N. Colonna, R. Sarmento and the n_TOF Collaboration cern.ch/nTOF

1. n_TOF Collaboration Meeting December 16th 2010, CNA Seville • Results on 236U(n,f) with FIC-0 N. Colonna, R. Sarmento and the n_TOF Collaboration www.cern.ch/nTOF

2. Outline Motivation Experimental setup Data analysis Results Conclusions

3. Motivation

4. The search for energy alternatives to the fossil fuels lead to a ”renaissance” of the interest in nuclear energy production. The current research on this area is directed towards the sustainability of nuclear reactor technology for energy production. • Improved operational safety • Economically competitive • Reduced nuclear waste

5. Solutions: • ADS: Accelerator-Driven Systems • Generation-IV Nuclear Reactors • Th-232 fuel cycle: U-236 build-up • Objective: • To know the U-236 neutron induced fission cross-section with a relative uncertainty below 5%.

6. Experimentaldata U-236 fission cross-section data from EXFOR (Experimental Nuclear Reaction Data)

7. Experimentaldata ENERGY WIDTH WIDTH-ERR EV MILI-EV MILI-EV 5.4500E+00 2.9000E-01 5.4500E+00 1.3000E-03 1.0E-04 5.4500E+00 1.7000E-03 1.0E-04 2.9900E+01 1.6000E-01 3.4000E+01 1.8000E-01 4.3700E+01 4.3000E-01 7.1100E+01 2.9000E-01 8.6400E+01 3.0000E-01 1.2080E+02 3.4000E-01 1.2470E+02 2.1000E-01 1.9400E+02 5.0000E-01 2.1400E+02 3.2000E-01 2.7240E+02 4.0000E-01 2.8820E+02 4.8000E-01 3.0250E+02 4.6000E-01 3.7100E+02 4.2000E-01 3.7900E+02 3.0000E-01 4.1500E+02 5.9000E-01 1.2688E+03 8.2000E-01 3.0E-01 * 1.2817E+03 7.7000E+00 5.0E+00 * 1.2917E+03 9.3000E-01 1.1E-01 * 2.9589E+03 1.4000E+00 6.0E-01 6.3000E+03 1.0800E+01 6.0E+00 * 1.0400E+04 4.6000E+00 2.6E+00 * Ressonance parameters: fission width 1972 - Theobald et al., Geel (width-error corresponds to 2.5% of the width) 1994 - Parker et al., Los Alamos 2008 - Wagemans et al., Geel *resonances observed but no resonance parameters Scarce and ambiguous data

8. Experimentaldata U-236/U-235 fission cross-section data from EXFOR (Experimental Nuclear Reaction Data)

9. Evaluateddata U-236 fission cross-section data from ENDF (Evaluated Nuclear Data File)

10. Summary Shortcomings attributed to the U-236 fission cross-section evaluated data 1. The thermal cross-section - discrepancy by two orders-of-magnitude 2. The 5.45 eV resonance width - discrepancy by two orders-of-magnitude 3. The resonance region is filled with false U-236 resonances 4. Intermediate energy resonances are missing 5. No agreement on the absolute value up to few hundred keVs

11. Experimental setup

12. Experimentalsetup • 1.56 x 1018 protons from dedicated beam • 4.52 x 1017 protons from parasitic beam • no-beam runs • fission colimator • FIC-0 detector • digitization by flash-ADCs working at a 250 MHz sampling rate. • four U-236 samples • two U-235 samples

13. Experimentalsetup FIC-0 One single fragment is detected per fission event - 2π detection efficiency. The other fission fragment is absorbed in the Al electrode backing the sample to measure.

14. Experimentalsetup FIC-0 • The ionization chamber operates in the ion saturation region - no avalanche multiplication occurs. • The fission cross‐section is measured by detecting the fission fragments (FF) - electrons and ion pairs are produced in the gas by the FF and this charge is collected by applying a voltage between the electrodes. • The gas was chosen by its fast timing properties to avoid pile-up problems.

15. Data analysis

16. Event reconstruction: time, amplitude and baseline • 2. Energy calibration • 3. Background subtraction: α-particles and impurities • 4. Corrections: efficiency and dead-time • 5. Extraction of the neutron flux • 6. Extraction of the cross-section: ratio method

17. Eventreconstruction M. Calviani The signal reconstruction from the raw data was made by applying C++ routines developed using the ROOT framework and based on the Advanced Spectra Processing Function class TSpectrum. Dedicated files are created storing the amplitude, baseline, time information and area of the signal peaks, as well as the time and intensity of the proton bunch originating the respective spallation neutrons.

18. Eventreconstruction M. Calviani Amplitude and time-of-flight spectra of FF Fission fragment (FF) digitized signature

19. Energycalibration • U-235 data • Agreement found in all measured energy range • The same parameters were used for the U-236 (exception of flight-path) Normalized n_TOF U-235 counts + U-235 fission cross-section from data libraries

20. Backgroundsubtraction • α-particles: • Contribution up to 1 keV • Activity from the samples • U-235 impurity in the U-236 samples: • 0.05% in mass • Ressonance structure from 10 eV up to 1 keV

21. Detectionefficiency The intrinsic detection efficiency was calculated by performing computational simulations with FLUKA. The values were obtained for setting a threshold on the energy deposited by the fission fragments at 35 MeV. M. Calviani

22. Dead-timecorrection • Important above 100 keV • High for the U-235, less than 3% for theU-236 Correction factor cr - count-rate δt : dead-time of the detector 220 ns

23. Neutronflux c - FF counts b - Background counts ε - Detection efficiency Δdt - Dead-time correction factor N - Sample thickness σ : Evaluated cross-section Flux from FIC-0 + Flux from other fission measurement made with FIC-1

24. Neutron flux High energy correction • Above 400 keV - need for correction • Change of sampling rate in the TOF-to-energy calibration • Different fractions of signals from dedicated and parasitic beams were accounted

25. Cross-section c - FF counts b - Background counts ε - Detection efficiency Δdt - Dead-time correction factor N - Sample thickness σ : Evaluated cross-section Obtained from the ratio between the U-236 and U-235 counts

26. Results

27. Results: low energyE(eV)<10 ENDF/B-VII JENDL/AC-2008 Alekseev et al. n_TOF 5.45 eV resonance

28. Results:10<E(eV)<103 • U-236 resonances observed in the evaluations: not detected • Originally from non-subtracted background in previous measurements

29. Results: intermediate energy103<E(eV)<104 JENDL/AC-2008 n_TOF Resonance triplet at 1.25 keV measured with excelent energy resolution

30. Results: intermediate energy103<E(eV)<104

31. Results: intermediate energy103<E(eV)<104

32. Results: below threshold104<E(eV)<105.5

33. Results: below threshold104<E(eV)<105.5 Results at energies below the threshold: follow well the evaluation of JEFF-3.1

34. Results: around the threshold105.5<E(eV) Results at and above the threshold energy of 1 MeV: good agreement with previous experimental data

35. Conclusions

36. Cross-section measured for the first time from 200 meV up to the threshold energy • The low energy cross-section - up to the 5.45 eV resonance - in most evaluated data libraries is highly overestimated. No U-236 resonances were observed from 10 eV up to 1 keV - explanation to be found for low cross-section • Confirmation of the intermediate energy structure with resonances measured above 1 keV • Justification problem at the threshold energy • Ongoing work on the resonance fitting and parameter determination