High-resolution study of Np-237 fission cross section from 5 eV to 1 MeV

High-Resolution Study of 237Np Fission Cross Section from 5 eV to 1 MeV W. Furman18, P.Cennini4, V.Ketlerov19, A.Goverdovski19, V.Konovalov18, U.Abbondanno14, G.Aerts7, H.Álvarez24, F.Alvarez-Velarde20, S.Andriamonje7, J.Andrzejewski33, P.Assimakopoulos9, L.Audouin5, G.Badurek1, P.Baumann6, F. Bečvář 31, J.Benlliure24, E.Berthoumieux7, F.Calviño25, D.Cano-Ott20,R.Capote23, A.Carrillo de Albornoz30, V.Chepel17, E.Chiaveri4, N.Colonna13, G.Cortes25, D.Cortina24, A.Couture29, J.Cox29, S.David5, R.Dolfini15, C.Domingo-Pardo21, A. Dorochenko19, W.Dridi7, I.Duran24, M.Embid-Segura20, L.Ferrant5, A.Ferrari4, R.Ferreira-Marques17, L.Fitzpatrick4, H.Frais-Koelbl3, K.Fujii13, C.Guerrero20, I.Goncalves30, R.Gallino36, E.Gonzalez-Romero20, F.Gramegna12, E.Griesmayer3, F.Gunsing7, B.Haas32, R.Haight27, M.Heil8, A.Herrera- Martinez4, M.Igashira37, S.Isaev5, E.Jericha1, Y.Kadi4, F.Käppeler8, D.Karamanis9, D.Karadimos9, M.Kerveno6, P.Koehler28, D. Kolokolov19, E.Kossionides39, M.Krtička31, C.Lamboudis10, H.Leeb1, A.Lindote17, I.Lopes17, M.Lozano23, S.Lukic6, J.Marganiec33, L.Marques30, S.Marrone13, P.Mastinu12, A.Mengoni4, P.M.Milazzo14, C.Moreau14, M.Mosconi8, F.Neves17, H.Oberhummer1, S.O'Brien29, M.Oshima38, J.Pancin7, C.Papachristodoulou9, C.Papadopoulos40, C.Paradela24, N.Patronis9, A.Pavlik2, P.Pavlopoulos34, L.Perrot7, R.Plag8, A.Plompen16, A.Plukis7, A.Poch25, C.Pretel25, J.Quesada23, T.Rauscher26, R.Reifarth27, M.Rosetti11, C.Rubbia15, G.Rudolf6, P.Rullhusen16, J.Salgado30, L.Sarchiapone4, M.Sedysheva18, C.Stephan5, G.Tagliente13, J.L.Tain21, L.Tassan-Got5, L.Tavora30, R.Terlizzi13, G.Vannini35, P.Vaz30, A.Ventura11, D.Villamarin20, M.C.Vincente20, V.Vlachoudis4, R.Vlastou40, F.Voss8, H.Wendler4, M.Wiescher29, K.Wisshak8 The n_TOF Collaboration 1Atominstitut der Österreichischen Universitäten, Technische Universität Wien, Austria, 2Institut für Isotopenforschung und Kernphysik, Universität Wien, Austria, 3Fachhochschule Wiener Neustadt, Wiener Neustadt, Austria, 4CERN, Geneva, Switzerland, 5Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France, 6Centre National de la Recherche Scientifique/IN2P3 - IReS, Strasbourg, France, 7CEA/Saclay - DSM, Gif-sur-Yvette, France, 8Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany, 9University of Ioannina, Greece, 10Aristotle University of Thessaloniki, Greece, 11ENEA, Bologna, Italy, 12Laboratori Nazionali di Legnaro, Italy, 13Istituto Nazionale di Fisica Nucleare, Bari, Italy, 14Istituto Nazionale di Fisica Nucleare, Trieste, Italy, 15Università degli Studi Pavia, Pavia, Italy, 16CEC-JRC-IRMM, Geel, Belgium, 17LIP - Coimbra & Departamento de Fisica da Universidade de Coimbra, Portugal, 18Joint Institute for Nuclear Research, Frank Laboratory of Neutron Physics, Dubna, Russia, 19Institute of Physics and Power Engineering, Kaluga region, Obninsk, Russia, 20Centro de Investigaciones Energeticas Medioambientales y Technologicas, Madrid, Spain, 21Consejo Superior de Investigaciones Cientificas - University of Valencia, Spain, 22Universidad Politecnica de Madrid, Spain, 23Universidad de Sevilla, Spain, 24Universidade de Santiago de Compostela, Spain, 25Universitat Politecnica de Catalunya, Barcelona, Spain, 26Department of Physics and Astronomy - University of Basel, Basel, Switzerland, 27Los Alamos National Laboratory, New Mexico, USA, 28Oak Ridge National Laboratory, Physics Division, Oak Ridge, USA, 29University of Notre Dame, Notre Dame, USA, 30Instituto Tecnológico e Nuclear, Lisbon, Portugal, 31Charles University, Prague, Czech Republic, 32Centre National de la Recherche Scientifique/IN2P3 - CENBG, Bordeaux, France, 33University of Lodz, Lodz, Poland, 34Pôle Universitaire Léonard de Vinci, Paris La Défense, France, 35Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy, 36Dipartimento di Fisica Generale, Università di Torino and Sezione INFN di Torino, I-10125 Torino, Italy, 37Tokyo Institute of Technology, Tokyo, Japan, 38Japan Atomic