EFNUDAT – Measurements and Models of Nuclear ReactionsThe CERN Neutron Time-Of-Flight (n_TOF) facility is well suited to measure low cross sections as those of neutron-induced fission in subactinides. The cross section ratios of natPb and 209Bi relative to 235U and 238U were measured using PPAC detectors and a fragment coincidence method that allows us to identify the fission events. The present experiment provides first results for neutron-induced fission up to 1 GeV. Good agreement is found with previous experimental data below 200 MeV. The comparison with proton-induced fission indicates that the limiting regime where neutron-induced and proton-induced fission reach equal cross sections is close to 1 Ge

Capote, Roberto

Durán, Ignacio

Lozano Leyva, Manuel Luis

Paradela, Carlos

Quesada Molina, José Manuel

Vicente, M.C.

Álvarez, H.

idUS. Depósito de Investigación Universidad de Sevilla

EPJ Web of Conferences 8, 07009 (2010)DOI: 10.1051/epjconf/20100807009c© Owned by the authors, published by EDP Sciences, 2010Measurements of high-energy neutron-induced fission ofnatPb and 209BiD. Tarrı´o1,a, L. Tassan-Got2, L. Audouin2, B. Berthier2, I. Duran1, L. Ferrant2, S. Isaev2,C. Le Naour2, C. Paradela1, C. Stephan2, D. Trubert2, U. Abbondanno3, G. Aerts4, H. ´Alvarez1,F. ´Alvarez-Velarde5, S. Andriamonje6, J. Andrzejewski7, P. Assimakopoulos8, G. Badurek9,P. Baumann10, F. Becva´r11, E. Berthoumieux6, F. Calvin˜o12, M. Calviani13,14, D. Cano-Ott5,R. Capote15,16, C. Carrapic¸o17,6, P. Cennini18, V. Chepel19, E. Chiaveri18, N. Colonna20, G. Cortes12,A. Couture21, J. Cox21, M. Dahlfors18, S. David1, I. Dillmann22, C. Domingo-Pardo23,24, W. Dridi6,C. Eleftheriadis25, M. Embid-Segura5, A. Ferrari18, R. Ferreira-Marques19, K. Fujii3, W. Furman26,I. Gonc¸alves17, E. Gonza´lez-Romero5, F. Gramegna13, C. Guerrero5, F. Gunsig6, B. Haas27,R. Haight28, M. Heil22, A. Herrera-Martinez18, M. Igashira29, E. Jericha9, Y. Kadi18, F. Ka¨ppeler22,D. Karadimos8, D. Karamanis8, M. Kerveno10, P. Koehler30, E. Kossionides31, M. Krticka11,C. Lampoudis25,6, H. Leeb9, A. Lindote19, I. Lopes19, M. Lozano16, S. Lukic10, J. Marganiec7,S. Marrone20, T. Martı´nez5, C. Massimi32, P. Mastinu13, A. Mengoni15,18, P.M. Milazzo3,C. Moreau3, M. Mosconi22, F. Neves19, H. Oberhummer9, S. O’Brien21, M. Oshima33, J. Pancin6,C. Papachristodoulou8, C. Papadopoulos34, N. Patronis8, A. Pavlik35, P. Pavlopoulos36, L. Perrot6,M.T. Pigni9, R. Plag22, A. Plompen37, A. Plukis6, A. Poch12, J. Praena13, C. Pretel12, J. Quesada16,T. Rauscher38, R. Reifarth28, C. Rubbia39, G. Rudolf10, P. Rullhusen37, J. Salgado17, C. Santos17,L. Sarchiapone18, I. Savvidis25, G. Tagliente20, J.L. Tain23, L. Tavora17, R. Terlizzi20, G. Vannini32,P. Vaz17, A. Ventura40, D. Villamarin5, M.C. Vicente5, V. Vlachoudis18, R. Vlastou34, F. Voss22,S. Walter22, M. Wiescher21, and K. Wisshak22 (The n TOF Collaboration (www.cern.ch/ntof))a e-mail: diego.tarrio@usc.esThis is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, whichpermits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited.1 Universidade de Santiago de Compostela, Spain2 Centre National de la Recherche Scientifique/IN2P3-IPN, Orsay, France3 Istituto Nazionale di Fisica Nucleare, Trieste, Italy4 CEA/Saclay – IRFU, Gif-sur-Yvette, France5 Centro de Investigaciones Energeticas Medioambientales y Tecnologicas, Madrid, Spain6 CEA/Saclay – DSM/DAPNIA, Gif-sur-Yvette, France7 University of Lodz, Lodz, Poland8 University of Ioannina, Greece9 Atominstitut der ¨Osterreichischen Universita¨ten, Technische Universita¨t10 Centre National de Recherche Scientifique/IN2P3 – IreS, Strasbourg, France11 Charles University, Prague, Czech Republic12 Universidad Politecnica de Catalunya, Barcelona, Spain13 Istituto Nazional di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy14 Dipartimento di Fisica, Universita` di Padova, Italy15 International Atomic Energy Agency (IAEA), Nuclear Data Section, Vienna, Austria16 Universidad de Sevilla, Spain17 Instituto Tecnolo´gico e Nuclear (ITN), Lisbon, Portugal18 CERN, Geneva, Switzerland19 LIP – Coimbra, Departamento de