The importance of improving the accuracy on the capture cross-section of 238U has been addressed by the Nuclear Energy Agency, since its uncertainty significantly affects the uncertainties of key design parameters for both fast and thermal nuclear reactors. Within the 7th framework programme ANDES of the European Commission three different measurements have been carried out with the aim of providing the 238U(n,γ) cross-section with an accuracy which varies from 1 to 5%, depending on the energy range. Hereby the final results of the measurement performed at the n_TOF CERN facility in a wide energy range from 1 eV to 700 keV will be presented

Altstadt, S.

Andrzejewski, J.

Audouin, L.

Barbagallo, M.

Belloni, F.

Berthoumieux, E.

Bečvář, F.

Billowes, J.

Boccone, V.

Bosnar, D.

Brugger, M.

Bécares, V.

Calviani, M.

Calviño, F.

Cano-Ott, D.

Carrapiço, C.

Cerutti, F.

Chiaveri, E.

Chin, M.

Colonna, N.

Cortés, G.

Cortés-Giraldo, M. A.

Diakaki, M.

Domingo-Pardo, C.

Dressler, R.

Durán, I.

Eleftheriadis, C.

Ferrari, A.

Fraval, K.

Furman, V.

Ganesan, S.

García, A. R.

Giubrone, G.

González, E.

Gonçalves, I. F.

Goverdovski, A.

Griesmayer, E.

Guerrero, C.

Gunsing, F.

Gómez-Hornillos, M. B.

Göbel, K.

Heftrich, T.

Hernández-Prieto, A.

Heyse, J.

Jenkins, D. G.

Jericha, E.

Kadi, Y.

Karadimos, D.

Katabuchi, T.

Ketlerov, V.

Khryachkov, V.

Kivel, N.

Koehler, P.

Kokkoris, M.

Kroll, J.

Krtička, M.

Käppeler, F.

Lampoudis, C.

Langer, C.

Leal-Cidoncha, E.

Lederer, C.

Leeb, H.

Leong, L. S.

Lerendegui-Marco, J.

Losito, R.

Mallick, A.

Manousos, A.

Marganiec, J.

Martínez, T.

Massimi, C.

Mastinu, P.

Mastromarco, M.

Mendoza, E.

Mengoni, A.

Milazzo, P. M.

Mingrone, F.

Mirea, M.

Mondelaers, W.

Paradela, C.

Pavlik, A.

Perkowski, J.

Plompen, A. J. M.

Praena, J.

Quesada, J. M.

Rauscher, T.

Reifarth, R.

Riego-Perez, A.

Robles, M.

Rubbia, C.

Ryan, J. A.

Sabaté-Gilarte, M.

Sarmento, R.

Saxena, A.

Schillebeeckx, P.

Schmidt, S.

Schumann, D.

Sedyshev, P.

Tagliente, G.

Tain, J. L.

Tarifeño-Saldivia, A.