Energy Research Institute, Tokai-mura, Japan, 39NCSR, Athens, Greece, 40National Technical University of Athens, Greece Abstract. A series of measurements of 237Np fission cross section have been performed at the CERN spallation neutron facility n_TOF which covers a wide energy range from 1 eV up to 250 MeV. A fast ionization chamber (FIC) was used as a fission fragment detector with registration efficiency of not less than 97 %. Particular attention was paid to correct the fission 1039 CP769, International Conference on Nuclear Data for Science and Technology, edited by R. C. Haight, M. B. Chadwick, T. Kawano, and P. Talou © 2005 American Institute of Physics 0-7354-0254-X/05/$22.50 cross section with use of 235U standard. Total experimental uncertainties are determined to be at the level of 3%. Analysis of the experimental data in the restricted neutron energy from 5 eV up to 1 MeV showed a systematic deviation from evaluated data (ENDF/B-VI). This discrepancy amounts to up to a factor 3 for resolved resonances in the neutron energy range of 5 eV - 2 KeV, and is in good agreement with some previous experiments. A similar disagreement at the level of 6-7% was found for higher energies around the threshold (En = 300 keV-1 MeV). This energy range is essential for the transmutation of neptunium in ADS or fast reactors. It is concluded that an updated evaluation of nuclear data for 237Np is required. INTRODUCTION Neptunium-237 is the most important actinide in the problem of spent fuel transmutation. While the development of power facilities of a new generation like fast reactors or accelerator driven systems (ADS) need accurate nuclear data, large discrepancies in the fission cross section data available up to now in the resolved resonances region have been detected. This situation motivated experimental activities devoted to repeat fission cross section measurements in a wide neutron energy range, aiming at obtaining high accuracy cross section data. This experimental programme has been setup at the CERN neutron time- of-flight facility n_TOF[1]. This is a spallation neutron source where neutrons are generated by the interaction of the 20 GeV proton beam from the CERN PS accelerator with a massive water-cooled lead target. The spallation neutrons have an energy spectrum that covers the range 1 eV to at least 250 MeV with intensity sufficient for the irradiation of thin neptunium samples installed in an ionization chamber. The time of flight (TOF) technique gives high-energy resolution (up to 10-4) with a width of the proton bunch of less than 7 ns and a flight path of 186m. These experimental conditions allowed us to perform detailed studies in the resolved resonance region and simultaneously in the fast region. The results of the analysis reported here cover the neutron energy range from 20 eV to approximately 700 KeV, chosen to point out the most important discrepancies between experimental and evaluated nuclear data for this isotope. EXPERIMENTAL DETAILS The 237Np and 235U targets were installed back-to- back in a multiple ionization chamber. All targets were fabricated from high purity material using the painting technique and analyzed with alpha-counting with semiconductor detector in good geometry. The total masses were 12.82±0.08 mg for neptunium and 31.8±0.2 mg for uranium. A fast parallel plate ionization chamber (FIC0) was used as a fragment detector. The detector chamber is a cylindrical stainless steel assembly 30 cm long and 40 cm in diameter with 2 mm Al windows. The operating gas was a mixture 90%Ar + 10% CF4 at a pressure of 600 mbar. The distance between electrodes was chosen to be 5 mm to get fast signals with 300 V high voltage supply. Our past experience in the use of analogous chambers showed a time resolution of 2-3 ns with minimal overlapping of fission fragments and α-particles well suited for the n_TOF facility. This is sufficient for n_TOF timing and neutron flux. The registration efficiency of the detector was near 98%. The