Fisica da Universidade de Coimbra, Portugal20 Istituto Nazionale di Fisica Nucleare, Bari, Italy21 University of Notre Dame, Notre Dame, USAArticle available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20100807009EPJ Web of Conferences22 Karlsruhe Institute of Technology (KIT), Campus Nord, GmbH (FZK), Institut fu¨r Kernphysik,Germany23 Instituto de Fı´sica Corpuscular, CSIC-Universidad de Valencia, Spain24 GSI, Darmstadt, Germany25 Aristotle University of Thessaloniki, Greece26 Joint Institute for Nuclear Research, Frank Laboratory of Neutron Physics, Dubna, Russia27 Centre National de la Recherche Scientifique/IN2P3-CENBG, Bordeaux, France28 Los Alamos National Laboratory, New Mexico, USA29 Tokyo Institute of Technology, Tokyo, Japan30 Oak Ridge National Laboratory, Physics Division, Oak Ridge, USA31 NCSR, Athens, Greece32 Dipartamento di Fisica, Universita` de Bologna, and Sezione INFN di Bologna, Italy33 Japan Atomic Energy Research Institute, Tokai-mura, Japan34 National Technical University of Athens, Greece35 Faku¨ltat fu¨r Physik, Universita¨t Wien, Austria36 Poˆle Universitaire Le´onard de Vinci, Paris, La De´fense, France37 CEC-JRC-IRMM, Geel, Belgium38 Departament of Physics – University of Basel, SwitzerlandAbstract. The CERN Neutron Time-Of-Flight (n TOF) facility is well suited to mea-sure low cross sections as those of neutron-induced fission in subactinides. The crosssection ratios of natPb and 209Bi relative to 235U and 238U were measured using PPAC de-tectors and a fragment coincidence method that allows us to identify the fission events.The present experiment provides first results for neutron-induced fission up to 1 GeV.Good agreement is found with previous experimental data below 200 MeV. The compari-son with proton-induced fission indicates that the limiting regime where neutron-inducedand proton-induced fission reach equal cross sections is close to 1 GeV.1 IntroductionThe development of Accelerator Driven Systems (ADS) requires a good knowledge of the cross sec-tions for a variety of reactions. In particular, neutron-induced fission cross sections of natPb and 209Biare of special interest since they will probably be the main compound materials of the spallationtarget [1]. In consequence, the neutron spectrum and the radioactivity produced in the target will besignificantly aﬀected by neutron-induced fission of Pb and Bi.Another application for 209Bi is in neutron fluence monitors. Thanks to its high fission thresholdof about 20 MeV it can be used even in a background of low energy neutrons. The fact that bismuth isa monoisotopic and non-radioactive material suggests that the 209Bi(n,f) reaction might be considereda well-suited standard cross section [2]. However, due to the technical diﬃculties to obtain neutronbeams of high energy, the available evaluations of (n,f) cross section have been made using experi-mental data below 200 MeV. The present work at the CERN n TOF facility provides first results of theneutron-induced fission cross sections of natPb and 209Bi in an extended energy range up to 1 GeV.2 The experimental setupThe present experiment has been performed at the n TOF (neutron Time-Of-Flight) facility at CERN,where a very intense neutron beam is produced by spallation reactions on a lead target, using a20 GeV/c proton beam from the PS (Proton Synchrotron) at CERN. The water of the cooling sys-tem acts as a moderator to produce the extended energy range of the neutron flux from thermal up toseveral GeV. Up to now, n TOF is the only facility that reaches such high neutron energies. The 185 mflight path between the spallation target and the experimental area allows one to measure the neutron07009-p.2EFNUDAT Workshop “Measurements and Models of Nuclear Reactions”, Paris, 2010Fig. 1. Schematic view of the PPAC detection setup used in this experiment.energy with a high accuracy (6% around 1 GeV and 0.01% around 1 eV). A more detailed descriptionof the n TOF facility can be found in Ref. [3].The experimental setup used in this experiment consisted in a reaction chamber especially de-signed for