Description and outlook of the 50,53Cr(n,γ) cross section measurement at n_TOF and HiSPANoS

Description and outlook of the 50,53Cr(n,γ) cross section measurement at n_TOF and HiSPANoS P. Pérez-Maroto1, C. Guerrero1, A. Casanovas2, B. Fernández1, O. Aberle3, V. Alcayne4, S. Altieri5,6, S. Amaducci7, J. Andrzejewski8, V. Babiano-Suarez9, M. Bacak3, J. Balibrea Correa9, C. Beltrami5, S. Bennett10, A. P. Bernardes3, E. Berthoumieux11, R. Beyer12, M. Boromiza13, D. Bosnar14, M. Caamaño15, F. Calviño2, M. Calviani3, D. Cano-Ott4, D. M. Castelluccio16,17, F. Cerutti3, G. Cescutti18,19, S. Chasapoglou20, E. Chiaveri3,10, P. Colombetti21,22, N. Colonna23, P. Console Camprini17,16, G. Cortés2, M. A. Cortés- Giraldo1, L. Cosentino7, S. Cristallo24,25, S. Dellmann26, M. Di Castro3, S. Di Maria27, M. Diakaki20, M. Dietz28, C. Domingo-Pardo9, R. Dressler29, E. Dupont11, I. Durán15, Z. Eleme30, S. Fargier3, B. Fernández1, B. Fernández-Domínguez15, P. Finocchiaro7, S. Fiore16,31, V. Furman32, F. García-Infantes33,3, A. Gawlik-Rami˛ega8, G. Gervino21,22, S. Gilardoni3, E. González-Romero4, F. Gunsing11, C. Gustavino31, J. Heyse34, W. Hillman10, D. G. Jenkins35, E. Jericha36, A. Junghans12, Y. Kadi3, K. Kaperoni20, G. Kaur11, A. Kimura37, I. Knapová38, M. Kokkoris20, Y. Kopatch32, M. Krtiˇ cka38, N. Kyritsis20, I. Ladarescu9, C. Lederer-Woods39, J. Lerendegui-Marco9, G. Lerner3, A. Manna17,40, T. Martínez4, A. Masi3, C. Massimi17,40, P. Mastinu41, M. Mastromarco23,42, E. A. Maugeri29, A. Mazzone23,43, E. Mendoza4, A. Mengoni16,17, V. Michalopoulou20, P. M. Milazzo18, R. Mucciola24,44, F. Murtas†45, E. Musacchio-Gonzalez41, A. Musumarra46,47, A. Negret13, A. Pérez de Rada4, N. Patronis30,3, J. A. Pavón-Rodríguez1,3, M. G. Pellegriti46, J. Perkowski8, C. Petrone13, E. Pirovano28, J. Plaza del Olmo4, S. Pomp48, I. Porras33, J. Praena33, J. M. Quesada1, R. Reifarth26, D. Rochman29, Y. Romanets27, C. Rubbia3, A. Sánchez-Caballero4, M. Sabaté-Gilarte3, P. Schillebeeckx34, D. Schumann29, A. Sekhar10, A. G. Smith10, N. V. Sosnin39, M. E. Stamati30,3, A. Sturniolo21, G. Tagliente23, A. Tarifeño- Saldivia2, D. Tarrío48, P. Torres-Sánchez33, E. Vagena30, S. Valenta38, V. Variale23, P. Vaz27, G. Vecchio7, D. Vescovi26, V. Vlachoudis3, R. Vlastou20, A. Wallner12, P. J. Woods39, T. Wright10, R. Zarrella17,40, P. Žugec14, and The n_TOF Collaboration 1Universidad de Sevilla, Spain 2Universitat Politècnica de Catalunya, Spain 3European Organization for Nuclear Research (CERN), Switzerland 4Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain 5Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy 6Department of Physics, University of Pavia, Italy 7INFN Laboratori Nazionali del Sud, Catania, Italy 8University of Lodz, Poland 9Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Spain 10University of Manchester, United Kingdom 11CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 12Helmholtz-Zentrum Dresden-Rossendorf, Germany 13Horia Hulubei National Institute of Physics and Nuclear Engineering, Romania 14Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 15University of Santiago de Compostela, Spain ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). 16Agenzia nazionale per le nuove tecnologie (ENEA), Italy 17Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 18Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy 19Department of Physics, University of Trieste, Italy 20National Technical University of Athens, Greece 21Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Italy 22Department of Physics, University of Torino, Italy 23Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy 24Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Italy 25Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Teramo, Italy 26Goethe University Frankfurt, Germany 27Instituto Superior Técnico, Lisbon, Portugal 28Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 29Paul Scherrer Institut (PSI), Villigen, Switzerland 30University of Ioannina, Greece 31Istituto Nazionale di Fisica Nucleare, Sezione di Roma1, Roma, Italy 32Affiliated with an institute covered by a