Neutron capture cross section of 234 U

1. Neutron capture cross section of 234U Walid DRIDI CEA/Saclay for the n_TOF Collaboration

2. Outline • Analysis procedure : • Subtraction of background • Dead time correction (MC simulation) • Detection efficiency (MC simulation) • Neutron flux • SAMMY Analysis • Results • Conclusion

3. 234U(n,γ) cross sections availables Transmission measurement of James et al.(1) The alone capture measurement published : Experiment of Muradyan et al.(2) 2sd capture measurement capture performed in 2004: Experiment in Los Alamos (2) Proceeding ISINN-7, JINR p.292 (1999) (1) Physical review C15, p.2023, 1977

4. Number of γcapture determination Subtraction background

5. Electronics PM BaF2 crystal MC simulation with GEANT4 • GEANT4 MC simulations include: • Very detailed TAC geometry (validated with multi-γ-rays sources) • 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).

6. MC simulation:TAC response to 234U The 5.16 eV of 234U resonance is reproduced by MC simulation

7. Dead time correction factor done on the experimental data Factor deduced from MC simulation Dead time Correction • The simulationstudy of dead time effects is done by studying the efficiency loss due to missed events. Efficiency loss

8. 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 : The correction factor equal to 0,99, close to 1, presents another proof of validation of our simulations. The TAC efficiency for Ncluster>1:

9. Beam interception Fraction (CIEMAT) Neutrons flux

10. Capture Yield of 234U

11. Analysis procedure : • Subtraction of background • Dead time correction (MC simulation) • Detection efficiency (MC simulation) • Neutron flux • SAMMY Analysis • Results • Conclusion

12. 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

13. Low energy region Our best fit is not able to fairly reproduce the shape of the first resonance with very low residual : Why ?

14. Sample modeling in SAMMY Ti-Al-U3O8 Ø10 Sample is modeling as an homogenous mixture of all components Sample of 234U in SAMMY Real geometry sample 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)

15. Some fitted resonances 250 < En < 400 eV 10 < En < 100 eV 750 < En < 1000 eV 1200 < En < 1500 eV

16. Analysis procedure : • Subtraction of background • Dead time correction (MC simulation) • Detection efficiency (MC simulation) • Neutron flux • SAMMY Analysis • Results • Conclusion

17. iis the number of found resonance εi statistic uncertainty done by SAMMY Level statistics (1/3) Average radiation width < Гγ >

18. Integral for distribution is : N0 total number levels without threshold Results N0= 136 ± 2 ( 123 observed levels), Average level spacing Level statistics (2/3) Reduced neutron width distribution Porter et Thomas distribution 10.92 ± 0.2 eV from Mughabgab

19. S0 is the slope the histogram presenting the cumulated value of according to energy S0 = (0,88 ± 0,03) . 10-4 Level statistics (3/3) S-wave strength fonction S0 ΔE, the range of the energy study i, the resonance number

20. Capture cross section for 234U Comparison between our results with ENDF/B-VI.8 and JENDL3.3

21. Capture cross section for 234U Average cross sections Comparison between our results with Muradyan Excellent agreement except on the first resonance

22. 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

23. Thank you for your attention

24. With : 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 Capture measurement principle To determine the neutron capture cross section of an isotope, is by the determination of experimental capture yield : Y(En)

25. Event selection criteria • Multiplicitéγ: • 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

26. Number of γ capture determination Raw Data : • Events selection criteria : Nclust > 1 • Energy spectra of three components : 1.) Background due to the Ti canning 2.) Activity 3.) 32.7 mg 234U + 1.) + 2.)

27. Empty canning All signals Number of γ capture determination Subtraction background

28. The negative resonance

29. The 1st resonance Thikness Effect Doppler broadening Гγeffect ГnEffect

30. 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.