The 236U neutron capture cross-section measured at the n-TOF CERN facility

EPJ Web of Conferences 146, 11054 (2017) DOI: 10.1051/epjconf/201714611054 ND2016 The 236U neutron capture cross-section measured at the n TOF CERN facility M. Mastromarco1,a, M. Barbagallo1, M.J. Vermeulen2, N. Colonna1,S.Altstadt 3, J. Andrzejewski4, L. Audouin5, V. B ´ ecares6,F.Be ˇ cv´ aˇ r7, F. Belloni8, E. Berthoumieux8, J. Billowes9, D. Bosnar10, M. Brugger11,F.Calvi ˜ no12, M. Calviani11, D. Cano-Ott6, C. Carrapic¸o13, F. Cerutti11, E. Chiaveri11,8,M.Chin 11,G.Cort ´ es12,M.A.Cort ´ es-Giraldo14, M. Diakaki15, C. Domingo-Pardo16,I.Dur ´ an17, N. Dzysiuk18, C. Eleftheriadis19, A. Ferrari11,K.Fraval 8,V.Furman 20, M.B. G´ omez-Hornillos12, S. Ganesan21,A.R.Garc ´ ıa6, G. Giubrone16, I.F. Gonc¸alves13, E. Gonz´ alez6, A. Goverdovski22, E. Griesmayer23, C. Guerrero11, F. Gunsing8, P. Gurusamy21,T.Heftrich 3, S. Heinitz24,A.Hern ´ andez-Prieto11,12, J. Heyse25, D.G. Jenkins2, E. Jericha23,F.K ¨ appeler27, Y. Kadi11, D. Karadimos15, T. Katabuchi28, V. Ketlerov22, V. Khryachkov22, P. Koehler29, M. Kokkoris15,J.Kroll 7,M.Krti ˇ cka7, C. Lampoudis8, C. Langer3, E. Leal-Cidoncha17, C. Lederer30,H.Leeb 23, L.S. Leong5, J. Lerendegui-Marco14,M.Licata 31,32,R.Losito 11, A. Manousos19, J. Marganiec4, T. Mart´ ınez6, C. Massimi31,32, P. Mastinu18, E. Mendoza6, A. Mengoni33, P.M. Milazzo34, F. Mingrone31,M.Mirea 35, W. Mondelaers25, C. Paradela17,A.Pavlik 30, J. Perkowski4, A.J.M. Plompen25, J. Praena14, J.M. Quesada14, T. Rauscher36,R.Reifarth 3, A. Riego-Perez12, M. Robles17, F. Roman35, C. Rubbia11,J.A.Ryan 9, M. Sabat´ e-Gilarte11,14, R. Sarmento13, A. Saxena21, P. Schillebeeckx25, S. Schmidt3, D. Schumann24, P. Sedyshev20, G. Tagliente1,J.L.Tain 16, A. Tarife˜ no-Saldivia16,D.Tarr ´ ıo17, L. Tassan-Got5, A. Tsinganis11, S. Valenta7, G. Vannini31,32, V. Variale1,P.Vaz 13, A. Ventura31, R. Versaci11, V. Vlachoudis11,R.Vlastou 15,A.Wallner 37,T.Ware 9, M. Weigand3,C.Weiss 23, T. Wright9, P. ˇ Zugec10, and the n TOF Collaboration 1Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 2University of York, UK 3Goethe University Frankfurt, Germany 4University of Lodz, Poland 5Institut de Physique Nucl´ eaire, CNRS-IN2P3, Univ. Paris-Sud, Universit´ e Paris-Saclay, 91406 Orsay Cedex, France 6Centro de Investigaciones Energeticas Medioambientales y Tecnol´ ogicas (CIEMAT), Spain 7Charles University, Prague, Czech Republic 8CEA Saclay, Irfu, Gif-sur-Yvette, France 9University of Manchester, UK 10 University of Zagreb, Croatia 11 European Organization for Nuclear Research (CERN), Switzerland 12 Universitat Polit` ecnica de Catalunya, Spain 13 Instituto Superior T´ ecnico, Lisbon, Portugal 14 Universidad de Sevilla, Spain 15 National Technical University of Athens, Greece 16 Instituto de F´ ısica Corpuscular, Universidad de Valencia, Spain 17 University of Santiago de Compostela, Spain 18 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy 19 Aristotle University of Thessaloniki, Thessaloniki, Greece 20 Joint Institute for Nuclear Research (JINR), Dubna, Russia 21 Bhabha Atomic Research Centre (BARC), India 22 Institute of Physics and Power Engineering (IPPE), Obninsk, Russia 23 Technische Universit¨ at Wien, Austria 24 Paul Scherrer Institut (PSI), Villingen, Switzerland 25 European Commission, Joint Research Centre, Geel, Retieseweg 111, 2440 Geel, Belgium 26 Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 27 Tokyo Institute of Technology, Japan 28 Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA 29 University of Vienna, Faculty of Physics, Vienna, Austria 30 Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 31 Dipartimento di Fisica e Astronomia, Universit` a di Bologna, Italy 32 Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy ae-mail: [email protected] 