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The n_TOF Collaboration , cern.ch/nTOF

The n_TOF Collaboration , www.cern.ch/nTOF. CERN EN/STI Student’s Coffee (17 th April 2012) An n_TOF experiment: the complete process from the data taken to the published results Carlos Guerrero CERN Physics Dpt. Fellow.

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The n_TOF Collaboration , cern.ch/nTOF

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  1. The n_TOF Collaboration,www.cern.ch/nTOF CERN EN/STI Student’s Coffee (17th April 2012) An n_TOF experiment: the complete process from the data taken to the published results Carlos Guerrero CERN Physics Dpt. Fellow

  2. The complete process from the data taken to the published results • PHYSICS MOTIVATION BEHIND THE N_TOF EXPERIMENT AT CERN • ONE PARTICULAR EXPERIMENT: • Motivation • How we make such an experiment: • Neutron source • Detectors • Data taking • Analysis • What we do after!

  3. Physics Motivation behind the n_TOF Experiment at CERN NUCLEAR ASTROPHYSICS: stellar nucleosynthesis Neutron capture and (n,a) cross section of stable (& radioactive) medium mass isotopes playing a role in the s- and r-processes responsible of the creation of elements heavier than Fe in the starts. NUCLEAR TECHNOLOGIES: ADS, Gen-IV and Th fuel cycle, Nuclear Medicine Neutron capture and fission cross sections of Actinides and Fission Fragments in the thermal, epithermal and fast energy range. Also reactions of interest for Nuclear Medicine (for instance 33Sn,a) BASIC NUCLEAR PHYSICS: levels densities, g-ray strength funct. and angular distributions Time-of-flight measurements with dedicated detectors provide very valuable information on basic nuclear physics quantities. DETECTOR DEVELOPMENT AND TESTING, IRRADIATION WITH NEUTRONS: CVD diamonds, Fiber Bragg Gratings, Optical Fiber dosimeters, GEMs, MediPix, … The n_TOF Collaboration 38 Research Institutions from Europe, Asia and USA. Based at CERN: 5 FTE + 3-5 short term visitors E. Chiaveri (Spokesperson), E. Berthoumieux (Techn. Coord.) , C. Guerrero (Run Coord.), C. Weiss (PhD), A. Tsinganis (PhD) (Also V. Vlachoudis and M. Calviani)

  4. MOTIVATION

  5. Neutrons and production of nuclear energy (& radioactive waste) 244, 245Cm 1.5 Kg/yr Figura Nucleosintesi (frecce che si muovono) Foto FIC 241Am:11.6 Kg/yr 243Am: 4.8 Kg/yr 239Pu: 125 Kg/yr 237Np: 16 Kg/yr +Energy LLFP 76.2 Kg/yr FP Quantities refer to yearly production in 1 GWe LW reactor

  6. How do the competing (n,g) (n,f) cross sections look like? 235U (even-odd): FISSILE 238U (even-even): NON-FISSILE Fast neutrons Fast neutrons Thermal neutrons Thermal neutrons Similar behavior for all isotopes with ODD number of neutrons: 239,241Pu, …, 245Cm Similar behavior for all isotopes with EVEN number of neutrons: 237Np, …, 240,242Pu, 241,243Am

  7. Neutrons and production of nuclear energy (& radioactive waste) Existingreactorshave low burn-upefficiencyand produce largeamountofradioactivewaste. Using fast neutrons it is possible to fission all actinides, thus reducing the high level waste!! Generation IV reactors based on recycling of thespent fuel (in particular actinides). Another possibility is to use dedicated burners (Accelerator Driven Systems[ADS]) In all cases, a large reduction of actinide inventory is achieved by means of neutron-induced reactions with FAST NEUTRONS (subsequent captures and fission).