cooperation agreement with CERN 33University of Granada, Spain 34European Commission, Joint Research Centre (JRC), Geel, Belgium 35University of York, United Kingdom 36TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria 37Japan Atomic Energy Agency (JAEA), Tokai-Mura, Japan 38Charles University, Prague, Czech Republic 39School of Physics and Astronomy, University of Edinburgh, United Kingdom 40Dipartimento di Fisica e Astronomia, Università di Bologna, Italy 41INFN Laboratori Nazionali di Legnaro, Italy 42Dipartimento Interateneo di Fisica, Università degli Studi di Bari, Italy 43Consiglio Nazionale delle Ricerche, Bari, Italy 44Dipartimento di Fisica e Geologia, Università di Perugia, Italy 45INFN Laboratori Nazionali di Frascati, Italy 46Istituto Nazionale di Fisica Nucleare, Sezione di Catania, Italy 47Department of Physics and Astronomy, University of Catania, Italy 48Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden Abstract. Chromium is a very relevant element regarding criticality safety in nuclear reactors because of its presence in stainless steel, an important structural material. Currently, there are serious discrepancies between the different evaluations regarding the neutron capture cross sections of 50Cr and 53Cr, most probably related to the difficulty of reducing and then estimating the very large neutron scattering effects on the shape of the resonances. In this context, there is a recent entry in the Nuclear Energy Agency (NEA) High Priority Request List (HPRL) to measure these reactions between 1 and 100 keV with an accuracy of 8-10%. In response to this request, we have performed a time-of-flight experiment at CERN n_TOF (Switzerland) and a complementary activation experiment on 50Cr at 30 and 90 keV at CNA HiSPANoS (Spain). The experiments are presented herein, together with a discussion on the quality of the preliminary data and the results to be expected. 1 Introduction and motivation Nuclear criticality safety is among the main priorities nowadays regarding nuclear power [1]. As criticality safety aims at preventing an uncontrolled self-sustained nuclear chain reaction, ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 2 the knowledge of absorption cross sections in fuel elements and the structural materials is crucial for the calculations. In this context, the sizable abundance (11-26%) of chromium in stainless steel makes some criticality benchmarks that include large amounts of chromium (e.g., PU-MET-INTER-002 or HEU-COMP-INTER-005/4=KBR-15/Cr) very sensitive to its cross section, in particular capture on 50Cr and 53Cr [2, 3]. These criticality benchmarks show clearly that the current differences in the order of 30% between evaluations have a significant impact of about 1000 pcm in the corresponding criticality safety calculations. For this reason, considering that the differences in the evaluations are due to the limited accuracy of the current cross section data, the Nuclear Energy Agency (NEA) [4] included an entry in its High Priority Request List (HPRL) to measure the capture cross section of 50Cr and 53Cr between 1 and 100 keV with an accuracy of 8-10%. There have been several measurements of this reaction in the past [5–9]. The aim of these measurements was, in general, to accumulate enough statistics up to a quite high neutron energy (several hundreds keV), hence the samples used were quite thick. Given the scattering-to-capture cross section ratio of about 300 in Cr isotopes, the associated multiplescattering corrections (neutrons suffering one or more collisions inside the sample before being captured) are between 50% and 80% in previous experiments, which seems to be the reason behind the disagreement among them. It should also be mentioned that the majority of previous experiments did not employ low neutron sensitivity detectors to limit the effect of the large neutron scattering background. In order to respond to the NEA-HPRL call, two experiments have been designed to overcome the issues of previous experiments. In the time-of-flight experiment, in addition to using low neutron