First Results of the 140Ce(n,γ)141Ce Cross-Section Measurement at n_TOF

universe Communication First Results of the 140Ce(n,γ)141Ce Cross-Section Measurement at n_TOF Simone Amaducci 1,2,* , Nicola Colonna 3, Luigi Cosentino 1, Sergio Cristallo 4,5, Paolo Finocchiaro 1, Milan Krtiˇcka 6, Cristian Massimi 7,8, Mario Mastromarco 9, Annamaria Mazzone 3,10, Alberto Mengoni 11, Stanislav Valenta 6, Oliver Aberle 9, Victor Alcayne 12, Józef Andrzejewski 13, Laurent Audouin 14, Victor Babiano-Suarez 15, Michael Bacak 9,16,17, Massimo Barbagallo 3,9, Samuel Bennett 18, Eric Berthoumieux 17, Jon Billowes 18, Damir Bosnar 19, Adam Brown 20, Maurizio Busso 4,21, Manuel Caamaño 22, Luis Caballero-Ontanaya 15, Francisco Calviño 23, Marco Calviani 9, Daniel Cano-Ott 12, Adria Casanovas 23, Francesco Cerutti 9, Enrico Chiaveri 9,18, Guillem Cortés 23, Miguel Cortés-Giraldo 24, Lucia-Anna Damone 3,25, Paul-John Davies 18, Maria Diakaki 9,26, Mirco Dietz 27, Cesar Domingo-Pardo 15, Rugard Dressler 28, Quentin Ducasse 29, Emmeric Dupont 17, Ignacio Durán 22, Zinovia Eleme 30, Beatriz Fernández-Domínguez 22, Alfredo Ferrari 9, Valter Furman 31, Kathrin Göbel 32, Ruchi Garg 27, Aleksandra Gawlik 13, Simone Gilardoni 9, Isabel Gonçalves 33, Enrique González-Romero 12, Carlos Guerrero 24, Frank Gunsing 17, Hideo Harada 34, Stephan Heinitz 28, Jan Heyse 35, David Jenkins 20, Arnd Junghans 36, Franz Käppeler 37, Yacine Kadi 9, Atsushi Kimura 34, Ingrid Knapova 11, Michael Kokkoris 26, Yuri Kopatch 31, Deniz Kurtulgil 32, Ion Ladarescu 15, Claudia Lederer-Woods 27, Helmut Leeb 16, Jorge Lerendegui-Marco 24, Sarah-Jane Lonsdale 27, Daniela Macina 9, Alice Manna 7,8, Trinitario Martínez 12, Alessandro Masi 9, Pierfrancesco Mastinu 38, Emilio-Andrea Maugeri 28, Emilio Mendoza 12, Veatriki Michalopoulou 9,26, Paolo Milazzo 39, Federica Mingrone 9, Javier Moreno-Soto 17, Agatino Musumarra 1,40, Alexandru Negret 41, Francisco Ogállar 42, Andreea Oprea 41, Nikolas Patronis 30, Andreas Pavlik 43, Jarosław Perkowski 13, Luciano Piersanti 4,5, Cristina Petrone 41, Elisa Pirovano 29, Ignacio Porras 42, Javier Praena 42, José-Manuel Quesada 24, Diego Ramos-Doval 14, Thomas Rauscher 44,45, René Reifarth 32, Dimitri Rochman 28, Carlo Rubbia 9, Marta Sabaté-Gilarte 9,24, Alok Saxena 46, Peter Schillebeeckx 35, Dorothea Schumann 28, Adhitya Sekhar 18, Gavin Smith 18, Nikolay Sosnin 18, Peter Sprung 28, Athanasios Stamatopoulos 26, Giuseppe Tagliente 3, José Tain 15, Ariel Tarifeño-Saldivia 23, Laurent Tassan-Got 9,26,14, Benedikt Thomas 32, Pablo Torres-Sánchez 42, Andrea Tsinganis 9, Jiri Ulrich 28, Sebastian Urlass 9,36, Gianni Vannini 7,8, Vincenzo Variale 3, Pedro Vaz 33, Alberto Ventura 7, Diego Vescovi 4,5,47, Vasilis Vlachoudis 9, Rosa Vlastou 26, Anton Wallner 48, PhilipJohn Woods 27, Tobias Wright 18, Petar Žugec 19 and on behalf of the n_TOF Collaboration   Citation: Amaducci, S.; Colonna, N.; Cosentino, L.; Cristallo, S.; Finocchiaro, P.; Krtiˇcka, M.; Massimi, C.; Mastromarco, M.; Mazzone, A.; Mengoni, A.; et al. First Results of the 140Ce(n,γ)141Ce Cross-Section Measurement at n_TOF. Universe 2021,7, 200. https://doi.org/ 10.3390/universe7060200 Academic Editors: Sergio Cristallo and Paolo Ventura Received: 31 March 2021 Accepted: 17 May 2021 Published: 17 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 1INFN Laboratori Nazionali del Sud, 95125 Catania, Italy 2Dipartimento di Fisica e Astronomia, Università di Catania, 2, 95131 Catania, Italy 3Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy 4Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, 06123 Perugia, Italy 5Istituto Nazionale di Astrofisica—Osservatorio Astronomico d’Abruzzo, 64100 Collurania, Italy 6Charles University, 11000 Prague, Czech Republic 7Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, 