Measurement of the neutron capture cross section of the fissile isotope 235U with the CERN n-TOF total absorption calorimeter and a fission tagging based on micromegas detectors

EPJ Web of Conferences 146, 11021 (2017) DOI: 10.1051/epjconf/201714611021 ND2016 Measurement of the neutron capture cross section of the fissile isotope 235U with the CERN n TOF total absorption calorimeter and a fission tagging based on micromegas detectors J. Balibrea-Correa4, E. Mendoza4, D. Cano-Ott4,M.Krti ˇ cka6, S. Altstadt1, J. Andrzejewski2, L. Audouin3, V. B ´ ecares4, M. Barbagallo5,F.Be ˇ cv´ aˇ r6,F.Belloni 7, E. Berthoumieux7, J. Billowes8, V. Boccone, D. Bosnar9, M. Brugger10,F.Calvi ˜ no11, M. Calviani10, C. Carrapic¸o12, F. Cerutti10, E. Chiaveri10,7,M.Chin 10, N. Colonna5, G. Cort´ es11,M.A.Cort ´ es-Giraldo13, M. Diakaki14, C. Domingo-Pardo15, R. Dressler16,I.Dur ´ an17, C. Eleftheriadis18, A. Ferrari10,K.Fraval 7,V.Furman 19,K.G ¨ obel1,C.Guerrero 10,M.B.G ´ omez-Hornillos11, S. Ganesan20,A.R.Garc ´ ıa4, G. Giubrone15,I.F.Gonc¸alves12,E.Gonz ´ alez4, A. Goverdovski21, E. Griesmayer22,F.Gunsing 7,T.Heftrich 1, S. Heinitz16,A.Hern ´ andez-Prieto10,11,J.Heyse 23, D.G. Jenkins24, E. Jericha22,F.K ¨ appeler25,Y.Kadi 10, D. Karadimos14, T. Katabuchi26, V. Ketlerov21, V. Khryachkov21,N.Kivel 16, P. Koehler27, M. Kokkoris14,J.Kroll 6, C. Lampoudis7, C. Langer1, E. Leal-Cidoncha17, C. Lederer28,H.Leeb 22, L.S. Leong3, J. Lerendegui-Marco13,M.Licata 29,30, R. Losito10, A. Mallick20, A. Manousos18, J. Marganiec2,T.Mart ´ ınez4,C.Massimi 29,30, P. Mastinu31, M. Mastromarco5, A. Mengoni32, P.M. Milazzo33, F. Mingrone29,M.Mirea 34, W. Mondelaers23, C. Paradela17,A.Pavlik 28, J. Perkowski2, A.J.M. Plompen23, J. Praena13, J.M. Quesada13, T. Rauscher35,R.Reifarth 1, A. Riego-Perez11, M. Robles17, C. Rubbia10, J.A. Ryan8, M. Sabat´ e-Gilarte10,13, R. Sarmento12, A. Saxena20, P. Schillebeeckx23, S. Schmidt1, D. Schumann16, P. Sedyshev19, G. Tagliente5,J.L.Tain 15,A.Tarife ˜ no-Saldivia15,D.Tarr ´ ıo17,L.Tassan-Got 3, A. Tsinganis10, S. Valenta6, G. Vannini29,30,V.Variale 5,P.Vaz 12, A. Ventura29, M.J. Vermeulen24, V. Vlachoudis10,R.Vlastou 14,A.Wallner 36, T. Ware8, M. Weigand1,C.Weiss 22,T.Wright 8,P. ˇ Zugec9, and the n TOF Collaboration 1Goethe University Frankfurt, Germany 2University of Lodz, Poland 3Institut de Physique Nucl´ eaire, CNRS-IN2P3, Univ. Paris-Sud, Universit´ e Paris-Saclay, 91406 Orsay Cedex, France 4Centro de Investigaciones Energeticas Medioambientales y Tecnol´ ogicas (CIEMAT), Spain 5Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 6Charles University, Prague, Czech Republic 7CEA Saclay, Irfu, Gif-sur-Yvette, France 8University of Manchester, UK 9University of Zagreb, Croatia 10 European Organization for Nuclear Research (CERN), Switzerland 11 Universitat Polit` ecnica de Catalunya, Spain 12 Instituto Superior T´ ecnico, Lisbon, Portugal 13 Universidad de Sevilla, Spain 14 National Technical University of Athens, Greece 15 Instituto de F´ ısica Corpuscular, Universidad de Valencia, Spain 16 Paul Scherrer Institut (PSI), Villingen, Switzerland 17 University of Santiago de Compostela, Spain 18 Aristotle University of Thessaloniki, Thessaloniki, Greece 19 Joint Institute for Nuclear Research (JINR), Dubna, Russia 20 Bhabha Atomic Research Centre (BARC), India 21 Institute of Physics and Power Engineering (IPPE), Obninsk, Russia 22 Technische Universit¨ at Wien, Austria 23 European Commission, Joint Research Centre, Geel, Retieseweg 111, 2440 Geel, Belgium 24 University of York, UK 25 Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 26 Tokyo Institute of Technology, Japan 27 Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA 28 University of Vienna, Faculty of Physics, Vienna, Austria 29 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 30 Dipartimento di Fisica e Astronomia, Universit` a di Bologna, Italy 31 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy 32 Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy 33 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy c The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 146, 11021 (2017) DOI: 10.1051/epjconf/201714611021 ND2016 34 Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 