sensitivity C6D6detectors, we have used (very) thin and “thick” samples to keep the neutron multiple-scattering to a minimum and still achieve, by measuring at the high neutron flux CERN n_TOF-EAR1 facility [10, 11], enough statistics in the region up to 100 keV. This is complemented in the case of 50Cr by an activation measurement at CNA HiSPANoS [12] with quasi-Maxwellian neutron beams of 30 and 90 keV. 2 Time-of-Flight experiment at n_TOF-EAR1 The n_TOF facility has been in operation at CERN since 2001. The CERN Proton Synchrotron (CPS) sends 20 GeV/c proton pulses into a cylindrical lead target with a repetition rate of 0.8 Hz on average, producing neutrons by spallation. The neutrons are then moderated in borated water and travel 185 meters horizontally towards the EAR1 experimental area, 20 meters vertically towards EAR2 and 3 meters horizontally towards the NEAR station. The measurement presented herein was performed in EAR1 because of its superior energy resolution, which allows resolving resonances up to higher energies. When the neutrons arrive to EAR1, after traversing the SiMon monitor [13], they impact in the sample, which is surrounded by four carbon fiber C6D6detectors [14] in order to observe the γ-rays cascades following neutron capture reactions. The material of the samples was purchased to Trace Sciences International, consisting of fine powder of enriched Cr2O3, 94.6(4)% for 50Cr and 97.7(2)% for 53Cr. The powder was pressed into a 20 mm diameter PEEK capsule, achieving the thicknesses listed in Table 1. In addition, some auxiliary samples were also measured: natCr, a dummy (PEEK capsule), empty and 197Au. A total number of 4.6·1018 protons impinged in the lead target during the 42 days of the experiment. Cr is a relatively light nucleus and hence it features a very hard capture cascade, with a maximum γ-ray emission energy of 8.5 MeV for 50Cr and 9.7 MeV for 53Cr. This can be observed in Fig. 1, which shows the energy deposited spectra of the γ-cascade measured by the C6D6detectors for the thin samples of both isotopes. These energy spectra will be used ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 3 Table 1. Characteristics of the detectors and samples used in this work and in the previous measurements. The thicknesses have been minimized to reduce the multiple-scattering corrections while providing the necessary statistics in a reasonable beam time. Stieglitz Beer Kenny Brusegan Guber Our work (1971) (1975) (1977) (1986) (2011) (2022) (n,γ) detector Liquid Sc. Liquid Sc. C6F6C6D6C6D6C6D6 50Cr nat(10−3at/barn) 8 18 5/8 7 - 0.6/1.9 53Cr nat(10−3at/barn) 14 14 8/12 12/60 14 1.2/6.0 for validating the simulations of the detector response and the decay cascades necessary to implement the Pulse Height Weighting Technique (PHWT) [15]. 2 4 6 8 10 12 14 Deposited energy (MeV) 4− 10 3− 10 2− 10 1− 10 1 10 protons 12 10⋅Counts/bin/7 Cr-50 thin Dummy Cr-50 thin (back sub) 2 4 6 8 10 12 14 Deposited energy (MeV) 4− 10 3− 10 2− 10 1− 10 1 10 protons 12 10⋅Counts/bin/7 Cr-53 thin Dummy Cr-53 thin (back sub) Figure 1. Deposited energy spectra of the 50Cr thin sample (left) and the 53Cr thin sample (right) γ-rays cascades on the C6D6detectors. 3 10 4 10 5 10 Energy (eV) 3− 10 2− 10 protons 12 10⋅Counts/bin/7 Cr thick 50 Cr thin 50 Dummy Empty 3 10 4 10 5 10 Energy (eV) 3− 10 2− 10 protons 12 10⋅Counts/bin/7 Cr thick 53 Cr thin 53 Dummy thick Dummy Empty Figure 2. Counting rate per nominal proton pulse of 50Cr (left) and 53Cr (right), for both thin and thick samples. For the 53Cr thick sample it was necessary to use a bigger capsule to fit all the material inside. The first resonance at 1.6 keV corresponds to 52Cr, the main contaminant in the enriched samples. ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 4 The measured time-of-flight distributions for every Cr sample and the background measurements in the energy range of interest are displayed in Fig. 2. A difference in the background contribution and in the shape of the main resonance is visible between the thin and the thick sample, specially in 53Cr as it is the most massive sample. It is caused by neutron scattering, illustrating the importance of the thickness of the sample in this respect. The capture data shows individual resonances with limited but enough statistics in the full energy rage of interest, i.e. up to 100 keV. Having achieved this by means of very thin samples, it is expected that the quality and scope of the data will provide what is requested in the corresponding NEA HPRL entry. 350Cr activation at HiSPANoS 51Cr is an unstable isotope with T1/2=27.7 days, decaying by electron capture and emitting a 320.1 keV γ-ray (Iγ=9.91(1)%). This allows for a neutron activation measurement to obtain an integral cross section complementary to the time-of-flight experiment. The MACS (Maxwellian Averaged Cross Section) of 50Cr has been measured at the HiS- PANoS [12] facility at CNA [16]. A quasi-Maxwellian neutron spectrum (QMNS) with a temperature of 30 keV has been produced as usual via the 7Li(p,n)7Be reaction with 1912 keV protons [17]. However, the energy range of interest goes up to 100 keV , but it is not straightforward to produce directly a high energy QMNS, for instance 90 keV. This has been achieved by a linear combination of different neutron spectra, none of them a QMNS, resulting from irradiation of a Li target with different proton energies, as proposed in Ref. [18]. In this case, we have produced a QMNS with 90 keV, which has also a high interest for s-process stellar nucleosynthesis [19], from the combination of irradiations with protons of 1912, 2040, 2080, 2120, 2150 and 2250 keV, resulting in the QMNS displayed in Fig. 3. The contribution of each spectrum has been chosen to minimize the difference between the SACS and the MACS90 of 197Au. It is not the definitive choice as it might not be the most optimal one for the 50Cr cross section, with resonances at higher energies than 197Au. This is under investigation at the moment. 0 100 200 300 400 500 600 Energy (keV) 1− 0.5− 0 0.5 1 1.5 2 2.5 Neutrons (arb. units) = 2040 keV p E = 2080 keV p E = 2120 keV p E = 2150 keV p E = 2250 keV p E = 1912 keV p E Final spectrum 90 keV MB Figure 3. Quasi-Maxwellian 90 keV neutron spectrum produced with a combination of several spectra from different proton energies. These were obtained by simulating the 7Li(p,n) reaction with PINO. The activation was performed at HiSPANoS, using the 3 MV Tandem Pelletron accelerator of CNA. A thick metallic Li target was inserted into an aluminum target holder with ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 5 Figure 4. Experimental set-up for the activation measurement at HiSPANoS: (left) 6Li-glass detectors at three different angles to characterize the neutron spectra, along with a LaBr3detector to measure the 7Be production, (center) 197Au sample at the face of the refrigerated aluminum target and (right) 50Cr sample with two gold foils on both sides. a 500 µm backing, similar to that reported in Ref. [20], that allows for water or air cooling, being the latter sufficient to avoid the melting of the Li target at the 6 µA beam current applied. The neutron spectrum produced by every proton energy used in the activation was characterized by time-of-flight with three 6Li-glass detectors placed at 0º, 40º and 80º (Fig. 4, left), at either 50 or 100 cm from the Li target. The measured spectra will be validated with PINO and SimLiT-GEANT4 [21] simulations before the corresponding SACS and MACS values can be extracted from the ratios of activities of 51Cr (from 50Cr(n,γ) reactions) and the references 198Au (from 197Au(n,γ) reactions) and 7Be (from 7Li(p,n) reactions). A gold foil was activated alone for every proton energy (see Fig. 4, center), as an additional cross-check of the neutron spectra produced. All the activations were measured after each irradiation with a LaBr3detector calibrated with radioactive sources complemented with GEANT4 simulations. The number of activated nuclei in each sample and the corresponding SACS and MACS are calculated using the procedure developed in Ref. [17]. The data analysis is still at a preliminary stage, but the analysis of a single 197Au foil under a QMNS of 30 keV has been almost completed. This simple case is meant as a validation, as the MACS of 197Au at 30 keV is a standard cross section [22]. Using the methodology mentioned above, we have obtained a preliminary MACS30 for 197Au of 600(23) mb. This result is already compatible within 3% and 2% with the values of 612(6) mb from KADONIS [18] and 620(11) mb from the standard cross section [22]. The nice agreement for 197Au allows to be optimistic about the results for 50Cr, for which the aimed accuracy is 8-10%. 