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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Outline
Analysis procedure :
Subtraction of background
Dead time correction (MC simulation)
Detection efficiency (MC simulation)
Neutron flux
SAMMY Analysis
Results
Conclusion
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Subtraction background
Number of γ capture determination
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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).
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
MC simulation:TAC response to 234U
The 5.16 eV of 234U resonance is reproduced by MC simulation
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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:
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Neutrons flux
Beam interception Fraction (CIEMAT)
007,0198,0εBeam
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Capture Yield of 234U
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Analysis procedure :
Subtraction of background
Dead time correction (MC simulation)
Detection efficiency (MC simulation)
Neutron flux
SAMMY Analysis
Results
Conclusion
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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 ?
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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)
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Some fitted resonances
10 < En < 100 eV 250 < En < 400 eV
750 < En < 1000 eV 1200 < En < 1500 eV
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIAAnalysis procedure :
Subtraction of background
Dead time correction (MC simulation)
Detection efficiency (MC simulation)
Neutron flux
SAMMY Analysis
Results
Conclusion
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Level statistics (2/3)
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Level statistics (3/3)
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Capture cross section for 234U
Comparison between our results with ENDF/B-VI.8 and JENDL3.3
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Capture cross section for 234U
Average cross sections
Comparison between our results with Muradyan
Excellent agreement except on the first resonance
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Thank you for your attentionThank you for your attention
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Number of γ capture determination
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 :
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
Empty canning
All signals
Subtraction background
Number of γ capture determination
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
The negative resonance
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
The 1st resonanceDoppler broadening
Гn Effect
Thikness Effect
Гγ effect
Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006
DAPNIA
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.
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