towards the evaluation

GROUPEMENT DE RECHERCHECEA - CNRS - EDF - AREVA NP

Maëlle KervenoGEDEPEON Workshop : Nuclear Data Evaluation, Issy-les-Moulineaux, France 29-30 November, 2010

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Measurement of actinides (n,xn ) cross sections :Towards the evaluation

Maëlle Kerveno,

Catalin Borcea, Philippe Dessagne, Jean Claude Drohé, Estelle Gasser, Erwin Jericha, Habib Karam, Arjan J. Koning, Alexandru Negret, Andreas Pavlik, Arjan J. M. Plompen, Chariklia Rouki, Gérard Rudolf, Mihai Stanoiu, Jean Claude Thiry

GROUPEMENT DE RECHERCHE CEA - CNRS - EDF - AREVA NP



2

Introduction : general context

New concepts of reactors-> fast reactors -> accelerator driven systems

Gen IV reactor systems

New fuel cycle-> 238U / 239Pu-> 232Th / 233U

In reactor, (n,xn) reactions (x ≥ 1) contribute to -> Energy loss mechanism-> Neutron multiplication-> Production of radioactive isotopes Bibliography in data bases shows that

improvement of the knowledge of (n,xn) process is necessary.

Important needs of new nuclear dataover a wide range of

nuclei, energy and reactions

One of the challengesis

measurement’s accuracy

NEA Nuclear Data High Priority Request List



3

Introduction : experimental method

How to study (n,xn) reactions?

-> Direct measurement of secondary neutrons-> Activation technique-> prompt -ray spectroscopy

(n,xn ) cross section measurements -> total (n,xn) cross section :Need of theoretical model…

but(n,xn ) cross sections :- can be measured using “white” neutron beam with the TOF technique,- provide exclusive measurements very restrictive for testing models.

Method: detection of the -rays stemming from the decay of excited states of nucleus created by the (n,xn) reaction.

Case of 235U and 232Th



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Experimental set-up

30mUranium Target

e-

f = 800 Hz

White neutrons beamfew eV ➔ 20 MeV

GELINA, IRMM, Geel, Belgium

Pulsed Neutron Beam

Fission Chamber

Sample

HPGeplanar detectors

150°

110° 110°

150°

Prompt spectroscopy



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Experimental set-up

BEAM

Digital acquisition systemTNT2 Card @IPHC

Fission ChamberHPGe planar Detector

Autofill Nitrogen system

FP16 – 30 m

Shielding: lead castle

-> Electromagnetic field(from the accelerator) insulation

-> Background reduction



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Data Analysis

TOF and spectra

232Th(n,3n )230Th

232Th(n,n’ )232Th

232Th(n,2n )231Th

-flash

Radioactivity

232Th



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Energy (keV) E= 6 - 20 MeV

Energy (keV) E=1- 6 MeV

Data Analysis

TOF and spectra

10 2

10 3

10 4

10 5

10 6

0 5000 10000 15000

Temps (ns)

flash

En = 20 MeV

En = 6 MeV

En = 1 MeV

radioactivity

Time (ns)

235U case



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Data Analysis

Cross section calculation

)(cos4

)(0

i

ii

tot Padd

)(

dd)(

dd4 *

2*2

*1

*1 wwtot

w1

= 0.3479 for θ1

= 30.56° or 149.44° andw2

= 0.6571 for θ2

= 70.12° or 109.88°

Pulsed Neutron Beam

Fission Chamber

Sample

HPGeplanar detectors

150°

110° 110°

150°

tNn

dd

nat

det)(

Differential cross section can be expressed by a finite sum of Legendre polynomials :

Measurement at the polynomial nodesGauss quadrature :

t timemeasuremen : efficiency HPGe :

fluxneutron : sample in the atoms ofnumber :

detected ofnumber : det

t

Nn

n

at



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Results : 235U

TALYS % Exp data* (n,n’

):

good

agreement in amplitude but the shape is not well reproduced* (n,2n ) :

quite good

agreement in the shape but factor of 0.455 in amplitude

D.P. McNabb* exp data % Our exp data* factor of 0.455 in amplitude but McNabb* data are normalized by an unknown factor

*D.P.McNabb, et al. Conf.: Symp.on

Capt.Gamma

Ray Spectroscopy, Santa Fe, NM 1999, p.384 (1999)



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Results : 235U

TALYS Considerations:Fissionable isotopes: How good is the TALYS fission cross section fit?

And what about the total (n,n’) and (n,2n) cross sections?



