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The n_TOF Collaboration , cern.ch/nTOF

The n_TOF Collaboration , www.cern.ch/nTOF. CERN-INTC-2011-045 / INTC-P-309 Measurements of neutron-induced capture and fission reactions on 235 U: cross sections, a ratios, photon strength functions and prompt g -ray from fission

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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-INTC-2011-045 / INTC-P-309 Measurements of neutron-induced capture and fission reactions on 235U: cross sections, a ratios, photon strength functions and prompt g-ray from fission C. Guerrero (CERN), E. Berthoumieux (CEA) and D. Cano-Ott (CIEMAT) on behalf of The n_TOF Collaboration

  2. 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 Competition between fission and capture 237Np: 16 Kg/yr +Energy LLFP 76.2 Kg/yr FP Quantities refer to yearly production in 1 GWe LW reactor

  3. Neutrons and production of nuclear energy (& radioactive waste) Existingreactorshave low burn-upefficiencyand produce largeamountofradioactivewaste. Generation IV or ADS reactors based on recycling of thespent fuel (in particular actinides). In all cases, a large reduction of actinide inventory is achieved by means of neutron-induced reactions (subsequent captures and fission). DIFF. FUEL COMPOSITION → DIFF. NEUTRON ENERGY RANGE → POORLY KNOWN s(n,x)

  4. Effects of the 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) Sensitive mainly to the 235U(n,g) below 2.25 keV (RRR) Source: NEA-WPEC 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.

  5. Status of the235U capture cross section and a ratio + From comparison between evaluations and data *From NEA-HPRL

  6. Prompt g-ray emission from fission & capture The g−heating at the centre of a typical fast reactor core: 20% from the γ produced in capture (fairly well known) 40% from the prompt γ emitted by fission fragments, 30% from the delayed γ-rays produced by fission products, 10% from the inelastic scattering reactions (fairly well known) Characteristics of γ-ray emission are MULTIPLICITY, TOTAL ENERGY and SPECTRUM SHAPE. Present calculations based on data from the 70s, which are in 20-30% disagreement with each other and do not contain any information on possible correlations with the neutron energy. Characteristics 235U(n,f) prompt g-ray emission now in NEA High Priority Request List: New data for the 235U case will be of direct use in reactor calculations and will also help to test and fine-tune the physics models used for many other actinides for which no data are available yet. • The TAC response to capture reactions contains basic information on the properties of the compound nucleus (A+1Z*): • - Level densities • - Transition Probabilities (Photon Strength Functions) • Investigation on other actinides already carried out at n_TOF

  7. Difficulty of measuring s(n,g) of fissile isotopes Measurements of AZ s(n,g) Detection of g-ray cascade from the compound nucleus A+1Z If AZ fissile: g-rays also produced in fission reactions!

  8. Experimental set-up: capture and fission detectors combined! • The n_TOF Total Absorption Calorimeter • 4p geometry (high tot. absorption effic.) • 40 BaF2 crystals (segmentation) • Good energy resolution (back. discrimin.) • Used extensively for (n,g) on actinides • The n_TOF MicroMegas • Two gas regions: conversion & amplification • High gain and low noise • Possibility for several samples in parallel. • Used extensively for (n,f) and (n,a)

  9. CERN-INTC-2010-037 ; INTC-I-105 Validation of simultaneous measurement of capture and fission reactions at n_TOF TAC Measurement performed in September 2010 BaF2 module Total - fission-backg. Fission Total – fission Background Fission HV (drift) MGAS neutrons C. Guerrero et al., A new set-up for the simultaneous measurements of neutron-induced capture and fission reactions at n_TOF, Proc. Int. Conf. ANIMMA-2011 (Belgium, June 2011 ) C. Guerrero et al., Simultaneous measurement of neutron-induced capture and fission reactions at CERN, submitted to the n_TOF Ed. Board (to be submitted to NIM-A) MGAS with 3 235U samples Capture dominated resonances MGAS signals High fission contribution Signal+Mesh Agreement with evaluations within 3-4% Goal: study the URR in 2012!! 3 MGAS detectors each equipped with a 1 mg 235U sample

