Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006 DAPNIA Neutron capture cross section of 234 U 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 234 U(n, γ ) cross sections availables Energy Uncertainty of σ capt (%) Pt therm - 0,50 eV5 0,50 – 450 eV10 After 2,0 keV20 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, sd 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) BaF 2 crystal PM Electronics Event reconstruction similar to experimental data, including : Energy resolution Threshold effects Dead 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 234 U The 5.16 eV of 234 U 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 (N cluster > 1) and the counting rate equal to zero, corrected by a factor of correction : The correction factor equal to 0,99, close to 1, presents another proof of validation of our simulations. The TAC efficiency for N cluster>1 :

Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006 DAPNIA Neutrons flux Beam interception Fraction (CIEMAT)

Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006 DAPNIA Capture Yield of 234 U

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 U(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 : N 234U = 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 resonancen_TOF ENDF/B-VI.8 JENDL3.3 Г γ (meV)38, ,6 Why ?

Walid DRIDI, CEA/Saclay n_TOF Collaboration Meeting, Paris December 4-5, 2006 DAPNIA Sample of 234 U in SAMMY Real geometry sample Sample modeling in SAMMY Sample is modeling as an homogenous mixture of all components Ti-Al-U 3 O 8 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 < E n < 100 eV 250 < E n < 400 eV 750 < E n < 1000 eV1200 < E n < 1500 eV

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 Level statistics (1/3) n_TOFENDF/B-VI.8JENDL ,4 ± 1,5 meV40 meV26 meV Average radiation width i is the number of found resonance ε i 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 Average level spacing N 0 = 136 ± 2 ( 123 observed levels), Results N 0 total number levels without threshold Integral for distribution is : ± 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 S 0 S 0 = (0,88 ± 0,03) S 0 is the slope the histogram presenting the cumulated value of according to energy 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 234 U 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 234 U 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 234 U capture cascade with G4 TAC detection efficiency by simulation 197 Au & 234 U 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 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(E n ) Capture measurement principle S – B : Number of capture counts without background Φ n (E n ) : Number of neutron per bunch impinging on the sample ε détecteur : Detector efficiency f : Correction factor ν γ : Events selection criteria With :

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 : N clust > 1  Energy spectra of three components : 1.) Background due to the Ti canning 2.) Activity 3.) 32.7 mg 234 U + 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 1 st resonance Doppler broadening Г n Effect Thikness Effect Г γ 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.