English

n_TOF Collaboration

KITopen

EPJ Web of Conferences 146, 11028 (2017) DOI: 10.1051/epjconf/201714611028ND2016High precision measurement of the radiative capturecross section of 238U at the n TOF CERN facilityF. Mingrone1,2,a, S. Altstadt3, J. Andrzejewski4, L. Audouin5, V. Be´cares6, M. Barbagallo7, F. Becˇva´rˇ8, F. Belloni9,E. Berthoumieux9, J. Billowes10, V. Boccone1, D. Bosnar11, M. Brugger1, F. Calvin˜o12, M. Calviani1, D. Cano-Ott6,C. Carrapic¸o13, F. Cerutti1, E. Chiaveri1,9, M. Chin1, N. Colonna7, G. Corte´s12, M.A. Corte´s-Giraldo14, M. Diakaki15,C. Domingo-Pardo16, R. Dressler17, I. Dura´n18, C. Eleftheriadis19, A. Ferrari1, K. Fraval9, V. Furman20, K. Go¨bel3,M.B. Go´mez-Hornillos12, S. Ganesan21, A.R. Garcı´a6, G. Giubrone16, I.F. Gonc¸alves13, E. Gonza´lez6, A. Goverdovski22,E. Griesmayer23, C. Guerrero1, F. Gunsing9, T. Heftrich3, A. Herna´ndez-Prieto1,12, J. Heyse24, D.G. Jenkins25, E. Jericha23,F. Ka¨ppeler26, Y. Kadi1, D. Karadimos15, T. Katabuchi27, V. Ketlerov22, V. Khryachkov22, N. Kivel17, P. Koehler28,M. Kokkoris15, J. Kroll8, M. Krticˇka8, C. Lampoudis9, C. Langer3, E. Leal-Cidoncha18, C. Lederer29, H. Leeb23,L.S. Leong5, J. Lerendegui-Marco14, R. Losito1, A. Mallick21, A. Manousos19, J. Marganiec4, T. Martı´nez6, C. Massimi2,30,P. Mastinu31, M. Mastromarco7, E. Mendoza6, A. Mengoni32, P.M. Milazzo33, M. Mirea34, W. Mondelaers24, C. Paradela18,A. Pavlik29, J. Perkowski4, A.J.M. Plompen24, J. Praena14, J.M. Quesada14, T. Rauscher35, R. Reifarth3, A. Riego-Perez12,M. Robles18, C. Rubbia1, J.A. Ryan10, M. Sabate´-Gilarte1,14, R. Sarmento13, A. Saxena21, P. Schillebeeckx24, S. Schmidt3,D. Schumann17, P. Sedyshev20, G. Tagliente7, J.L. Tain16, A. Tarifen˜o-Saldivia16, D. Tarrı´o18, L. Tassan-Got5,A. Tsinganis1, S. Valenta8, G. Vannini2,30, V. Variale7, P. Vaz13, A. Ventura2, M.J. Vermeulen25, V. Vlachoudis1,R. Vlastou15, A. Wallner36, T. Ware10, M. Weigand3, C. Weiss23, T. Wright10, P. ˇZugec11, and the n TOF Collaboration1 European Organization for Nuclear Research (CERN), Switzerland2 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy3 Goethe University Frankfurt, Germany4 University of Lodz, Poland5 Institut de Physique Nucle´aire, CNRS-IN2P3, Univ. Paris-Sud, Universite´ Paris-Saclay, 91406 Orsay Cedex, France6 Centro de Investigaciones Energe´ticas Medioambientales y Tecnolo´gicas (CIEMAT), Spain7 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy8 Charles University, Prague, Czech Republic9 CEA Saclay, Irfu, Gif-sur-Yvette, France10 University of Manchester, UK11 University of Zagreb, Croatia12 Universitat Polite`cnica de Catalunya, Spain13 Instituto Superior Te´cnico, Lisbon, Portugal14 Universidad de Sevilla, Spain15 National Technical University of Athens, Greece16 Instituto de Fı´sica Corpuscular, Universidad de Valencia, Spain17 Paul Scherrer Institut (PSI), Villingen, Switzerland18 University of Santiago de Compostela, Spain19 Aristotle University of Thessaloniki, Thessaloniki, Greece20 Joint Institute for Nuclear Research (JINR), Dubna, Russia21 Bhabha Atomic Research Centre (BARC), India22 Institute of Physics and Power Engineering (IPPE), Obninsk, Russia23 Technische Universita¨t Wien, Austria24 European Commission, Joint Research Centre, Geel, Retieseweg 111, 2440 Geel, Belgium25 University of York, UK26 Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany27 Tokyo Institute of Technology, Japan28 Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA29 University of Vienna, Faculty of Physics, Vienna, Austria30 Dipartimento di Fisica e Astronomia, Universita` di Bologna, Italy31 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy32 Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy33 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy34 Horia Hulubei National Institute of Physics and Nuclear Engineering, Romaniaae-mail: federica.mingrone@cern.chc© 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, 11028 (2017) DOI: 10.1051/epjconf/201714611028ND201635 Department of Physics, University of Basel, Switzerland36 Australian National University, Canberra, AustraliaAbstract. The importance of improving the accuracy on the capture cross-section of 238U has been addressedby the Nuclear Energy Agency, since its uncertainty significantly affects the uncertainties of key designparameters for both fast and thermal nuclear reactors. Within the 7th framework programme ANDES of theEuropean Commission three different measurements have been carried out with the aim of providing the238U(n,γ ) cross-section with an accuracy which varies from 1 to 