  8. Open Problems: Uncertainties in the capture 235U cross section The criticality of current and fast future reactors must be known within 0.3-0.5% for operation/safety. (FCA) Fast Critical Assembly (JAEA) GOAL: Measure 235U s(n,g) below 2.5 keV (First Goal: demonstrate feasibility) Sensitive mainly to the 235U(n,g) below 2.25 keV (RRR) Soft Hard Differences up to 2% in the measured and calculated criticality values for FCA (JAERI, Japan) assemblies with different hardness are due to 235U s(n,g) below 2.5 keV.

  9. GOAL: Measure 235U s(n,g) below 2.5 keV The concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. Reaction products I (neutrons/cm2/s) n (atoms/cm2) • Measuring the neutron cross sections requires: • A facility providing a neutron beam (The n_TOF facility). • A highly pure sample. • A detection system for counting the reactions (The TAC+MGAS set-up). • The analysis tools to determine the measured cross section with the required accuracy.

  10. The n_TOF facility

  11. The n_TOF Facility at CERN: a view n_TOF 185 m flightpath neutron time-of-flight experiment neutronstart neutronstop time 185 meters Booster 1.4 GeV Pb Spallation Target ProtonBeam 20 GeV/c 7x1012ppp PS 20 GeV NeutronBeam 10oprod. angle Linac 50 MeV

  12. The n_TOF Facility at CERN Experimental Area Access Area & Escape line & DAQ

  13. Detection system for (n,g) reactions [fissile isotopes] BUT… Total Absorption Calorimeter (TAC) for (n,g) 40 BaF2 crystals 4p geometry (95% coverage) 16% energy resolution at 662 keV Used for s(n,g) of actinides since 2004 g-rays n (n,el) reaction: neutrons (n,g) reaction: g-rays (n,f) reaction: Fission fragments g-rays [similar to (n,g)!!!] neutrons Results: distributions Esum, mcr & En C. Guerrero et al., NIM-A 608 (2009) 424-433

  14. Detection system for (n,g) reactions [fissile isotopes] We need to detect capture and fission reactions simultaneously! Total Absorption Calorimeter (TAC) for (n,g) MicroMegas (MGAS) for (n,f) 40 BaF2 crystals 4p geometry (95% coverage) 16% energy resolution at 662 keV Used for s(n,g) of actinides since 2004 Based on Bulk technology Double stage GAS detector: conversion +amplification ~90% efficiency for Fission Fragments (FF) Used for neutron monitoring since 2009 n FF1 g-rays n FF2 Use of TAC and MGAS in “anticoincidence”!! Results: distributions Esum, mcr & En Results: distributions Amp. & En C. Guerrero et al., NIM-A 608 (2009) 424-433 S. Andriamonje et al., NIM-A 481 (2002) 120–129

  15. Detection system for (n,g) reactions [fissile isotopes]

  16. Data taking at n_TOF Sounds familiar to n_TOF shifters? n_TOF DAQ n_TOF Run Control n_TOF Event Display

  17. Analysis and Results

  18. Analysis in coincidence of TAC and MGAS detectors Time and energy calibration of the TAC. Coincidence algorithm among the TAC crystals. Time calibration of the MGAS detectors with respect to the TAC Loop runs over all MGAS events (ordered in time for each neutron pulse) Loop over the TACselecting coincident candidates (with TOFTACЄ[TOFMGAS-300 ns, TOFMGAS+50]) Select TAC fission event. If more than one candidate, select the highest energy and multiplicity FWHM~20 ns

  19. Information from (n,g) [and fission g-rays] with the TAC Collimators g Protons (20 GeV) Neutrons (meV-GeV) g g Sweeping magnet Neutron energy Deposited energy Multiplicity

  20. RESULTS: deposited energy and multiplicity distributions Deposited energy (mcr>2) and multiplicity (Esum>3)distributions corresponding to resonances: 98% Sn(236U)~6.5 MeV All – fission – backg. Fission All – fission Background Fission 98% 68% 67% - 33% of fission events fall above Esum>7.5 MeV - 100% of capture events fall below Esum<7.5 MeV - 32% of fission events fall above mcr>9 - 98% of capture events fall below mcr<9