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, 11054 (2017) DOI: 10.1051/epjconf/201714611054 ND2016 33 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 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 236U isotope plays an important role in nuclear systems, both for future and currently operating ones. The actual knowledge of the capture reaction of this isotope is satisfactory in the thermal region, but it is considered insufficient for Fast Reactor and ADS applications. For this reason the 236U(n, γ) reaction crosssection has been measured for the first time in the whole energy region from thermal energy up to 1 MeV at the n TOF facility with two different detection systems: an array of C6D6detectors, employing the total energy deposited method, and a 4πtotal absorption calorimeter (TAC), made of 40 BaF2crystals. The two nTOF data sets agree with each other within the statistical uncertainty in the Resolved Resonance Region up to 800 eV, while sizable differences (up to ≃20%) are found relative to the current evaluated data libraries. Moreover two new resonances have been found in the n TOF data. In the Unresolved Resonance Region up to 200 keV, the n TOF results show a reasonable agreement with previous measurements and evaluated data. 1. Introduction 236U plays an important role in nuclear systems, both innovative or already in use. In the current reactors based on U/Pu it is important in the neutron balance in the core and in the equilibrium fuel composition. Regarding the future reactors based on the Th/U cycle, 236U plays the same role as 242Pu in the traditional fuel cycle, therefore its contribution to the fraction of absorbed neutrons is relevant. The actual knowledge of the capture cross-section of 236U is considered satisfactory in the thermal region, but insufficient for Fast Reactors and ADS applications; in these cases the target accuracy is 10% [1], which is therefore the aim of the present measurement. Major data libraries are based on a few experimental measurements characterized by poor accuracy and resolution and covering incomplete energy ranges. Apart for the low-energy 1/v region, where the major evaluations like JENDL-4.0, ENDF/B-VII.1 and JEFF-3.2 show a smooth trend and are in agreement between each other within a few percent, discrepancies are observed between evaluated data and experimental results, in particular in the Resolved Resonance Region (RRR), which are well above the required accuracy. Moreover, the RRR is limited to 1.5 keV in most of the libraries, while it extends up to 4.0 keV in JENDL-4.0. In the Unresolved Resonance Region (URR), evaluated cross-section below 200 keV are consistent to each other, although experimental data show some discrepancies, while above 200 keV both evaluations and experimental data show large variations. The present situation is therefore unacceptable for future nuclear applications. This situation motivated a new measurement of the capture cross-section of 236U at the CERN n TOF facility [2]. To reduce systematic uncertainties, the measurement was carried out with two independent detection techniques based on a Total Absorption Calorimeter [3] and a pair of C6D6 detectors [4]. 2. The experimental setup The C6D6detectors, characterized by a low neutron sensitivity and γ-ray effciency, are mounted at 135◦with respect to the sample; they rely on the use of the Pulse Height Weigting Technique [5] which requires that at most one γ-ray per capture event is detected. The second detection system (TAC), made of 40 BaF2crystals and characterized by high geometric and intrinsic effciciency, allows detection and reconstruction of the full γ-ray cascade of the capture event. However, due to its large sensitivity to neutrons and to the γ-flash, it was not possible to collect