5%, depending on the energy range. Herebythe final results of the measurement performed at the n TOF CERN facility in a wide energy range from 1 eVto 700 keV will be presented.1. IntroductionNowadays, nuclear energy represents one of a limitednumber of options available at scale to reduce greenhouse-gas emissions. In this framework, the design of newnuclear reactors rests its foundations upon accurate andprecise nuclear data, a list of the most urgent requirementsof which has been compiled by the Nuclear Energy Agency[1]. This includes, the measurement of the 238U(n,γ )reaction cross-section because of its key role in thedesign calculations of nuclear reactors, governing thebehaviour of the reactor core. In particular, in the fastregion of the neutron spectrum, which is fundamental forthe calculation of fast reactors, inconsistencies betweenpublished experimental data can reach 15%, and themost recent evaluations disagree with each other. Theassessment of nuclear data uncertainties for innovativereactor systems shows that the uncertainty in the radiativecapture cross-section of 238U should be further reduced to1-3% in the energy region from 20 eV to 25 keV.To achieve these requirements, within the 7th frame-work programme ANDES of the European Commissionthree independent measurements have been carried out,two at n TOF and one at JRC-Geel [2]. The final resultsof the 238U(n,γ ) measurement performed at the n TOFCERN facility will be here presented. It was carriedout with a detection system constituted of two liquidscintillators. In the data analysis, special attention wasdevoted to the identification of all sources of backgroundand to the accurate determination of the various systematicuncertainties.2. Experimental setupAt the neutron time-of-flight facility of CERN, n TOF,neutrons are generated in spallation reactions by a pulsed20 GeV/c proton beam impinging on a lead block. Theneutron source is surrounded by 4 cm of water plus 1 cm ofborated water, which serves as a coolant and as a moderatorof the originally fast neutron spectrum [3]. The resultingwhite neutron beam ranges from thermal energies up to1 GeV. The neutrons travel through a vacuum beam lineto the experimental area, located at the nominal distanceof 185 m from the spallation target. This very long flightpath provides an excellent energy resolution, allowing toresolve closely spaced neutron resonances.The experimental technique used for this measurementis the Total Energy technique, which requires that theefficiency of the detection system is directly proportionalto the total radiative energy emitted by the capture event.To achieve this proportionality a low solid angle detectionsystem has been used, consisting in two C6D6 liquidscintillators, one commercial BICRON and one custommade (Forschungszentrums Karlsruhe−FZK) [4], placedface to face at 90◦ with respect to the neutron beamdirection. Afterwards, the response of the detector hasbeen weighted applying the so-called weighting function,which has been calculated by means of the pulse-heightweighting technique [5].The 238U sample used is an extremely pure metallicplate, approximately rectangular in shape, with an area of1621.2 ± 0.1 mm2 and a mass of 6.125 ± 0.002 g. In orderto cover the same fraction of the neutron beam, the othersamples measured for background evaluation have beenprepared with the same geometry.3. Data reductionDuring the measurement, the stability has been checkedboth for the neutron flux and for the scintillators, andruns that showed a deviation in the counting rate ofmore than 3.5% have been rejected. An accurate study ofthe calibrations between the flash-ADC channels and theenergy deposited by the γ -rays in the detectors has beenperformed on a weekly basis.To calculate the weighting functions (WF) required bythe total energy detection technique, the detector responseshave been simulated with the GEANT4 toolkit [7] andconvoluted with the experimental resolution. Two WF havebeen produced: one considering an homogenous emissionof the γ -rays, used for the analysis of both the resolvedand unresolved resonance region, the other considering anexponential attenuation along the neutron beam directionto take into account the effects of neutron transport withinthe sample, not negligible in the saturated resonances thatare exploited to extract the normalization factor.The capture yield is the observable quantity measuredin a capture reaction, and it is related to the weightedcounting rate C by [6]:Yexp(En) = N C(En) − B(En)Aεφn(En) , (1)where B(En) indicates the contribution of the background,ε the efficiency of the detection system, A the effectivearea of the sample intercepted by neutron beam and φnthe incident neutron flux. To compare the experimentalyield to its expected value, the geometry of the wholesystem (including the effective area of the sample) and theabsolute value of the neutron fluence are included into asingle and energy-independent normalization factor N .To reach the aimed precision for the cross section,a very accurate and precise characterization of thebackground that affects the measurements is fundamental.2EPJ Web of Conferences 146, 11028 (2017) DOI: 10.1051/epjconf/201714611028ND2016Figure 1. 