ionization detector was equipped with fast electronics including time-digital converters (TDC) and wave-form digitizers (WFD, or FADC). TDC are used for long times covering the neutron energy range from 0.1 eV to 1 MeV. FADC have been used for measurements above 20 KeV. The energy calibration was performed using the resolved resonances of 235U(n,f) and an example for a limited energy range is presented in Fig. 1. The results show very good agreement with known experimental data. This allowed us to normalize the experimental fission yield for 237Np(n,f) reaction into absolute fission cross section as a function of neutron energy. 8 1012141618202224 1 10 100 U235(n,f) Moore78 Deruytter90 Wagemans78 cross-section, barn En, eV 100 1000 10000 present FIGURE 1. Comparison of 235U fission cross section with the existing experimental data (from EXFOR). RESULTS AND DISCUSSION The results of the fission cross-section measurements around well-known resonance cluster at the neutron energy En = 39.9 eV are presented in Fig. 2 together with other available data [2,3,4]. Disagreement between different data sets looks too large to satisfy practical needs. The list of calculated fission cross section integrals Aλ = ∫ σf dEn for neutron energies between 38 and 42 eV is presented in Table 1. A factor of 3 higher value is found in a comparison of the present data and the data of Plattard et al. [3]. 1040 38.0 38.5 39.0 39.5 40.0 40.5 41.0 41.5 42.0 0 2 4 6 8 10 12 14 16 237Np(n,f) En, eV FISSION CROSS-SECTION, bn POMMARD CERN'2003 Plattard e.a. ENDF FIGURE 2. 237Np(n,f) cross-section between 38 and 42 eV. TABLE 1. A comparison of fission cross-section integrals from different experiments. The ENDF/B-VI value is lower than the value of Plattard et al. (see Fig. 2). Author Aλ, b*eV Comment n_TOF 2003 8.34 ± 0.10 Spallation source Brown et al. [2] 10.2 ± 2.5 POMMARD Plattard et al. [3] 2.88 ± 0.03 Linac, Saclay Dermenjiev et al. [4] 9.14 ± 1.37 IBR-30, Dubna The extremely good n_TOF energy resolution allowed us to get more accurate and detailed data above 600 eV. This is shown in Fig. 3 where the present data are compared with the data of Plattard et al. [3]. Resonance doublets around En = 1150, 1270, 1320 eV are clearly resolved. The determination of more accurate resonance parameters is possible. This work is in progress with SAMMY code. The experimental data for fast neutrons are shown in Fig. 4. The ratio of the fission cross sections of 237Np to that of 235U is used as experimental observable. Agreement between the different data sets is good excluding the region above 450 KeV. To clarify the situation fission cross-sections measured with only TOF technique are presented in Fig. 5. Obviously, all TOF experiments show higher fission cross sections than contained in present in evaluated data libraries. The discrepancy is of the order of 6-7%. REFERENCES 1. CERN n_TOF Facility: Performance Report- CERN-SL- 2002-053 ECT. 2. W.K. Brown et al. – Nucl. Phys. A156, 609 (1970). 3. S. Plattard – Ph.D. thesis, Saclay, 1973. 4. E. Dermenjiev et al. – Phys. At. Nucl. 57, 1362 (1994). 1041 1120 1160 1200 1240 1280 1320 0.2 0.4 CERN Plattard e.a. Fission Cross-Section, a.u En, e V 800 1000 1200 1400 1600 1800 2000 0.2 0.4 0.6 CERN Plattard e.a. Fission Cross-Section, a.u En, eV FIGURE 3. Resolution effects in 237Np fission cross section (upper part) and general view between 700 and 2000 eV. 250000 300000 350000 400000 450000 500000 550000 0.0 0.2 0.4 0.6 237Np(n,f) CERN'2003 Behrens'1980 Meadows'1983 ENDF Henkel'1957 Klema'1972 Kanda'1985 Fursov'1978 Fission X-section Ratio En, eV FIGURE 4. Fission cross-section of 237Np(n,f) relative Uranium-235. 560000 580000 600000 620000 640000 660000 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Jiacoletti ENDF CERN Plattard Behrens Fission Cross-Section, barn Neutron Energy, eV FIGURE 5. Fission cross-section of 237Np(n,f) relative 235U in the energy range up to 660 KeV. Only experimental data from TOF measurements are shown for comparison. 1042