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Results : 232Th

TALYS % Exp data* Total cross sections:Fission : ~ ok(n,n’) (n,2n) : small underestimation of the code predictions* (n,n’

)cross

sections for states in

ground_state

band :amplitude okbut overestimation above En

= 7 MeV



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Results : 232Th

TALYS % Exp data* (n,n’

):

agreement is

less good than for - transition within the

ground_state

band

J.H. Dave* exp data % Our exp data* agreement is very good up to En

=2 MeV

(high limit of the J.H. Dave* exp data)

*J.H.Dave, et al. Nuclear Science and Engineering, Vol.91, p.187 (1985)

Preliminary results (data from only 2 detectors) : -transitions from few exciting bands to the ground_state band for the (n,n’ ) process compared to TALYS and another experiment



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Results : 232Th

TALYS % Exp data* (n,2n ):

shape is well reproduced but factor of 2.58 in amplitude

* (n,3n ):

shape is well reproduced but factor of 0.72 in amplitudeNote: the uncertainties are rather large because of the combined

effects of the lower cross sections and the low neutron incident

flux



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235U:* New experimental data in disagreement with the only published one (n,2n )* Highlighted the TALYS difficulties to predicted with confidence (n,xn ) cross section (even if the total reaction cross section is well calculated)

232Th: * New experimental data in agreement with the published one (n,n’ )* Confirmed that, in TALYS, semi-classical exciton model simply fails to properly describe the spin distribution that accompanies the pre-equilibrium process (En

≥ 7 MeV).

@GELINA / FP16 – 30 m:* We have measured (n,xn ) reaction cross sections (x≥1) on 235U and 232Th* Uncertainties reached : 5 – 7 % (see J.C Thiry thesis 09/2010)

Results



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Towards the evaluation?

What about uncertainties?

!



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tNn

dd

nat

det)(

Cross section of a given γ ray transition [b]

Detected hits in a given ray:- Good statistics- Low background

Number of atoms in target

Incident neutron flux [s-1·cm-2]Fission chamber characteristics

detection efficiency- Measurements- Simulations

Acquisition time [s]




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parametersUncertainties

minUncertainties

maxcomments

Ndet(for one

detector)1% 15%

Long running time(235U: 1248h ; 232Th: 375h )

Neutron flux dependence

Nat 1.2 % (232Th) 2.7 % (235U)Precise mass, thickness, … measurements

n2.3 %

1 MeV <En

< 2 MeV

2.9 % En

< 1 MeV,

En

> 2 MeV

Simulation + calibration measurements

3 %

100 keV <E

< 1 MeV6 %

E

< 100 keVSimulation + calibration measurements

vary from 5% to 7% in the 0.5 – 9 MeV neutron energy rangeBut can reach up to 20 % in the high neutron energy range(where the neutron flux is low)



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Neutron flux : With this set-up, it’s certainly optimized

-> multi layers fission chamber (235U)?-> another independent neutron flux measurements with an another reference cross section, H(n,p)?

-> ~1.5 % ?


Is it possible to do better?

exp/ s

im

‐ray energy (keV)

HPGe detector efficiency :some improvements are still possible at low energy

-> by carrying out optimization of the simulation (dead zones, multiple diffusions)

-> ~1 % ? Total uncertainty in (n,n’) domain -> ~ 3%



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Total inelastic scattering cross section is the sum of the cross section carried by all transitions that directly decay to the ground-stateIn general case:

-Requires a good knowledge of spectroscopic parameters-Practically, the deduced inelastic cross section is a lower limit for the total inelastic cross section-> model prediction

From (n,xn ) to (n,xn) cross sections?

Measured (n,xn )

cross section

total (n,xn)

cross sectionSpectroscopicParameters

ELiE

i ki

ikinn

x

i

i LLpsgLpLLEE

)(

1,'' )(

.).(),()(

gs

Li

Lki

0AX

Theoretical model



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We have measured several (n,n’ ) cross sections and especially some more intense transitions to the ground_state

232Th case

The deduced 232Th(n,n’) cross section (from the measured (n,n’ ) ones) is very close to the total232Th inelastic cross section



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TALYS 1.0 (P.Romain, CEA/DAM)

235U(n,2n) case

We have measured only ~50% of the total cross section …

Strong model dependence-> What is the uncertainty???



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Conclusions

Accuracy business : we will continue our investigations and efforts to provide the (n,xn ) cross sections as accurate as possible !!!!

Deduced total reaction cross section : collaboration with theoreticians is essential

Dear evaluators and experimentalists colleagues : this workshop is the opportunity to discuss about our work so feel free to comment, to review…



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Outlooks

@GELINA / FP16 – 30 m* ANDES project (7ème PCRD)238U(n,xn ) reaction cross sections measurement -> NEA Nuclear Data High Priority Request List (2 – 3%)

* 182W, 183W, 184W, 186W(n,xn ) reaction cross sections measurement-> experimental data are very scarce-> “more simple” tests of model code

@SPIRAL2 / NFSDay one experiment proposal: “Comparison between activation and prompt spectroscopy as means of (n,xn) cross section measurements” (case of 90Zr(n,3n))

@ theoretical aspects* Collaboration with A.J. Koning (NRG, Petten)and P. Romain (CEA, Bruyères

le Châtel)

for TALYS

calculations* Collaboration with M. Sin (University of Bucharest)

for EMPIRE

calculations

Phys. Rev C80 014603 (2009)



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Collaboration, Financial support, Acknowledgement

Geel, Belgium

A. Plompen, C. Rouki, M. Staniou, J.C. Drohé

Strasbourg, France

Ph. Dessagne, E. Gasser, M. Kerveno, G.Rudolf, J.C. Thiry

C. Borcea, D. Deleanu, A.L. Negret

Bucharest, Romania

The authors thank the team of the GELINA facility for the preparation of the neutron beam and for their strong support day after day….

towards the evaluation

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