238U(n,γ ) capture yield compared with the totalbackground and its individual components discussed in the text.The yields have been obtained separately for each one of the twodetectors and summed afterwards.Five different contributions have been identified andcharacterized both through experimental measurement andMonteCarlo simulations. In Fig. 1 these component areshown with respect to the 238U capture yield. The timeindependent component of the background, due to thenatural and sample radioactivity and to air activation,have been evaluated in a beam-off measurement. Thebackground related only to the neutron beam has beenmeasured without any sample in-beam to take into accountall the sources of background in the experimental area.Three background components have been associated to thepresence of the sample in-beam: first of all, the backgrounddue to γ -rays coming from sample-scattered neutronscaptured in the environmental material (the so-calledneutron sensitivity). Although the detectors exploited forthis measurement have been optimized to minimize thiskind of background, its contribution cannot be neglectedsince it follows the same energy dependence as the truecapture events and may therefore compromise the analysisof the resonances. To properly evaluate it, GEANT4simulations have been performed with a completedescription of the geometry of the experimental area [8].Secondly, we evaluated the background contribution due toin-beam γ -rays through a n+natPb measurement, properlyscaled to take into account the differences in areal densityand charge number between the uranium and lead samples.The third sample-related background component comesfrom the γ -rays emitted in both fission events and decaysof fission fragments. This has been evaluated by meansof a complete set of GEANT4 simulations describing theexperimental area and the sample.To validate the background level, it has beenadditionally evaluated with an independent methodexploiting the presence of Ag, W, Co and Al black-resonance filters in beam. The resulting background hasbeen properly scaled to take into account the attenuationof both the neutron beam and the in-beam photons. Thetwo background yields agree for 3 < En < 100 keV within5% except near the big Al resonances, because the totalamount of aluminum in the windows of the beam line isnot precisely known.The neutron flux for the n TOF facility has beenevaluated combining together the results of five differentFigure 2. n TOF neutron flux in 2012 (blue) compared with the2011 evaluated flux. The ratio is plotted in the bottom panel.Table 1. The three different normalization factors obtained fromthe saturated resonances for the two C6D6 scintillators.Detector N1 N2 N3Bicron 0.842 0.843 0.850FZK 1.008 0.995 1.001detectors, as described in Ref. [9]. For the 2012measurement campaigns not all detectors were available.However during the entire measurement the flux at thesample position was controlled by a thin 6Li foil that isplaced in beam and viewed by a silicon detector [10]. Ascan be seen in Fig. 2, the absolute value of the flux is 13%lower in 2012 with respect to the evaluated flux, unlessa loss of efficiency occurred, possibly due to a change ofthe position of the SiMON detector or to a degenerationof the 6Li converter foil. Nevertheless, above 0.1 eV theflux remains constant in shape within 2% (bottom panel ofFig. 