Results on 241Am(n,2n)240Am Reaction Cross Section

TUNL and Duke Univ.

First Results on 241Am(n,2n)240Am Reaction Cross Section

TUNL and Duke Univ.A. Tonchev et al. PRC in preparation.

Summary

TUNL and Duke Univ.

Complete cross section measurements of the 241Am(n,2n)240Am reaction.

Preliminary data from TUNL support the recent statistical prediction from LANL (2005 Kawano) Intense, pulsed, monoenergetic TUNL neutron source provides

powerful capabilities for Science-Based Stockpile Stewardship and Nuclear Science.

More than 160 new parities measured in nuclei: 40Ar, 88Sr, 92Zr, 96Mo,112,124Sn, 138Ba, 140Ce, 172,174,176Yb, 164,162Dy, 207Pb.

Strong TUNL-LANL-LLNL Collaboration Effort

Participants*

TUNL and Duke Univ.

Los Alamos

E. BondJ. FitzPatrickR.S. RundbergA. SlemmonsD.J. VieiraJ.B. Wilhelmy

Livermore

J.A. BeckerD. Dashdorj*

J. KenneallyR.A. MacriD. ShaughnessyM.A StoyerCh.Y. Wu

TUNLke: Duke: A.S. Crowell B. Fallin C.R. Howell A. Hutchison A.P. Tonchev W. Tornow

Univ. of N. Carolina: H.J. Karwowski C.T. Angell N. Carolina State: J.H. Kelley D. Dashdorj*

NC A&T: R. S. Pedroni

*(n,2n) measurements supported by the NNSA Stewardship Science Academic Alliance and by DOE/NNSA LANL & LLNL

Participants

TUNL and Duke Univ.

Extra Viewgraphs

TUNL and Duke Univ.

240Am Gamma-ray Signature

TUNL and Duke Univ.

- 889 keV line is partially contaminated by a weak 241Am line, but the 988 keV line is clean

- Measured 2000-3000 events after a 1-2 day irradiation and 1day count time

- Measured 11 energies in March, Nov., Dec. 2006 runs

Total amount of used beam time: 120 days

Collaborators from 2 National Labs and 3 Universities

www.tunl.duke.edu/groups/nnsa

Completed Experiments

TUNL and Duke Univ.

Time Reaction Energies [MeV]

May 2003 238U(n,2n )237U 6, 10

Aug 2004 238U(n,2n )237U 8, 10, 14

Feb 2005 90Zr(n,n' )90Zr 6

Feb 2005 89Yb(n,n' )89Yb 6

Feb 2005 112Sn(n,n' )112Sn 6.5, 7.5, 8.0

Feb 2005 124Sn(n,n' )124Sn 6.5, 7.5, 8.0

May 2005 235U(n,n' )235U 5

Jun 2005 235U(n,2n' )234U 12

Jun 2005 natHf(n,xn' ) 12

Jun 2005 16O(n,n' )16O 7

Jun 2005 12C(n,n' )12C 7

Dec 2005 235U(n,2n' )234U 12

Time Reaction Energies [MeV]

Jan 2006 235U(n,2n' )234U 10, 8

Jan 2006 181Ta(n,2n' )180Ta 14.5

Feb 2006 140Ce(n,2n' )139Ce 14.5

Mar 2006 94Zr(n,n' ) 5.0

Mar 2006 241Am(n,2n) 8-14

Jun 2006 natCu, natPb(n,n‘ ) 8,12

Oct 2006 235,238U(n, 2n ) 10

Nov 2006 241Am(n,2n) 8-16

Dec 2006 241Am(n,2n) 8-16

Dec 2006 235U(n, 2ng) 12

Jan 2007 235U(n,2ng) 8

Feb 2007 235U(n,2ng) 10

Time Reaction En [MeV] Time Reaction En [MeV]

Neutron Monitors

TUNL and Duke Univ.

Tact = 12.5 h

n-flux fluctuationn-monitors / BCI

NRF Spectra on 138Ba

Primary transitions to the 2+ states

138Ba

En = 8.5 MeV

0+

2+

E2

2+

8434 keV

8074

1-

1-

TUNL and Duke Univ.

143622182639

E = 8.50.08 MeV

Branching ratio (1- 2+) 3% !

2+

Decay Scheme of Am Isotopes

TUNL and Duke Univ.

Absolute Flux Measurements at HIS

TUNL and Duke Univ.

Compton Scattered Setup Compton Scattered Spectra

NNSA Synergistic Activities at TUNL

TUNL and Duke Univ.

NNSA (n,xn) activity: low-energy introduction to nuclear experimental techniques for the Research Experience for

Undergraduates (REU) program at TUNL

Average 8-10 REU students every summer For the past three years one or two has been involved in the TUNL NNSA project Diversification of the NNSA setup for basic research:

nuclear structure nuclear astrophysics neutrino and dark matter

Synergistic activities at TUNL

Beam Correlated Measurements

TUNL and Duke Univ.

Start Stop

n-beam

TUNL pulsed n-beam

~2ns ON / 400ns Off

0

200

400

6000

600

1200

1800

800 900 1000 1100 1200 1300

200

400

600

200 400 600 800

200 400 600 800

200 400 600 800

Fe *U Fe

Fe

Fe

*

*

*

U

*

* ***

*

**

*UU FeFe

Fe

TOF

Energy (keV)

Co

un

ts

TOF

TOF

G-BG

G

U

Laser Compton Backscattering

UNDULATOR

E2

E1

TUNL and Duke Univ.


CAARI 2004

UNDULATOR

E2

E1

P

TUNL and Duke Univ.


CAARI 2004

UNDULATOR

E1

P

E2

γ

TUNL and Duke Univ.


CAARI 2004

UNDULATOR

E2

E1

P

TUNL and Duke Univ.

TUNL and Duke Univ.

precision measurements of the (n,2n ) reaction cross sections on actinides

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