  21. RESULTS: neutron energy distributions Under the selected conditions, the counts/pulse information can be directly related to the corresponding cross sections

  22. RESULTS: Detection efficiencies: eMGAS(n,f), eTAC(n,f) and eTAC(n,g) With two different detectors and two different types of reactions to detect, it is important to define clearly the different efficiencies that play a role in the measurement and their interrelations. • When a neutron capture occurs, it can only be detected in the TAC → eTAC(n,g) When a fission reaction occurs, it can be detected: • in both detectors, → eMGAS(n,f) ∙ eTAC(n,f) • in none of them, → (1-eMGAS(n,f)) ∙ (1-eTAC(n,f)) • only in the MGAS→ eMGAS(n,f) ∙ (1-eTAC(n,f)) • only in the TAC. → (1-eMGAS(n,f)) ∙ eTAC(n,f) eMGAS(n,f), eTAC(n,f) and eTAC(n,g) • The efficiency for detecting fission reactions in each detector is independent from the other, but the calculation from experimental data requires that these four probabilities are properly taken into account.

  23. RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g) • Calculation of eFTMG(n,f) • MC simulations • Samples are 318 mg/cm2, nearly identical to those of the 235U samples (316 mg/cm2) used in FIC, for • which simulations with FLIKA give eMC(n,f)~0.94 (6% losses due to absorption in the sample). • Experimentally: • Fission events produce high-energy, high-multiplicity TAC events. Assumption → eTAC ~100% for such events. Then, the detection efficiency of the FTMGs can be calculated as the ratio of tagged to allevents for multiplicities higher than ~10 (no capture events). eexp(n,f)~0.90 eMC(n,f)~0.94 & eexp(n,f)~0.90 eFTMG(n,f)~0.92 Esum>7 MeV mcr>3

  24. RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g) • Calculation of eTAC(n,f) A coincident event in the TAC is found for 97% of the FTMG events (MGASamp>20 channels). This value represents the TAC efficiency for fission events, eTAC(n,f), and is very similar to the efficiency of eTAC(n,g)=0.974(4) for capture events in 197Au (from GEANT4 Monte Carlo simulations). The efficiencyeTAC(n,f) depends on the analysis conditions for the deposited energy and multiplicity values. • Efficiency of the TAC for detecting fission events under different conditions in deposited energy and crystal multiplicity.

  25. RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g) • Calculation of eTAC(n,g) Geant4 simulation of the TAC • The detection efficiency eTAC(n,g) can be calculated accurately by means of Monte Carlo simulations when both the experimental set-up and the details of the capture cascades are properly considered. eTAC(n,g)=0.70(3) [Esum>3 MeV and mcr>2] C. Guerrero et al., NIM-A 671 (2012) 108-117

  26. RESULTS: Capture and Fission Cross Sections of 235U

  27. RESULTS: Capture and Fission Cross Sections of 235U C. Guerrero et al. (The n_TOF Collaboration), Simultaneous measurement of neutron-induced capture and fission reactions at CERN, European Physical Journal A 48 (2012) 29

  28. The work is not over after finalizing the analysis! ⓪ Summer 2010: Data taken at n_TOF ① December 2010 / April 2011: Results presented to the n_TOF Collaboration ② June 2011: Results presented to the scientific community in the ANIMMA Int. Conf. You are welcome to visit nTOF any time!!! ③ October 2011: New proposal submitted (and accepted) to the CERN INTC committee: “Measurements of neutron-induced capture and fission reactions on U-235: cross sections and alpha ratios, photon strength functions and prompt gamma-ray from fission” ④ December 2011: Paper submitted to the European Physical Journal A ⑤ February 2012: Paper accepted for publication in the Eur. Phys. J. A 48 (2012) 29 ⑥ June 2012: New measurement (see ③) scheduled at n_TOF

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