Study of Neutron-Induced Fission Cross Sections of U, Am, and Cm at n_TOF

Study of Neutron-Induced Fission Cross Sections of U, Am, and Cm at n_TOF Cite as: AIP Conference Proceedings 1265, 477 (2010); https:// doi.org/10.1063/1.3480244 Published Online: 05 August 2010 P. M. Milazzo, U. Abbondanno, G. Aerts, H. Alvarez, F. Alvarez-Velarde, S. Andriamonje, J. Andrzejewski, P. Assimakopoulos, L. Audouin, G. Badurek, P. Baumann, F. Becčvář, F. Belloni, E. Berthoumieux, F. Calviño, M. Calviani, D. Cano- Ott, R. Capote, C. Carrapiço, P. Cennini, V. Chepel, E. Chiaveri, N. Colonna, G. Cortes, A. Couture, J. Cox, M. Dahlfors, S. David, I. Dillman, C. Domingo-Pardo, W. Dridi, I. Duran, C. Eleftheriadis, M. Embid-Segura, L. Ferrant, A. Ferrari, R. Ferreira-Marques, K. Fujii, W. Furman, I. Goncalves, E. Gonzalez-Romero, F. Gramegna, C. Guerrero, F. Gunsing, M. Heil, A. Herrera-Martinez, E. Jericha, F. Käppeler, Y. Kadi, D. Karadimos, D. Karamanis, M. Kerveno, P. Koehler, E. Kossionides, M. Krtička, C. Lamboudis, H. Leeb, A. Lindote, I. Lopes, M. Lozano, S. Lukic, J. Marganiec, S. Marrone, T. Martinez, C. Massimi, P. Mastinu, A. Mengoni, M. Mosconi, F. Neves, H. Oberhummer, S. O’Brien, J. Pancin, C. Papachristodoulou, C. Paradela, N. Patronis, A. Pavlik, P. Pavlopoulos, L. Perrot, R. Plag, A. Plukis, A. Poch, J. Praena, C. Pretel, J. Quesada, R. Reifarth, C. Rubbia, G. Rudolf, J. Salgado, C. Santos, L. Sarchiapone, I. Savvidis, C. Stephan, G. Tagliente, J. L. Tain, L. Tassan-Got, L. Tavora, R. Terlizzi, G. Vannini, P. Vaz, A. Ventura, D. Villamarin, M. C. Vincente, V. Vlachoudis, R. Vlastou, F. Voss, S. Walter, M. Wiescher, K. Wisshak, and The n_TOF collaboration ARTICLES YOU MAY BE INTERESTED IN Measurement of the Neutron Induced Fission Cross Section on Transuranic (TRU) Elements at the n_TOF Facility at CERN AIP Conference Proceedings 947, 43 (2007); https:// doi.org/10.1063/1.2813848 The cross section at the CERN n-TOF facility AIP Conference Proceedings 1377, 459 (2011); https:// doi.org/10.1063/1.3628445 ASTROPHYSICS AT n_TOF FACILITY AIP Conference Proceedings 1265, 160 (2010); https:// doi.org/10.1063/1.3480156 AIP Conference Proceedings 1265, 477 (2010); https://doi.org/10.1063/1.3480244 1265, 477 © 2010 American Institute of Physics. Study of Neutron-Induced Fission Cross Sections of U, Am, and Cm at n_TOF P.M. Milazzo 1 , U. Abbondanno 1 , G. Aerts 2 , H. Alvarez 3 , F. Alvarez- Velarde 4 , S. Andriamonje 2 , J. Andrzejewski 5 , P. Assimakopoulos 6 , L. Audouin 7 , G. Badurek 8 , P. Baumann 9 , F. Bečvář 10 , F. Belloni 1 , E. Berthoumieux 2 , F. Calviño 11 , M. Calviani 12 , D. Cano-Ott 4 , R. Capote 13,14 , C. Carrapiço 15 , P. Cennini 16 , V. Chepel 17 , E. Chiaveri 16 , N. Colonna 18 , G. Cortes 11 , A. Couture 19 , J. Cox 19 , M. Dahlfors 16 , S. David 9 , I. Dillman 7 , C. Domingo-Pardo 20 , W. Dridi 2 , I. Duran 3 , C. Eleftheriadis 21 , M. Embid-Segura 4 , L. Ferrant 2 , A. Ferrari 16 , R. Ferreira-Marques 17 , K. Fujii 1 , W. Furman 22 , I. Goncalves 16 , E. Gonzalez-Romero 4 , F. Gramegna 12 , C. Guerrero 4 , F. Gunsing 2 , M. Heil 7 , A. Herrera-Martinez 6 , E. Jericha 9 , F. Käppeler 7 , Y. Kadi 16 , D. Karadimos 6 , D. Karamanis 6 , M. Kerveno 10 , P. Koehler 23 , E. Kossionides 24 , M. Krtička 10 , C. Lamboudis 21 , H. Leeb 9 , A. Lindote 17 , I. Lopes 17 , M. Lozano 14 , S. Lukic 9 , J. Marganiec 5 , S. Marrone 18 , T. Martınez 4 , C. Massimi 25 , P. Mastinu 12 , A. Mengoni 13 ,M. Mosconi 7 , F. Neves 17 , H. Oberhummer 9 , S. O’Brien 19 , J. Pancin 2 , C. Papachristodoulou 6 , C. Paradela 3 , N. Patronis 6 , A. Pavlik 26 , P. Pavlopoulos 27 , L. Perrot 2 , R. Plag 7 , A. Plukis 2 , A. Poch 11 , J. Praena 12 , C. Pretel 11 , J. Quesada 13 , R. Reifarth 7 , C. Rubbia 28 , G. Rudolf 10 , J. Salgado 15 , C. Santos 15 , L. Sarchiapone 16 , I. Savvidis 21 , C. Stephan 2 , G. Tagliente 18 , J. L. Tain 20 , L. Tassan-Got 2 , L. Tavora 15 , R. Terlizzi 18 , G. Vannini 25 , P. Vaz 15 , A. Ventura 29 , D. Villamarin 4 , M.C. Vincente 4 , V. Vlachoudis 16 , R. Vlastou 24 , F. Voss 7 , S. Walter 7 , M. Wiescher 19 , and K. Wisshak 