4 Summary and conclusions Chromium is quite abundant in stainless steel, used as a structural material in nuclear reactors. Criticality benchmarks show large sensitivity to Cr neutron capture, dominated by 50,53Cr between 1 and 100 keV, changing the criticality in the order of 1000 pcm for a 30% change in the cross section. The current evaluations have discrepancies of around 30%, a difference that is not reported or considered in the corresponding estimated uncertainties. The NEA opened an entry in its HPRL to measure these cross sections within 8 to 10% accuracy. As a response, a measurement of both isotopes was successfully performed at the n_TOF facility at CERN. Because 51Cr is unstable, a complementary 50Cr activation measurement was performed at the HiSPANoS facility at CNA, with the aim to obtain the MACS at a temperature of kT =30 ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 6 and 90 keV. The preliminary results from both experiments show high quality data, hence soon a new data set of 50,53Cr (n,γ) cross sections will be released. Hopefully this new data set can be considered in forthcoming evaluations, contributing to reduce the uncertainty of safety calculations. Acknowledgements These measurements have received funding from the Euroatom research and training programme 2014-2018 under grant agreement No 847594 (ARIEL), and from the Spanish national projects RTI2018-098117-B-C21 and PID2021-123879OB-C21. References [1] J.B. Briggs et al., Nuclear science and engineering 145, 1 (2003) [2] V. Koscheev et al., Use the results of measurements on KBR facility for testing of neutron data of main structural materials for fast reactors, in EPJ Web of Conferences (EDP Sciences, 2017), Vol. 146, p. 06025 [3] A. Trkov et al., Tech. rep. (2018) [4] Dupont, E. et al., EPJ Web Conf. 239, 15005 (2020) [5] R. Stieglitz et al., Nuclear Physics A 163, 592 (1971) [6] H. Beer, R.R. Spencer, Nuclear Physics A 240, 29 (1975) [7] M. Kenny et al., Neutron capture by the chromium isotopes (Australian Atomic Energy Commission, 1977) [8] A. Brusegan et al., Radiation Effects 93, 297 (1986) [9] K.H. Guber et al., Tech. rep., Oak Ridge National Lab.(ORNL), Oak Ridge, TN (United States) (2011) [10] C. Guerrero et al., The European Physical Journal A 49, 1 (2013) [11] R. Esposito et al., Physical Review Accelerators and Beams 24, 093001 (2021) [12] B. Fernández et al., HiSPANoS facility and the new neutron beam line for TOF measurements at the Spanish National Accelerator Lab (CNA), in Journal of Physics: Conference Series (IOP Publishing, 2020), Vol. 1643, p. 012033 [13] S. Marrone et al., Nucl. Instrum. Methods A 517, 389 (2004) [14] R. Plag et al., Nucl. Instrum. Methods A 496, 425 (2003) [15] U. Abbondanno et al., Nucl. Instrum. Methods A 521, 454 (2004) [16] J. Gómez-Camacho et al., The European Physical Journal Plus 136, 273 (2021) [17] W. Ratynski, F. Käppeler, Physical Review C 37, 595 (1988) [18] R. Reifarth et al., The European Physical Journal Plus 133, 424 (2018) [19] R. Käppeler et al., Reviews of Modern Physics 83, 157 (2011) [20] J. Praena, P. Jiménez-Bonilla, Two absolute integral measurements of the 197Au (n, γ) stellar cross-section and solution of the historic discrepancies, in EPJ Web of Conferences (EDP Sciences, 2020), Vol. 239, p. 19002 [21] M. Friedman et al., Nucl. Instrum. Methods A 698, 117 (2013) [22] A.D. Carlson et al., Nuclear Data Sheets 148, 143 (2018) ,01004 (2024) EPJ Web of Conferences https://doi.org/10.1051/epjconf/202429401004 294 WONDER-2023 7