this purpose housing 10 PPAC (Parallel Plate Avalanche Counter) detectors. Each PPAChas a central anode surrounded by two cathodes with a low-pressure gas filling the gaps between theelectrodes. The cathodes are segmented in orthogonal directions so that the fission fragments trajec-tory can be reconstructed. Thin targets were placed in between the PPACs as shown in Fig. 1, wheretargets of 235U and 238U were included as references.A detailed characterisation of the targets used in this experiment, including the values of the totalmasses, and their chemical composition has been performed by means of Rutherford BackscatteringSpectroscopy (RBS). Additional measurements of the α-activity of the radioactive samples providedan independent measurement of the target masses and of their spatial distribution.3 Data analysisFission events were identified by coincident detection of the fragments within a time window of 10 ns.This constraint discards most of the background due to the α-activity of the radioactive targets and tothe emission of light particles in spallation reactions.For a single target, the number of detected fission events is given by:Ci (E) = Φi (E) · Ni · σi (E) · εi (E) (1)where Φ(E) is the neutron fluence (in n/cm2) integrated for the full measuring time, N is the totalnumber of atoms in the target, σ(E) is the fission cross section and ε(E) is the detection eﬃciency.Therefore, the cross section ratio of two of the targets can be calculated from Equation 1, as:σi (E)σ j (E) =Ci (E)C j (E) ·Φ j (E)Φi (E) ·NjNi· ε j (E)εi (E) (2)The PPACs and the targets are so thin that the neutron flux attenuation is less than 1%, as demonstratedby MCNP calculations [4] so that the ratio Φ j(E)/Φi(E) can be assumed as 1 for samples of the samesize. However, the reference samples of 235U and 238U were 8 cm in diameter, while natPb and 209Biwere spread over the whole backing surface. Using the cathode signals it is possible to map the fissionevents over the target area and, in this way, to obtain the correction on the neutron fluence for diﬀerentsamples, when needed, as it can be seen in Fig. 2 for 209Bi and 235U targets. The figure also shows thatthe beam spot is shifted upward with respect to the nominal position of the neutron beam, representedby the black circles.The detection eﬃciencies ε(E) for the various targets diﬀer slightly due to diﬀerences in the back-ings and in the sample thicknesses. A simulation combining Monte-Carlo methods and numericalcalculations for the stopping power was developed to estimate these corresponding ratios ε j(E)/εi(E),where the diﬀerent mass distribution of the fission fragments was also taken into account. The eﬃ-ciency ratios εU235(E)/εPb(E) and εU235(E)/εBi209(E) were estimated to 0.85, while the εU238(E)/εPb(E)and εU238(E)/εBi209(E) ratios were estimated to 0.88. Due to the limited angular acceptance of the ex-perimental setup, our data have been corrected using the parameterization proposed in Ref. [5] for the07009-p.3EPJ Web of ConferencesFig. 2. Position of the fission events in the 209Bi and in the 235U samples. The black circles represent the nominalposition of the neutron beam. Fig. 3. Ratios of the neutron-induced fission cross sections of natPb/235U and 209Bi/235U obtained in this work(solid symbols), compared with some previous data from [6,7] (open symbols). The solid and dashed lines arethe corresponding ratios for proton-induced fission from the systematic of Ref. [8].anisotropy. The overall contribution of systematic uncertainties has been estimated to be lower than12%, including the eﬀect of the linear momentum transferred from the incident neutrons to the nuclei,what has been calculated to be much lower than the other components of the systematic uncertainty.4 ResultsWe have measured the neutron-induced fission cross sections of natPb and 209Bi relative to 235U andto 238U from threshold up to 1 GeV. The ratios of the (n,f) cross sections of natPb and 209Bi withrespect to 235U are shown in Fig. 3. The error bars represent the statistical uncertainties only, beingthe systematic related