7 (the n_TOF collaboration) 1 INFN, Trieste, Italy; 2 CNRS/IN2P3 - IPN, Orsay, France ; 3 Universidade de Santiago de Compostela, Spain; 4 CIEMAT, Madrid, Spain; 5 University of Lodz, Poland; 6 University of Ioannina, Greece; 7 Forschungszentrum Karlsruhe GmbH, Germany; 9 Atominstitut der Österreichischen Universitäten,Technische, Wien, Austria; 10 CNRS/IN2P3 - IReS, Strasbourg, France; 11 Universitat Politecnica de Catalunya, Spain; 12 INFN, Laboratori Nazionali di Legnaro, Italy; 13 IAEA, Vienna, Austria; 14 Universidad de Sevilla, Spain; 15 ITN, Lisbon, Portugal; 16 CERN, Geneva, Switzerland; 17 LIP & Universidade de Coimbra, Portugal; 18 INFN, Bari, Italy; 19 University of Notre Dame, USA; 20 CSIC - University of Valencia, Spain; 21 Aristotle University of Thessaloniki, Greece; 22 JINR, Dubna, Russia; 23 Oak Ridge National Laboratory, USA; 24 NSCR, Athens, Greece; 25 Università di Bologna and 477 CREDIT LINE (BELOW) TO BE INSERTED ON THE FIRST PAGE OF EACH PAPER EXCEPT THE ARTICLES ON PP. 144 – 147 and 178 - 179 CP1265, VIII Latin American Symposium on Nuclear Physics and Applications edited by R. Alcaron, H. F. Arellano, P. L. Cole, and A. J. Kreiner © 2010 American Institute of Physics 978-0-7354-0814-2/10/$30.00 CREDIT LINE (BELOW) TO BE INSERTED ONLY ON THE FIRST PAGE OF THE ARTICLES ON PP. 144 – 147 and 178 - 179 CP1265, VIII Latin American Symposium on Nuclear Physics and Applications edited by R. Alcaron, H. F. Arellano, P. L. Cole, and A. J. Kreiner 2010 American Institute of Physics 978-0-7354-0814-2/10/$30.00 INFN Bologna, Italy; 26 Institut für Facultät für Physik , Universität Wien, Austria; 27 Pôle Universitaire Léonard de Vinci, Paris La Défense, France; 28 Università di Pavia, Italy; 29 ENEA, Bologna, Italy; Abstract. Neutron induced fission cross sections of several isotopes have been measured at the CERN n_TOF spallation neutron facility. Between them some measurements involve isotopes ( 233 U, 241 Am, 243 Am, 245 Cm) relevant for applications to nuclear technologies. The n_TOF facility delivers neutrons with high instantaneous flux and in a wide energy range, from thermal up to 250 MeV. The experimental apparatus consists of an ionization chamber that discriminates fission fragments and α particles coming from natural radioactivity of the samples. All the measurements were performed referring to the standard cross section of 235 U. Keywords: Neutron cross section, neutron-induced fission reactions, n_TOF PACS: 25.85.-w,25.85.Ec,28.20.-v INTRODUCTION Progress in the field of Nuclear Technology requires a strong effort in order to improve the current knowledge of cross sections. Among them a better accuracy on fission neutron-induced cross sections is mandatory. The design of systems based on the Th/U fuel cycle [1], Accelerator Driven Systems (ADS) [2–4], and Gen-IV nuclear reactors [5] depends on a better determination of these cross sections. In particular, it would be possible to increase the efficiency of the fuel cycle, improving the fuel burn-up, and to upgrade the safety of future systems. Moreover, the presence of an higher fraction of actinides in the fuel mix would be possible. One has also to consider that nowadays one of the main limitation of nuclear energy programs is the nuclear waste treatment and storage. A significant fraction of the highlevel nuclear wastes is constituted by minor actinides, and between them Am and Cm isotopes. A possible solution to the problem of nuclear waste management could be the transmutation, via neutron induced fission of transuranic elements, both in subcritical and critical systems. In order to improve the reliability of evaluated databases [6] the n_TOF collaboration has performed several measurements of neutron induced cross sections (capture and fission). In this contribution some results on 233 U, 241,243 Am and 245 Cm are reported. In particular, the 233 U(n,f) cross section is crucial for the study of the Th/U fuel cycle, which interest resides in the abundance of the 232 Th seed and in the reduced production of long-lived actinides. Moreover, discrepancies up to 