data above a few tens of keV. The uranium sample consists of a high purity pressed disk pellet 99.85% enriched in 236U3O8with a total mass of 399 mg. Known contaminants are 0.05% of 235U and 0.1% of 238U. The pressed pellet is placed inside a disk shaped capsule made of high purity and normal aluminum. A capsule without sample inside, called “dummy”, was built with the same dimensions of the aluminum capsule. The sample was inserted between two thin kapton foils and this assembly was glued on a carbon fiber frame mounted on the remotely-controlled sample exchanger together with other samples used for the determination of the background: an “empty” sample, made of just the kapton foils, the dummy sample, a nat Pb sample for in-beam γrays and a natural gold sample used for normalization purposes. All samples were made with the same diameter as the U3O8pellet. 3. Results 3.1. The resolved resonance region The background-subtracted capture yield measured with the C6D6setup is shown in Fig. 1. The resolved resonance region was analyzed by means of the R-matrix code SAMMY. The yield has been parametrized via the Reich-Moore approximation; experimental effects as Doppler broadening, sample scattering, isotopic correction for contaminants, self-shielding and n TOF resolution function are properly taken into account within the SAMMY code. In the SAMMY fit a residual constant background was considered that was probably due to an underestimated contribution of the aluminum capsule. However, in the Resolved Resonanance Region, this component is only ≃0.04% in the extracted capture yield. In the fitting procedure for the Resonance Shape Analysis (RSA), the resonance energy (ER) and both partial widths (γand n) were allowed to vary. While the resulting 236U resonance parameters are not very accurate, this procedure provides an accurate value for the capture kernel, gγn/. As a starting point for the SAMMY fits, the resonance parameters from the JENDL-4.0 data library 2 EPJ Web of Conferences 146, 11054 (2017) DOI: 10.1051/epjconf/201714611054 ND2016 Neutron energy (eV) 1 10 2 10 3 10 4 10 5 10 Capture yield -4 10 -3 10 -2 10 -1 10 1 Figure 1. Capture yield obtained after normalization and background subtraction. The n TOF data shown here are collected with C6D6detection system. Neutron energy (eV) 30 32 34 36 38 40 42 44 Capture yield 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 6 D 6 C SAMMY JENDL-4.0 ENDF/B-VII.1 Figure 2. Comparison between C6D6data fitted by SAMMY and major data libraries. Neutron energy (eV) 75 75.5 76 76.5 77 77.5 78 78.5 Capture yield 0 0.002 0.004 0.006 0.008 0.01 6 D 6 C SAMMY JENDL-4.0 ENDF/B-VII.1 Figure 3. Structure at 76.7 eV in the C6D6data compared with major data libraries. The resulting SAMMY fit is also shown. were taken for the C6D6and from ENDF/B-VII.1 for the TAC analysis. Although structures are present, the lower statistics of C6D6comparated to the TAC data does not allow to analyze resonance above 800 eV with a statistical uncertainty on the capture kernel below 40%. An example of the fitted yield of C6D6data is shown in Fig. 2. Two structures at 76.7 and 362.9 eV not present in major data libraries have been found in the C6D6data set, as shown in Figs. 3and 4, and were confirmed by the TAC measurement. A possible candidate is a 237Np contamination in the sample as a result of the capture Neutron energy (eV) 356 358 360 362 364 366 368 Capture yield 0 0.002 0.004 0.006 0.008 0.01 6 D 6 C SAMMY JENDL-4.0 ENDF/B-VII.1 Figure 4. Structure at 362.9 eV of C6D6data compared with major data libraries. The resulting SAMMY fit is also shown. Neutron energy (eV) 0 100 200 300 400 500 600 700 800 Kernel ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 / TAC 6 D 6 C Figure 5. Comparison between the capture kernels