uncertainties below 12%. Good agreement is found with previous results fromRefs. [6,7] which cover the energy range below 200 MeV. Above this energy, only data for proton-induced fission are available. Therefore, the ratios obtained from the systematics presented in [8] forproton-induced fission are shown for comparison.To obtain the final cross sections of natPb(n,f) and 209Bi(n,f), the ratios were multiplied by the235U(n,f) and 238U(n,f) cross sections given by the JENDL/HE-2007 evaluation, the only one thatcovers the whole energy range of our data. The results obtained with the two reference cross sectionsare compatible. The final values of the cross sections are shown in Figs. 4 and 5.For both isotopes, natPb and 209Bi, our results are compatible with previous measurements be-low 200 MeV in Refs. [7,9,10]. The presently available evaluations agree well with our data below07009-p.4EFNUDAT Workshop “Measurements and Models of Nuclear Reactions”, Paris, 2010 Fig. 4. Neutron-induced fission cross section of natPb obtained in this work, compared with previous data from[7,9]. Experimental data [11] and systematics [8] for proton-induced fission are also shown. Fig. 5. Neutron-induced fission cross section of 209Bi obtained in this work, compared with previous data from[9,10]. The parametrization from Smirnov et al. [9] and the recommended standard cross section of Carlsonet al. [2] do not reproduce our data above 200 MeV. Experimental data [11] and systematics [8] for proton-inducedfission are also shown.200 MeV, but deviate significantly at higher energies. In both cases the parameterization proposed bySmirnov et al. [9] is shown, while for the 209Bi(n,f) case also the recommended standard cross sectionof Carlson et al. [2] has been included. On the basis of the new results, a revision of the evaluations isclearly called for.Experimental data for proton-induced fission from [11], as well as the systematics of Prokofiev [8]are shown for comparison. It can be inferred that the (n,f) cross sections, even though they are lowerthan the (p,f) values at lower energy, tend to be compatible with around 1 GeV.5 Summary and conclusionsTaking advantage of the intense, white neutron source n TOF at CERN, and of the timing capabilitiesof the PPAC detectors, the neutron-induced fission cross sections of natPb and 209Bi have been mea-sured, for the first time, from threshold up to 1 GeV. The results are in good agreement with previousdata and existing evaluations below 200 MeV but deviate significantly at higher energies. Therefore,07009-p.5EPJ Web of Conferencesthe evaluations should be revised above 200 MeV to comply with the present experimental values. Theresults tend to be compatible with the (p,f) values around 1 GeV.References1. OECD-NEA, “Accelerator and Spallation Target Technologies for ADS Applications: StatusReport”, NEA-5421 (2005)2. A.D. Carlson, S. Chiba, F.J. Hambsch, N. Olsson and A.N. Smirnov, “Update to Nuclear DataStandards for Nuclear Measurements”, IAEA Report INDC (NDS), p. 368 (1997)3. The n TOF Collaboration, “CERN n TOF Facility: Performance Report”, CERN/INTC-O-011,CERN-SL-2002-053 ECT (2002)4. L. Ferrant, PhD. Work, Universite´ Paris-Sud (2005)5. V.P. Eismont, A.V. Prokofiev, I.V. Ryzhov et al., “Proc. of the 3rd International Conference onAccelerator Driven Transmutation technologies and Applications”, Praha, Czech Republic (1999)6. A.B. Laptev et al., “Proc. Of the 4 thInternational Conference on Fission and Properties ofNeutron-Rich Nuclei”, Sanibel Island, Florida (2007)7. O. Shcherbakov et al., Journ. Nucl. Sci. and Techn., Supp. 2, 230 (2002)8. A.V. Prokofiev, Nucl. Inst. and Meth. A, 463, 557 (2001)9. A.N. Smirnov et al., Phys. Rev. C, 70, 054603 (2004)10. R. Nolte et al., Nucl. Sci. Eng., 156, 197 (2007)11. A.A. Kotov et al, Phys. Rev. C, 74, 034605 (2006)07009-p.6

Measurements of high-energy neutron-induced fission of natPb and 209Bi

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Measurements of high-energy neutron-induced fission of natPb and 209Bi

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