15% between data from different measurements [7-10] are present in the fast neutron energy region for the 243 Am(n,f) cross section; stringent requirements for this cross section were found for Advanced Minor Actinides Burners (ADMAB), where a reduction of a factor 5 of the uncertainty is requested. Stronger improvements on accuracy are requested for the 243 Am(n,f) and 245 Cm(n,f) cross sections, where a reduction factor of 7 and 15, respectively, were demanded [11]. The measurements performed at the CERN n_TOF facility allowed to collect high accuracy data from thermal energy up to tenths of MeV, with this full energy range covered simultaneously, thanks to the favorable conditions offered by the neutron facility. 478 EXPERIMENTAL SET-UP The measurements were performed at CERN taking advantage of the neutron beam available at the n_TOF facility [12, 13]. The unprecedented high instantaneous neutron flux in combination with the low duty cycle, high resolution, and low background of the n_TOF neutron beam allows one to collect capture cross section data with good accuracy and with an excellent signal-to-background ratio. The pulsed neutron beam of the n_TOF facility is generated in spallation reactions in a massive lead target by 20 GeV protons [13]. The spallation neutrons are slowed down and moderated in the lead target and in a 5.8 cm thick layer of cooling water surrounding the target. The resulting neutron spectrum runs from thermal energies to 250 MeV. The neutron beam is transported through an evacuated flight path with collimators at 135 and 175 m to the measuring station at a distance of 185 m from the spallation target. The beamline extends 12 m beyond the experimental area to minimize the effect of back-scattered neutrons. Background due to fast charged particles is suppressed by a 1.5 T sweeping magnet, heavy concrete walls, and a 3.5 m thick iron shielding. Measurements of neutron-induced fission cross sections have been carried out using Fast Ionization Chambers (FIC) [14] built in collaboration between the Joint Institute of Nuclear Research (JINR, Dubna, Russian Federation), the Institute of Physics and Power Engineering (IPPE, Obninsk, Russian Federation), the Istituto Nazionale di Fisica Nucleare, and CERN. The detector consists of a stack of ionization chambers, assembled along the direction of the neutron beam, thus allowing the simultaneous measurement on several isotopes. Each chamber consists of three electrodes: a central one, 100 µm thick Al foil plated on both sides with sample material, and two 15 µm thick Al anode foils at a distance of 5 mm from the cathode, used to define the electric field. The electrodes are 12 cm in diameter, while the diameter of the sample deposit is 8 cm, so to match the size of the neutron beam. The detector is operated with a gas mixture of 90% Ar and 10% CF4 at 720 mbar pressure. The total mass of the measured samples is reported in Table 1, together with the sample activities. The samples were prepared by means of the painting technique [15]. All detected signals were recorded with fast digitizers; a sampling rate of 100 MSamples/s was chosen, which allow to extend the time-of-flight (TOF) range to 80 ms, corresponding to a minimum neutron energy of about 30 meV. Then, the data were processed and stored by the standard n_TOF data acquisition system [16]. TABLE 1. Mass and activity of the samples involved in the measurements. Sample Mass (mg) Activity 235U 31.8 0.2 kBq 233U 28.8 5 MBq 241Am 2.26 76 MBq 243Am 245Cm 4.80 1.71 7.4 MBq 0.2 GBq 479 DATA ANALYSIS Cross sections were extracted referring to 235 U, which is an accepted standard between 0.15 MeV and 200 MeV [6]. In particular, the neutron-induced fission cross sections are extracted by Eq. 1, 235 235 235 235 ),(),( A A m m N N fnfn x x x x ⋅⋅⋅= σσ (1) where – σ 235 (n,f) corresponds to the tabulated ENDF/B-VII.0 [6] cross section, – x stands for the investigated isotope, i.e. 233 U, 241 Am, 243 Am or 245 Cm, – N x is the number of fission events detected for isotope x, – m x is the mass (in grams) of isotope x, – A x is the