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(240,)242 Pu( n,f ) measurement update

(240,)242 Pu( n,f ) measurement update. n_TOF Annual Collaboration Meeting Manchester, May 7-9, 2013. Outline. Detectors Time-evolution of detector response Analysis Latest data Resonance region, fission threshold High-energy region Simulations Summary.

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(240,)242 Pu( n,f ) measurement update

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  1. (240,)242Pu(n,f) measurement update n_TOF Annual Collaboration Meeting Manchester, May 7-9, 2013

  2. Outline • Detectors • Time-evolution of detector response • Analysis • Latest data • Resonance region, fission threshold • High-energy region • Simulations • Summary 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  3. Detector deterioration: the case of 240Pu • Very high α-activity of the samples (~6.4MBq/sample) • An unexpected alteration of some detectors was observed after removal from the chamber • A distinct discoloration of the micromesh is visible in the 4 detectors used with the 240Pu samples • Physical damage to the micromesh is visible under inspection with a microscope • This leads to a deterioration of the electrical field and a severe reduction of the gain • A unique – if inadvertent – ageing test 3cm 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  4. Detector deterioration: the case of 242Pu • The α-activity of the 242Pu samples is considerably lower (~0.13MBq/sample) • A similar visible discoloration was NOT observed • …but the detectors have not been studied under a microscope • A slow but significant deterioration of the gain is observed! • Now visible as data covers a few months of beam-time • Beam-off runs in det. #1 (242Pu) • Evolution during 2012 run 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  5. Detector deterioration: the case of 242Pu • Interestingly, one of the 4 detectors shows a sudden deterioration in mid-June • …goes from “normal” running to something that resembles the damaged 240Pu detectors • This happens over 2 days… • June 13-15, 2012 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  6. Detector deterioration: the case of 242Pu Start of 2012 run 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  7. Detector deterioration: the case of 242Pu End of 2012 run 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  8. Detector deterioration: the case of 242Pu • The quality of alpha / fission fragment separation suffers accordingly Start of 2012 run 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  9. Detector deterioration: the case of 242Pu • The quality of alpha / fission fragment separation suffers accordingly End of 2012 run 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  10. How does this affects the analysis? • Data need to be analysed in smaller bunches • This complicates things a bit on the software side… • …and makes the uncertainty estimations somewhat more problematic… 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  11. 235U(n,f) & 242Pu(n,f) – recorded counts (1000 bins per energy decade – ~60% of total acquired data) Absorption dips Resonances Baseline oscillation Spontaneous fission background 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  12. 235U(n,f) cross section • Using the evaluated n_TOF neutron flux… 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  13. 242Pu spontaneous fission background • The spontaneous fission background dominates the low energy region and remains visible up to about 1 keV. • Still, several resonances can be observed above this background. • SF background fitted with appropriate function 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  14. High-energy analysis • One step added in the data-processing flow • Matching structure of ROOT files created conventionally • Using the same analysis routines to extract the cross sections Cross section PEAK SEARCH Raw data DST files ROOT files ANALYSIS * RunNumber * EventNumb * PSpulse * PulseInte * FIMGdetn * FIMGsatfl * FIMGindex * FIMGtof Raw data ROOT files COMPENSATION * Detn * Time * ampl * eve * PSPulse * prot 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  15. High-energy analysis • Micromegas response to γ-flash • Initial γ-flash signal (hundreds of ns) • Baseline oscillation after the γ-flash lasts several μs and affects the high-energy data (>MeV) • “Compensation method” • Oscillations recorded in adjacent detectors for the same proton bunch are almost identical • Baseline oscillation can be subtracted from adjacent detector • One step added in the data-processing flow • Raw data are cleaned, the same peak-search routine is used to process the clean data • Only dedicated pulses are used (t0 recognition difficult in parasitic pulses) 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  16. Resolved resonances – Threshold region (normalised to ENDF) • First resonance at ~2.7eV • Visible over spontaneous fission background • Resolved resonances • At ~780eV and ~1830eV, two resonances not attributable to any of the stated sample impurities (at a preliminary analysis) • Fission threshold and above • Max. energy limit will be pushed as high as possible 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  17. Resolved resonances – Threshold region (normalised to ENDF) • First resonance at ~2.7eV • Visible over spontaneous fission background • Resolved resonances • At ~780eV and ~1830eV, two resonances not attributable to any of the stated sample impurities (at a preliminary analysis) • Fission threshold and above • Max. energy limit will be pushed as high as possible 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  18. Simulations • Reproducing signals from α-particles and fission fragments • Simplified geometry of samples and active detector volume created in FLUKA • Energy deposition is scored event-by-event and transformed to charge • Electron drift velocity calculated for given gas mixture and drift voltage (GARFIELD) • We can thus calculate the current reaching the readout electrode and “pass” it through an appropriate circuit to reconstruct the signal • A large time window is selected in which we randomly add: • white noise • α-particle and fission fragment pulses (at appropriate rates) Gas V Sample dE/dz 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  19. Simulations • Left: fission fragment pulse-height spectrum • Comparison of the real amplitude distribution of simulated fission fragments with the distribution extracted by the pulse analysis routine • Right: how does α-pile-up (especially in 240Pu) affect the α-particle amplitude spectrum? • A long tail at higher amplitudes appears (just like in experimental data) • Loss of fission fragments inside sample: <5% for 235U, <1% for 240,242Pu • Depending on the amplitude cut, the correction (i.e. fraction fission fragments included in the cut) varies between 80-95% 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  20. Summary • Outstanding issues • Efficiency / amplitude cut correction • Final breakdown of run groups for analysis • Data merging, uncertainty calculation • High-energy region • Unidentified resonances • Proceed with 2011 data • How should results obtained over time be combined to obtain the correct uncertainties? 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

  21. The end 240,242Pu(n,f) measurement | AT n_TOF Analysis Group Meeting - Manchester, May 7-9, 2013

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