35 Department of Physics, University of Basel, Switzerland 36 Australian National University, Canberra, Australia Abstract.The accuracy on neutron capture cross section of fissile isotopes must be improved for the design of future nuclear systems such as Gen-IV reactors and Accelerator Driven Systems. The High Priority Request List of the Nuclear Energy Agency, which lists the most important nuclear data requirements, includes also the neutron capture cross sections of fissile isotopes such as 233,235Uand239,241Pu. A specific experimental setup has been used at the CERN n TOF facility for the measurement of the neutron capture cross section of 235U by a set of micromegas fission detectors placed inside a segmented BaF2 Total Absorption Calorimeter. 1. Introduction The neutron capture cross sections of fissile isotopes play an essential role for the understanding of the actual critical nuclear systems and the development of future systems such as Gen-IV reactors and Accelerator Driven Systems [1]. The actual nuclear data required is summarized in the High Priority Request List of the Nuclear Energy Agency [2]. The measurement of the neutron capture cross section in such isotopes is difficult due to the dominant γ-ray background produced in the competing neutron-induced fission reactions. A specific experimental setup [3] has been used in the horizontal beam line at the CERN n TOF neutron time- of-flight (TOF) facility [4,5] to tackle this problem. The concept of the technique was proven in 2010 in a test experiment [3] and the real measurement on 235Uwas performed in 2012. The aim of this paper is to describe the experimental setup, the data analysis carried out and the results obtained in the neutron energy range from 0.2 to 20 eV. 2. Experimental setup The setup consists of a set of Fission Tagging Micromegas (FTMG) [6] detectors placed inside the segmented n TOF Total Absorption Calorimeter (TAC) [7]. During the experiment, ten isotopically enriched samples of 235U3O8(41.1±0.1 mg) with thickness 7.6·10−7At/barns were used. The samples were manufactured at the Joint Research Center-Geel [8]. The 235U targets were grouped in a stack of eight samples and other two encapsulated inside two FTMG detectors. A second configuration with ten FTMG detectors, one per 235U target, was used to cross check the neutron capture data obtained at low neutron energies. The first configuration was preferred in the measurement for the better signal to background ratio. The ensemble was placed inside a sealed fission chamber filled with a gas mixture of 88% Ar, 10% CF4and 2% isobutane at 1 atm for the operation of the FTMG detectors. The fission chamber was located in the center of the TAC surrounded by a 5 cm thick borated polyethylene neutron absorber in order to reduce the amount of scattered neutrons detected by the TAC. The capture and the neutron-induced fission reactions on 235U targets were simultaneously measured by the TAC and the FTMG detectors as a function of TOF. 3. Experimental yield determination The neutron-induced fission and capture yields are experimentally determined as a function of the neutron energy as Yn,x(En)=Ctot,x(En)−Cbkg,x(En) εx(En)·φ(En)(1) where Ctot,x(En) and Cbkg,x(En) are the total and the background counts registered by the TAC (x=γ)orby the FTMG detectors (x=f), εx(En) is the corresponding detection efficiency, and φ(En) the neutron fluence. For the TAC, many background components have to be subtracted: (i) the background related to the material that intercepts the beam; (ii) the activity of the BaF2crystals, ambient background and sample activity; (iii) the promptfission γbackground; (iv) and the background induced by the prompt-fission neutrons. The background due to the interaction of the neutron beam with the 235U targets, i.e., due to elastic scattering and delayed β-decay of the fission fragments, was negligible. Dedicated measurements with the same experimental configuration, including all dead material layers without the 235U targets, were performed in order to estimate the components (i) and (ii). Component (iii) is obtained accurately via the coincident events between the FTMG and TAC (Ctagg), by selecting the events with high γ-ray multiplicity which correspond only to (n,f) reactions [3,9]. This background component is calculated as Ctagg/ε f, where εfis the fission detection efficiency. The background component (iv) was calculated by Monte Carlo simulations. In the FTMG detectors, the energy thresholds were chosen to remove the α-particles from the sample activity, thus neglecting the background in these detectors. The shape of the neutron fluence has been verified by the ratio between the 235U(n,f) cross section obtained from the FTMG detectors and ENDF/B-VII.1 obtaining differences within the 2% in the neutron range of interest. 