atomic number of isotope x Since the investigated isotopes and the 235 U reference samples are mounted in the same detector, they are exposed to the same neutron flux. The number of detected fission events obtained by Eq. 1 had to be corrected for self absorption and dead-time. The number of particles leaving the sample as well as their energy spectrum is determined by the sample thickness. Moreover, since the samples have slightly different thicknesses corresponding efficiency corrections were determined by means of detailed Monte Carlo simulations of the energy loss in the sample and in the gas volume. It was assumed that the fission events were uniformly distributed inside the sample and that the fragments were emitted isotropically. The FLUKA code was used for the simulations [17]. The loss of counts due to the deadtime induced by the reconstruction routine was calculated under the hypothesis of a non-paralyzable model. The efficiency and dead-time corrections were found to be of the order of a few percent, with a corresponding uncertainty of less than 1%. It is important to note that the data are not normalized to any previous result, as in some past measurements, but rely only on the standard 235 U(n,f) cross section. For the first time, the whole energy range from thermal to tenths of MeV is covered in a single measurement, thus minimizing possible systematic uncertainties related, for example, to the absolute normalization of the cross section RESULTS Accurate and high resolution neutron-induced cross sections were extracted. These results will contribute to improve the reliability of cross sections databases. Detailed information on the results can be found in the referenced cited in [18]. In the following few examples of the impact of present results will be given. Figure 1 shows how the high resolution of the n_TOF data would allow to extend the limit of resonances listed in current databases; this is particularly important to estimate self-shielding effects in reactor simulations. 480 FIGURE 1. The results of the 233 U(n,f) reaction around 600 eV [6, 19, 20]. Figure 2 shows a comparison between present data and previous evaluations in different energy ranges [6, 19] for the 233 U(n,f) cross section. This figure shows a general agreement with past measurements, with the relevant exception of the energy range between 100 eV and 10 keV where databases need to be revisited. FIGURE 2. 233 U: ratio between n_TOF results and previous data and evaluations over neutron energy decades [6, 19]. Another important example concerns the 243 Am(n,f) cross section. Present data allow solving a long-standing discrepancy of more than 15%; as shown in Figure 3 n_TOF data confirm current evaluations, against recent previous results [8]. FIGURE 3. 243 Am:comparison between present results and previous data [7-10]. 481 Measurements of 241 Am and 245 Cm fission cross sections were difficult because of the high α activity of the samples; this situation was also complicated by the presence of contaminants in the samples. In these cases it was possible to extract cross sections with larger uncertainties (≈10%) [18]. CONCLUSIONS AND PERSPECTIVES Taking advantage of the high instantaneous flux and the high energy resolution of the CERN n_TOF facility, neutron-induced fission cross sections of 233 U, 241 Am, 243 Am and 245 Cm have been measured. The results are important to resolve discrepancies in previous data, thus providing a reliable basis for future evaluations. Long-term plans of interest for fission measurements at n_TOF also include the construction of a short, 20 m flight path that would allow the study of very low mass samples and of very low cross sections, taking advantage of the unique characteristics of the n_TOF neutron beam. ACKNOWLEDGMENTS This work was supported by the EC under Contract FIKW-CT-2000-00107 and by the funding agencies of the participating institutes. REFERENCES 1. The fuel cycle - potential benefits and challenges, IAEA-TECDOC-1450 (2005). 2. C.D. Bowman et al., Nucl. 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