2). This constant behaviour allows to use the evaluatedflux to extract the yield for the 238U(n,γ ) measurementfrom 0.1 eV neutron energy, providing that the differencein intensity is taken into account in the normalizationprocedure.The 238U sample has been chosen in such a way thatthe first three resonances (En = 6.67, 20.9 and 36.7 eV,respectively) are saturated, and therefore the normalizationfactor N can be extracted by applying the saturatedresonance method. The expected capture yield has beenevaluated through a least-squares adjustment of theexperimental data with the SAMMY code [11]. The twodetectors have been treated separately, and in Table 1 thedifferent results are shown. The final normalization factorshave been chosen as the average of the three resonances,and the mean deviation of these factors of 1% has beenadopted as the normalization uncertainty.4. ResultsThe Resolved Resonance Region (RRR) in this work islimited up to 3 keV by the decrease of the signal-to-background ratio with neutron energy together with thelimited counting statistics. Within this energy range aresonance shape analysis of the s-wave resonances hasbeen performed by means of the multilevel multichannel3EPJ Web of Conferences 146, 11028 (2017) DOI: 10.1051/epjconf/201714611028ND2016Figure 3. 238U(n,γ ) cross section from this work compared toprevious measurements by Ullman et al. [13] and Kim et al. [14],and with the cross-section recommended by Carlson et al. [15].The top panel shows neutron energies from 3 to 80 keV, while thebottom panel from 80 to 700 keV. The evaluated cross sectionsfrom JEFF-3.2 is plotted for comparison.R-matrix code SAMMY [11], starting from the parametersreported in the JEFF-3.2 evaluated library [12]. Theresonance kernels (defined as κ = gnγ /(n + γ ))have been used in the comparison with evaluated libraries.In particular, the statistical distribution of kernel ratios ofthis work over JEFF-3.2 and this work over ENDF/B-VII.1is gaussian peaked at 0.99 and 1.00 respectively, with asigma of 0.06, illustrating the good agreement between thiswork and the evaluated libraries.For energies from 3 to 700 keV, the capture cross-section is obtained by applying to the experimental yielda correction factor that takes into account the sample-related effects, i.e. self-shielding and multiple scatteringfollowed by capture. In this Unresolved Resonance Region(top panel of Fig. 3) from 3 to 80 keV data from this workare in good agreement with previous measurements byUllman et al. [13] and Kim et al. [14], with the cross-section recommended by Carlson et al. [15], and withthe JEFF-3.2 evaluated cross-section. In the high energyregion from 80 to 700 keV, the cross-section from this workdeviates by more than a 20% from both the cross-sectionrecommended by Carlson et al. and from the JEFF-3.2evaluated cross-section, while it is still in fair agreementwith the cross section obtained by Ullman et al. [13].5. ConclusionsThe radiative capture cross-section of 238U has beenmeasured at the n TOF facility of CERN in the wideenergy range from 1 eV up to 700 keV.The very careful data reduction, with particularemphasis on the background evaluation and subtraction,has allowed to extract the experimental yield with very lowcorrelated uncertainties.The resonance shape analysis has been performed upto 3 keV, showing in general fair agreement with theevaluated libraries. In the URR results from this workare in fair agreement with previous measurements andevaluated libraries up to 80 keV, but are about 20% higherfrom 80 to 700 keV.References[1] NEA Nucler Data High Priority Request List,http://www.nea.fr/html/dbdata/hprl/[2] P. Schillebeeckx et al., Total and radiative capturecross section measurements for 238U at GELINA andn TOF (European Commission, 2013)[3] C. Guerrero et al., The European Physical Journal A49, 49 (2013)[4] R. Plag et al., Nucl. Instr. Methods Phys. Res. A 496,425 (2003)[5] A. Borella et al., Nucl. Instr. Methods Phys. Res. A577, 626 (2007)[6] P. Schillebeeckx et al., Nuclear Data Sheets 113,3054 (2012)[7] S. Agostinelli, J. Allison, K. Amako et al., Nucl. Instr.Methods Phys. Res. A 506, 250 (2003)[8] P. ˇZugec et al., Nucl. Instr. Methods Phys. Res. A760, 57 (2014)[9] M. Barbagallo et al., Eur. Phys. J. A 49, 156(2013)[10] S. Marrone et al., Nucl. Instr. Methods Phys. Res. A517, 389 (2004)[11] N.M. Larson, ORNL/TM-9179/R8, ENDF-364/R2(Oak Ridge National Laboratory, 2008)[12] A.J. Koning et al., Journal of the Korean PhysicalSociety 2, 1057 (2011)[13] J.L. Ullmann et al., Phys. Rev. C 89, 034603 (2014)[14] H.I. Kim et al., Eur. Phys. J. A 52, 170 (2016)[15] A.D. Carlson et al., Nucl. Data Sheets 110, 3215(2009)4

High precision measurement of the radiative capture cross section of $^{238}$U at the n_TOF CERN facility

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High precision measurement of the radiative capture cross section of $^{238}$ U at the n_TOF CERN facility

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High precision measurement of the radiative capture cross section of $^{238}$ U at the n_TOF CERN facility