4. Normalization The normalization of the neutron capture cross section has been performed to the accurately known integral 235U fission cross section in the neutron energy range from E1=7.0toE 2=11.0eV, E2 E1σf(E)dE =(246±1) barns·eV, using the ratio of capture and neutron-induced fission yields. The neutron capture cross section integral is obtained in the thin target approximation as E2 E1 σγ(E)dE =E2 E1 σf(E)dEE2 E1 Ctot,γ (E)−Cbkg,γ (E) εγ(E)·φ(E)dE 1 εfE2 E1 Ctot,f(E) φ(E)dE (2) 2 EPJ Web of Conferences 146, 11021 (2017) DOI: 10.1051/epjconf/201714611021 ND2016 where, σγ(E), and σf(E) are the capture and neutroninduced fission cross sections, respectively, and assuming that εfdoes not depend on the neutron energy. In this approximation, the normalization depends neither on the mass of the samples nor on the absolute value of the neutron intensity or the beam area covered by the 235U targets, thus reducing the systematic uncertainties. Thereby, the accuracy of the normalization depends only on the statistics achieved and the determination of the efficiency for the capture and fission events. 4.1. Fission detection efficiency The fission detection efficiency εf, defined as the probability to detect a neutron-induced fission reaction in any of the ten 235U samples with the FTMG detectors, was experimentally obtained from the ratio of the number coincidences between the FTMG detectors and the TAC to the total number of neutron-induced fission reactions detected by the TAC. This ratio is the detection efficiency of the FTMG detectors under the assumption that the probability of detecting a fission reaction in the FTMG detectors does not depend on its detection in the TAC. The number of neutron-induced fissions in the TAC was calculated taking into account only events with total deposited energies (Esum) larger than the neutron separation energy of 236U, Sn(236U) = 6.55 MeV, thereby excluding the 235U(n,γ) reactions. The background contribution in such conditions was obtained from the dedicated background measurements. The fission detection efficiency was calculated in different 235U resonances under different conditions on Esum and crystal multiplicity (mcr ), finding compatible results. The value obtained was εf=0.1847 ±0.0022, where the uncertainty includes both the statistical and systematic uncertainties. We have verified experimentally that εfis constant in the neutron energy range of interest. 4.2. TAC detection efficiency for 235U(n,γ) events The TAC efficiency for neutron capture events has been accurately calculated by means of Monte Carlo simulations following the methodology described in [10,11]. The 235U(n,γ) cascades were generated using the DICEBOX code [12] and simulated in the TAC+FTMG geometry with a C++ application [13] based on the GEANT4 toolkit [14,15]. The excellent agreement between the Monte Carlo simulations and the experimental data shown in Fig. 1permit the TAC efficiency determination with an accuracy of 1%. 