T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

1

Current status on experimental investigations on

prompt fission neutrons spectra

T. GranierCEA DAM DIF, Bruyères le Châtel, Arpajon,

France

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

23 experiments

• 239Pu PFNS (Los Alamos)

• 238U mean prompt neutron multiplicity measurement (Los Alamos)

• 238U and 235U PFNS (Bruyères le Châtel)

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

3239Pu PFNS

• Experiment performed in the framework of the CEA/DAM-DOE-NNSA cooperation agreement on basic sciences

• Collaborators:T. Granier, A. Chatillon, B. Laurent, J. Taieb, G. Bélier

CEA DAM DIF, Bruyères le Châtel, France

RC Haight, R.O Nelson, J. O’Donnell, M. Devlin, S. NodaLANL, Los Alamos, USA

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

4PFNS investigations at LANSCE/WNR

800 MeV protons Pulsed mode Spallation in W targets Unmoderated spallation neutrons

(WNR)

800 MeV protons

WNRLujan

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

5PFNS investigation at LANSCE/WNR

Collimatedn beam

Figaro (WNR)

Multi-plate fission chamber designed in CEA

• Nuclear physicists from CEA initiated an experimental program in Los Alamos to study PFNS

• FIGARO (Bob Haight) array of organic liquid scintillator (analogous to NE-213) n detectors

• Multi-plate fission chambers loaded with hundreds mg of actinide

• Double time of flight• n-γ discrimination based on pulse shape• Detection efficiency measured with 252Cf

TOF 2 / ~1 m

Neutron detectors

Fission chamber

TOF 1 / ~22 m

Collimated pulsed n beam

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

6Investigation of 239Pu at WNR

• 1st experiment: Phys. Rev. C 83 3 034604 (2011), S. Noda et al.• Newer results on 239Pu from a second, improved experiment have

need obtained (analysis by A. Chatillon)– Examples of PFNS obtained

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

7

• Measured PFNS are systematically lower than models above 5 MeV

• There seems to be independent indications that ENDF/B-VII PFNS are too hard

Investigation of 239Pu at WNR

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

8238U(n,f) Nu-bar

• Goal of the experiment: Fragment mass yields, Fragment kinetic energy, neutron multiplicity

• Collaboration CEA/DAM-JRC/IRMM-LANL• Experiment performed at LANSCE/WNR• IRMM double Frisch-Gridded ion chamber with 1 mg 238

sample• CEA fission neutron counter• Collaborators:

T. G., B. Laurent, G. Bélier, A. Chatillon, J. Taieb, H. DuarteCEA-DAM DIF, Bruyères le Châtel, France

F.-J. Hambsch, S. Oberstedt, P. Salvador CastineiraEC-JRC-IRMM, Geel, Belgium

F. Tovesson, A. Laptev, R.C. Haight, R.O. Nelson, J. O’Donnell

LANSCE-NS, Los Alamos National Laboratory, USA

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

9Experiment principle

• Conjunction of 2 detectors:– Double Frisch-gridded ion chamber:

• Double kinetic energies E1, E2 of post neutron fragments• emission angle of both post neutron fragments

– Neutron counter• Number of neutrons emitted in coincidence

• Time of flight is used to measure incident neutron energy

3He det.

Fission chamber

Poly moderator

WNRN beam

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

10LANSCE-WNR

Spallation neutrons “White” neutron source 0.1 < En < 200 MeV Pulsed mode: Time-of-flight

800 MeV protons

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

11LANCSE-WNR

Pulsed beam 800 MeV protons (200ps) makes broadspectrum of neutrons by spallation reactions

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

12The double kinetic energy method

• E1 and E2 (post neutron) measured• Conservation of mass and momentum

– At low energy and for preneutron fragments:• A1/A2=E2/E1 • A+1=A1+A2

– Corrections • Energy loss of fragments in target-substrate• Pulse height defect• Neutrons emitted vs. A (constrained by coincident neutron

measurement)• kinematics

– Iterative procedure required→ A1,A2 pre and post neutron

• Resolution on mass corresponds at best to a sigma of ~2 mass units

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

13The IRMM double Frisch-gridded ion chamber

• 238U sample, 1 mg, vacuum-evaporated onto thin polyimide film• Energy and angle are measured for both FF (summing method)• Precision pulse generator used for signal-drift monitoring• Counting gas: P-10 (90%Ar+10% CH4).• 1.05 bar gas pressure.• 0.1 l/min gas flow.