determined from the C6D6and TAC data. Neutron energy (eV) 0 100 200 300 400 500 600 700 800 Kernel ratio 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 / JENDL-4.0 6 D 6 C / ENDF/B-V.II.1 6 D 6 C Figure 6. Comparison between capture kernels obtained from the C6D6measurement with the corresponding values from data libraries. reaction on 236U(n, γ)237U followed by β-decay. However, since there is no evidence of other, stronger resonances of 237Np in the analyzed energy range, these resonances could be attributed to 236U. The capture kernel is sensitive to systematic effects related to the experimental setup, in particular to the neutron sensitivity. In the present analysis, these effects have been correctly taken into account, as demonstrated by Fig. 5, where the ratio between the capture kernels calculated from the resonance parameters obtained from C6D6and from the TAC data is shown. The good agreement between the two datasets and the resulting average difference of less than 1% confirms that the 3 EPJ Web of Conferences 146, 11054 (2017) DOI: 10.1051/epjconf/201714611054 ND2016 Neutron energy (eV) 4 10 5 10 ) cross-section (b)γ U(n, 236 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 6 D 6 C ENDF/B-VII.1 JENDL-4.0 Figure 7. Comparison between C6D6data and major data libraries in the URR. Neutron energy (eV) 4 10 5 10 ) cross-section (b)γ U(n, 236 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 6 D 6 C Kazakov (1988) Adamchuk (1987) Buleeva (1985) Carlson (1970) Barry (1961) Figure 8. Comparison between C6D6data and previous results in the URR. capture yield in both independent analyses was consistent and reliable. A comparison of the capture kernels measured at nTOF with the ones calculated from data libraries (see Fig. 6) shows a deviation of 16% relative to JENDL-4.0 and of 23% relative to ENDF/B-V.II.1. Between 800 eV and 1.5 keV, the resonance analysis has been performed only on the TAC data. Structures above 1.5 keV are present in both data sets, confirming the structures in the evaluated cross section of JENDL-4.0. However, in this case the low statistics makes it difficult to perform a resonance analysis above 1.5 keV neutron energy. 3.2. The unresolved resonance region Regarding the unresolved resonance region, a comparison between the C6D6average cross-section data and major data libraries shown in Fig. 7reveals that in this energy range the present results agree to better than 5% with major evaluations up to 700-800 keV. Due to the prompt γ-flash and inelastic reactions, data above this energy range are not very reliable. The two structures in the data at 5.9 and 34.7 keV are not due to the 236U(n, γ) reaction, but are consistent with the 27Al(n, γ) reactions in the capsule, which are affected by a sizable uncertainty. This result and the constant background found in the RRR, indicate that the contribution of the aluminum capsule to the total capture yield is underestimated. A comparison with previous results shown in Fig. 8, indicates that the present data, apart from the discrepancies due to the aluminum structures, agree better than 7% with most of the previous measurements up to 700–800 keV. As a final remark, it should be considered that the present data, affected by a systematic uncertainty around 7%, are certainly the most accurate obtained so far in this energy region that is particularly important for the development of fast reactors. References [1] V.G. Pronyavev, Assessment of Nuclear Data Needs for Thorium and other Advanced Cycles, INDC(NDS), IAEA (1999) [2] C. Guerrero et al., Eur. Phys. J. A 49, 27 (2013) [3] C. Guerrero et al., Nucl. Instr. and Meth. A 608, 424– 433 (2009) [4] R. Plag et al., Nucl. Instr. and Meth. A 496, 425–436 (2003) [5] A. Borella et al., Nucl. Instr. and Meth. A 577, 626– 640 (2007) 4