5. Results The 235U(n,γ) reaction rate is extracted by conditions on mcr and Esum, which maximize the signal to background ratio without compromising the statistics. The best results were obtained for 2<mcr <6 and 2.5<Esum(MeV) <7.0. Dead time corrections have been applied following a procedure similar to the methodology described in [16]. The ratio of the experimental resonance integrals to the evaluated values from the ENDF/B-VII.1 and the latest CIELO release (ib18o23g6cnu5ef1) are shown in Fig. 2and Fig. 3. The evaluated libraries were accurately broadened, processing the resonance parameters with (keV) sum E 0 1000 2000 3000 4000 5000 6000 7000 arb. units 0 0.05 0.1 0.15 0.2 0.25 -3 10× > 0 CR m > 1 CR m > 2 CR m > 3 CR m > 4 CR m Figure 1. Experimental 235U(n,γ) signature detected in the TAC for different mcr cuts compared with Monte Carlo simulations. The Monte Carlo simulations are plotted as dotted lines. En(eV) 0 2 4 6 8 10 12 14 16 18 20 ) ,ENDF/B-VII.1γ σ)/I( ,nTOFγ σI( 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 Statistical uncertainty Systematic uncertainty (bkg) Systamatic uncertainty (fission) Figure 2. Ratio between the 235U(n,γ) cross section measured at the CERN n TOF facility and the ENDF/B-VII.1 library in the 235U resonances from 0.2 eV to 20 eV. En(eV) 0 2 4 6 8 10 12 14 16 18 20 ) ,CIELOγ σ)/I( ,nTOFγ σI( 0.8 0.9 1 1.1 1.2 1.3 Statistical uncertainty Systematic uncertainty (bkg) Systamatic uncertainty (fission) Figure 3. Ratio between the 235U(n,γ) cross section measured at the CERN n TOF facility and the latest CIELO release in the 235U resonances from 0.2 eV to 20 eV. SAMMY8 code [17] and the experimental resolution. The black line represents the statistical uncertainty and the blue and the red lines represent the systematic uncertainties due to the subtraction of the beam and the prompt-fission background components, respectively. Other sources of systematic uncertainty were neglected in this neutron energy range. The n TOF neutron capture data is, on average, ∼6% larger than the ENDF/B-VII.1 evaluation in this neutron 3 EPJ Web of Conferences 146, 11021 (2017) DOI: 10.1051/epjconf/201714611021 ND2016 energy range. Larger discrepancies are observed with the CIELO evaluation as shown in Fig. 3. However, taking into account all the uncertainties, the n TOF data are still compatible with both libraries. 6. Summary and conclusions The 235U neutron capture cross section has been measured at the n TOF facility relative to the well known 235U neutron-induced fission cross section. The two critical parameters for the normalization have been determined: (i) the TAC detection efficiency for 235U(n,γ) events and (ii) the fission detection efficiency. Our capture cross section data is, on average, ∼6% larger than the ENDF/B-VII.1 evaluation in the neutron energy range from 0.2 to 20 eV. Larger discrepancies are observed with the latest CIELO library. The main sources of systematic uncertainties have been carefully determined: (i) the systematic effects associated with the fission γ-ray cascades; (ii) the beam background, and (iii) the dead time corrections. In the investigated neutron energy range component (i) is important. The 235U neutron capture cross section has been made available to the CIELO project and will be sent soon to EXFOR. This work was supported in part by the Spanish national company for radioactive waste management ENRESA, through the CIEMAT-ENRESA agreements on ’Transmutaci´ on de residuos radiactivos de alta actividad’ the Spanish Plan Nacional de I+D+i de F´ ısica de Part´ ıculas (project FPA2014-53290-C2-1) and the CHANDA project of the 7 th Framework Programme. References [1] A.J. Knoning et al. “CANDIDE: Nuclear data for sustainable nuclear energy” (2009) [2] Nuclear Energy Agency “Nuclear Data High Priority Request List ” Available at http://www.oecd-nea.org/dbdata/hprl/ hprlview.pl?ID=430 [3] C. Guerrero et al. Eur. Phys. J.A 48, 29 (2012) [4] C. Guerrero et al. Eur. Phys. J.A 49, 27 (2013) [5] F. Mingrone et al. EPJ Web of Conferences 122, 05001 (2016) [6] S. Andriamonje et al. J. of Korean Phys. Soc. 59, 1597–1600 (2011) [7] C. Guerrero et al. Nucl. Inst. Meth. A 608, 424–433 (2009) [8] Joint Research Center https://ec.europa.eu/ jrc/en [9] M. Jandel et al. Phys. Rev. Lett. 109, 202506 (2012) [10] C. Guerrero et al. Phys. Rev. C 85, 044616 (2012) [11] E. Mendoza et al. Phys. Rev. C 90, 034608 (2014) [12] F. Beˇ cv´ aˇ r Nuc. Inst. and Meth. A 417, 434–449 (1998) [13] C. Guerrero et al. Nuc. Inst. and Meth. A 671, 108–117 (2012) [14] S. Agostinelli et al. Nucl. Inst. and Meth. A 506, 250–303 (2003) [15] J. Allison et al. Nucl. Inst. and Meth. A 835, 186–225 (2016) [16] E. Mendoza et al. Nucl. Inst. and Meth. A 768, 55–61 (2014) [17] N.M. Larson, Oak Ridge National Laboratory, Oak Ridge, TN, Report No. ORNL/TM-9179/R7, 2006 4