238U

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

14The CEA fission neutron counter

• 30 3He tubes in 2 rings surrounding fission chamber• 3He tubes are within a polyethylene block to moderate fission neutrons• Neutrons are counted and time-stamped during 150 μs after each trigger• Efficiency ~25% for 252Cf (in this configuration)• Assembly is shielded against room return using poly+boron carbide• Insensitive to gamma rays

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

15In real life, at WNR

Experimental setup on 90 L flight path

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

16Neutron efficiency correction

• Counting efficiency vs neutron energy and emission angle studied by means of MC simulations using MCNPX

• Efficiency measured with 252Cf (in perfect agreement with simulations)

• For incident neutron energy below ~20 MeV, compatible with constant efficiency (excellent agreement with Fréhaut data)

• Above ~20 MeV, the BRIEFF code is used:– angle x energy double differential neutron spectra calculated

with BRIEFF code for different incident neutrons energies from 20 to 200 MeV used as neutron sources in MCNPX calculations

Emitted neutron energy (MeV)

Cos

ine

of e

mis

sion

ang

le

En=200 MeV

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

17238U Nu-bar measurement

• 12 days beam• 1 mg 238U target

Fréhaut et al., CEA Bruyères le Châtel 1980Ethvignot et al., LANL-CEA 2002This Work

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

18Preliminary fragment anisotropy data

• Fission fragment anisotropy in center of mass

0 1 2 3 4 5 6 7 8 9 10 11 120.0

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LANL 2010

A=

W(0

º)/W

(90º

)-1

En (MeV)

A=

W(0

º)/W

(90º

)-1

En (MeV)

Shpak, 1989

Vivés, 1997

A=

W(0

º)/W

(90º

)-1

En (MeV)

Kaufmann, 1996A=

W(0

º)/W

(90º

)-1

En (MeV)

A=

W(0

º)/W

(90º

)-1

En (MeV)

Ryzhov, 2005

Simmons, 1960

A=

W(0

º)/W

(90º

)-1

En (MeV)

Henkel, 1956

A=

W(0

º)/W

(90º

)-1

En (MeV)

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

19Preliminary fragment mass distribution data

• Preliminary mass distributions for 2 incident neutron energy groups

80 100 120 140 1600

1

2

3

4

5

6

7

Yie

ld (

%)

Pre-neutron Mass (amu)

En = 8 - 10 MeV

80 100 120 140 1600

1

2

3

4

5

6

7

Yie

ld (

%)

Pre-neutron Mass (amu)

En = 1.75 - 3.5 MeV

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

20New PFNS measurements at CEA• Van de Graaff 4 MV HVEC• Quasi monoenergetic neutrons

– 7Li(p,n) ~500 keV– T(p,n) : 1-3 MeV– D(d,n) : 3-7 MeV– T(d,n) : 15-20 MeV

• Time of flight method• Paraterphenyl detector (Kharkov)

– better sensitivity to low energy neutrons

• Detection efficiency: 252Cf + MC simulations

Paraterphenyldetector

n shielding(Li paraffin)

Fission chamber

TiT or TiD target

~0.9 m flight path

N-γ shadow bar

p or d beam

Lead shielding

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

21New PFNS measurements at CEA

• Goal: PFNS precision measurements for a few incident neutron energies

• Current experiments: 238U, 235U• Currently: flux limitations due to radiation protection issues

– Should be solved soon• Implementation of new shielding• New operation mode will allow us to run the accelerator at

high current

• Planned measurements: 235U, 239Pu, 232Th …

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

22238U(n,f) at 6.5 MeV, spectral shape

• Preliminary result in agreement with previous measurements by Kornilov and Lovchikova: effect of the opening of 2nd chance fission

• Major data libraries currently fail to describe this effect

BRC eval. 2011BRC LA model

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

23Perspectives

• Experiments with higher neutron flux in Bruyères le Châtel – New shielding– Operation after hour with extended exclusion area

• Experiments in Geel in collaboration with F-J Hambsch and S. Oberstedt– 1st test in mid 2012 : 235U at 0.5 MeV with fission chamber

• Remark: PFNS precision measurement in the domain 7-8 MeV can lead to information on the probability ratio 1st chance/2nd chance

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

24235U(n,f) at 6.5 MeV PFNS shape

• 235U(n,f), En=6.5 MeV, Spectral shape

This measurement

Eval. by V. Maslov

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

25A few words on the current PFNS evaluation in Bruyères

• As outlined by V. Maslov in last meeting : LA model fails at describing the 2nd chance PFNS

• B. Morillon and P. Romain now include the prefission spectrum calculated within the cross section evaluation using the Talys code (Hauser-Feschbach + Exciton model)

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

26

• Additional material

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

27Prompt fission neutrons

• Deexcitation of the fragments in flight

• When En > Bf, opening of 2nd chance fission (emissive fission, pre-fission neutrons)

• Neutron emission at scission (scission neutrons) probably exist

n A A+1

n A A+1 A

σ

T. Granier , CEA-DAM – IAEA ‘s CRP on PFNS, 2nd meeting, Vienna, 13-16 December 2011

28Examples of preliminary results

238U(n,f), En=6.5 MeV, spectral shape• Preliminary result confirms previous measurements by

Kornilov and Lovchikova: effect of the opening of 2nd chance fission

• Major data libraries fail to describe this effect