  10. Objectives and Beam Time Request • Required accuracy of the capture cross section: 5% (RRR) and 8% (URR) • BEAM TIME REQUEST so to reach statistical uncertainties lower than 3% • The constraints of the measurements are mainly: • Samples have to be thin (<300 mg/cm2) so that fission fragments can escape. • There is no space in the TAC for more than 8-10 samples, yielding a total mass of ~40 mg. • The counting statistics of the capture reactions is the liming factor, since the fission cross section and the efficiency of the MGAS detector are larger. • At least 6000 bins per decade are needed in order to resolve the resonances in the keV region. • The lowest counting rate per bin will correspond to the RRR in the keVregion. • A minimum of ~1000 counts integrated over each resonance are needed to reach the 3% stat. unc. • The information from several resonances will be combined for the study of g-ray emission from capture and fission and therefore the statistics acquired will not be a limiting factor. IRMM (now a member of n_TOF) will manufacture the samples and deliver them by June 2012

  11. Objectives and Beam Time Request

  12. Objectives and Beam Time Request Auxiliary measurements will help to identify and determine the different sources of background: • Sample-out beam-On measurements: a similar set-up without 235U samples will provide data on the overall background of the measurement. • Beam-off: a measurement without beam will serve to identify and quantify the room background. • 197Au: the measurement will be performed with a sample of 4.5 cm in diameter and serves for normalization purposes. • Single sample-upstream/sample-downstream: we plan to do two measurements leaving only one of the 235U samples in place: once the one upstream and another time only the one downstream. This will serve to quantify the dependence of the TAC response to the spatial origin of the g-ray emission. • Graphite: the measurement of a “pure-scatterer” sample will serve to determine the TAC response to scattered neutrons.

  13. Conclusions • Measurements of neutron-induced capture and fission reactions on 235U (FISSILE!!!): • Capture cross section • Fission cross section • a Ratio • Prompt g-ray from fission: g-ray heating in reactor cores. • Prompt g-ray from capture: Photon Strength Functions • GOAL: MEASURE BOTH CROSS SECTIONS WITHIN 5% • The n_TOF TAC and MGAS detectors will be combined • Individual detectors intensively used nowadays at n_TOF • Combined detection system tested successfully in 2010 • Samples will be produced at IRMM by June 2010 • <3% statistical uncertainty required in the full energy range • BEAM REQUEST: 4x1018protons (includes auxiliary backg. measurements)

  14. Additional slides

  15. Study of Photon Strength Functions on Actinides (TAC) Detection of the capture cascades from 234U, 237Np and240Pu with the TAC provide information regarding the corresponding compound nuclei. mcr=1 mcr=2 mcr=3 mcr=6 mcr=4 mcr=5

  16. Study of Photon Strength Functions on Actinides (TAC) Characteristics of the cascade depend on: levels (known+LD) & transition probabilities (PSF) J.L. Tain et al. NIM A 571 (2007) 719-727 Sn+En Statistical models (LD & PSF) (n,g) TAC Known level scheme ? A+1Z

  17. Study of Photon Strength Functions on Actinides (TAC) • DECAY GENerator (DECAYGEN) • Low energy level scheme (KNOWN!) • Level density (KNOWN!) • Photon Strength Functions (E1,M1,E2) Capture Cascades: Egi=1,mg • Geant4 MC simulation of the TAC • Detailed detector geometry • Geant4 Standard EM physics • Realistic event reconstruction Assumption on PSF Comparison Experimental TAC response (Esum, mcr) Simulated TAC response (Esum, mcr)

  18. Study of Photon Strength Functions on Actinides (TAC) 241Pu Pygmy (or spin-flip) mode associated to oscillations/vibrations neutron vs. protons PSF from RIPL-2 PSF from RIPL-2 Tuned PSF PSF from RIPL-2 PSF from RIPL-2 Tuned PSF PSF from RIPL-2 PSF from RIPL-2 Tuned PSF PSF from RIPL-2 PSF from RIPL-2 Tuned PSF PSF from RIPL-2 PSF from RIPL-2 Tuned PSF

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