walid dridi, cea/saclay n_tof collaboration meeting, paris december 4-5, 2006 dapnia neutron capture...

Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006

DAPNIA

Neutron capture cross section of 234U

Walid DRIDI

CEA/Saclay

for the n_TOF Collaboration


DAPNIA

Outline

Analysis procedure :

Subtraction of background

Dead time correction (MC simulation)

Detection efficiency (MC simulation)

Neutron flux

SAMMY Analysis

Results

Conclusion


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234U(n,γ) cross sections availables

Energy Uncertainty of σcapt(%)

Pt therm - 0,50 eV 5

0,50 – 450 eV 10

After 2,0 keV 20

The alone capture measurement published : Experiment of Muradyan et al.(2)

(2) Proceeding ISINN-7, JINR p.292 (1999)

Transmission measurement of James et al.(1)

(1) Physical review C15, p.2023, 1977

2sd capture measurement capture performed in 2004: Experiment in Los Alamos


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Subtraction background

Number of γ capture determination


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MC simulation with GEANT4

GEANT4 MC simulations include:

• Very detailed TAC geometry (validated with multi-γ-rays sources)

BaF2 crystal

PM

Electronics

• Event reconstruction similar to experimental data, including :

Energy resolution Threshold effectsDead time effects

• γ-rays cascade generator for (n, γ) processes using data for each nucleus (from J.L. Taín).


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MC simulation:TAC response to 234U

The 5.16 eV of 234U resonance is reproduced by MC simulation


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Dead time Correction

Efficiency loss

The simulation study of dead time effects is done by studying the efficiency loss due to missed events.

Factor deduced from MC simulation

Dead time correction factor done on the experimental data


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TAC detection efficiency

The detection efficiency which should be included in the analysis, is the efficiency found by simulation for the chosen criterion (Ncluster > 1) and the counting rate equal to zero, corrected by a factor of correction :

1 cluster

simulation

1 clusterTAC loss efficiency Simulated

TAC of loss efficiency Mesuredεε

%9,7899,07975,0εTAC

The correction factor equal to 0,99, close to 1, presents another proof of validation of our simulations.

The TAC efficiency for Ncluster>1:


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Neutrons flux

Beam interception Fraction (CIEMAT)

007,0198,0εBeam


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Capture Yield of 234U


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Analysis procedure :




Neutron flux

SAMMY Analysis

Results

Conclusion


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Normalization and sample thikness

The capture yield was normalized in order to reproduce the ENDF/B-VI.8 234U(n,γ) cross section in thermal energy range lower than 0,1 eV and the first resonance.

Procedure :

Normalization fitted

Neutron and radiation width are fixed (ENDF)

Sample thikness composition fitted

Results :

N234U = 0,950


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Low energy region

Our best fit is not able to fairly reproduce the shape of the first resonance with very low residual :

First resonance n_TOF ENDF/B-VI.8 JENDL3.3

Гγ (meV) 38,13 40 24,6

Why ?


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Sample of 234U in SAMMY

Real geometry sample Sample modeling in SAMMY

Sample is modeling as an homogenous mixture of all components

Ti-Al-U3O8

Ø10

Sample was inserted between two Al foils (0.15 mm) and encapsulated into 0.2 mm of Ti in order to fulfill the ISO 2919 certification (requested by the safety regulations at CERN)


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Some fitted resonances

10 < En < 100 eV 250 < En < 400 eV

750 < En < 1000 eV 1200 < En < 1500 eV


DAPNIAAnalysis procedure :




Neutron flux

SAMMY Analysis

Results

Conclusion


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Level statistics (1/3)

n_TOF ENDF/B-VI.8 JENDL-3.3

36,4 ± 1,5 meV 40 meV 26 meV

< Гγ >Average radiation width

i

i

i

i

i

2

2

1

1

i is the number of found resonance

εi statistic uncertainty done by SAMMY


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Reduced neutron width distribution

Porter et Thomas distribution

dxex

dxxdxxpx

22

2

1

Average level spacing eV2.00.111136

16.591.14900

D

N0= 136 ± 2 ( 123 observed levels), eV100,0120.987 30 n

g

Results

tx

t

t

xNdxxpNxN

2erf1

00

N0 total number levels without threshold

0

0

n

n

t g

gx

Enn

eV10

Integral for distribution is :

10.92 ± 0.2 eV from Mughabgab


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S-wave strength fonction S0

S0 = (0,88 ± 0,03) . 10-4

S0 is the slope the histogram presenting the cumulated value of according to energy0

n

i

in

n gED

gS 0

0

0

0

1

ΔE, the range of the energy study

i, the resonance number


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Capture cross section for 234U

Comparison between our results with ENDF/B-VI.8 and JENDL3.3


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Capture cross section for 234U

Average cross sections

Comparison between our results with Muradyan

Excellent agreement except on the first resonance


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Conclusions

We have proceeded the analysis by bin time for subtraction of background

Simulation of the TAC response to the 234U capture cascade with G4

TAC detection efficiency by simulation 197Au & 234U

Dead time correction by simulation

Absolute normalization (normalize to thermal cross section) using ENDF data

Capture Yield has been extracted up to ~1.5 keV

Several problems :

Some systematics errors may still remain in the data reduction procedure due to : Dead time, Pile-up, …

The Free Gas model is probably not well suited to describe the oxide target

Sample Modeling in SAMMY is inaccurate


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Thank you for your attentionThank you for your attention


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To determine the neutron capture cross section of an isotope, is by the determination of experimental capture yield : Y(En)

Capture measurement principle

),(

,,,,SY

detector

exp

n

nn

totalntotaln

nEf

E

EEBEEE

S – B : Number of capture counts without background

Φn(En) : Number of neutron per bunch impinging on the sample

εdétecteur : Detector efficiency

f : Correction factor

νγ : Events selection criteria

With :

detectiondetector

nnnEE

nTOFbeam


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Multiplicité γ :

• Improve signal at low energy

• Does not not change scattering to capture ration

Clustering improves capture relatively to ambient background but also γ and neutron scattering

Event selection criteria


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Events selection criteria : Nclust > 1

Energy spectra of three components :

1.) Background due to the Ti canning2.) Activity3.) 32.7 mg 234U + 1.) + 2.)

Raw Data :


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Empty canning

All signals

Subtraction background



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The negative resonance


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The 1st resonanceDoppler broadening

Гn Effect

Thikness Effect

Гγ effect


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Dead time modeling

Dead time corresponds to events missed

The interval time spectra between two succesive events impinging a

given detector : is supposing suited a Poissonian distribution

It could be seen by missing counts up to several µs.

walid dridi, cea/saclay n_tof collaboration meeting, paris december 4-5, 2006 dapnia neutron capture...

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