40127 Bologna, Italy 8Dipartimento di Fisica e Astronomia, Università di Bologna, 40129 Bologna, Italy 9European Organization for Nuclear Research (CERN), 1211 Geneva, Switzerland 10 Consiglio Nazionale delle Ricerche, 7, 00185 Bari, Italy 11 Agenzia Nazionale per le Nuove Tecnologie (ENEA), 40129 Bologna, Italy 12 Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), 28040 Madrid, Spain 13 University of Lodz, 90-137 Łód´z, Poland 14 Institut de Physique Nucléaire, CNRS-IN2P3, University Paris-Sud, Université Paris-Saclay, CEDEX, F-91406 Orsay, France 15 Instituto de Física Corpuscular, CSIC—Universidad de Valencia, 46980 Paterna, Spain 16 Technische Universität Wien, 1040 Vienna, Austria 17 CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 18 University of Manchester, Manchester M13 9PL, UK 19 Department of Physics, Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia 20 University of York, York YO10 5DD, UK 21 Dipartimento di Fisica e Geologia, Università di Perugia, 06123 Perugia PG, Italy 22 University of Santiago de Compostela, Santiago de Compostela, 15705 Compostela, Spain 23 Universitat Politècnica de Catalunya, 08034 Barcelona, Spain 24 Universidad de Sevilla, 41004 Sevilla, Spain Universe 2021,7, 200. https://doi.org/10.3390/universe7060200 https://www.mdpi.com/journal/universe Universe 2021,7, 200 2 of 11 Copyright: c 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 25 Dipartimento di Fisica, Università degli Studi di Bari, 70121 Bari, Italy 26 National Technical University of Athens, Athens, 10682, Greece 27 School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, UK 28 Paul Scherrer Institut (PSI), 5232 Villingen, Switzerland 29 Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 30 University of Ioannina, 451 10 Ioannina, Greece 31 Joint Institute for Nuclear Research (JINR), 141980 Dubna, Russia 32 Goethe University Frankfurt, 60323 Frankfurt am Main, Germany 33 Instituto Superior Técnico, 1049-001 Lisbon, Portugal 34 Japan Atomic Energy Agency (JAEA), Tokai-mura 319-1184, Japan 35 European Commission, Joint Research Centre, Geel, Retieseweg 111, B-2440 Geel, Belgium 36 Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 37 Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 38 Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, 35020 Legnaro, Italy 39 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, 34149 Trieste, Italy 40 Dipartimento di Fisica e Astronomia, Università di Catania, 95123 Catania, Italy 41 Horia Hulubei National Institute of Physics and Nuclear Engineering, P.O. Box MG-6, RO-76900 Bucharest, Romania 42 University of Granada, 18010 Granada, Spain 43 Faculty of Physics, University of Vienna, 1090 Vienna, Austria 44 Department of Physics, University of Basel, CH-4056 Basel, Switzerland 45 Centre for Astrophysics Research, University of Hertfordshire, Herts AL10 9AB, UK 46 Bhabha Atomic Research Centre (BARC), Maharashtra 400094, India 47 Gran Sasso Science Institute, 67100 L’Aquila, Italy 48 Australian National University, 2600 Canberra, Australia *Correspondence: [email protected]; Tel.: +39-3387997513 Abstract: An accurate measurement of the 140 Ce(n, γ ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the 140 Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in 140 Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the 140 Ce Maxwellian-averaged cross-section. Keywords: cerium; 140 Ce; neutron; capture; cross-section; s-process; n_TOF; MACS; nucleosynthesis 1. Introduction It has been well ascertained since the late 1950s that the vast majority of the elements above the iron peak are synthesized in stars, via sequences of neutron captures and β - decays [ 1 , 2 ]. Depending on the typical time elapsing between two consecutive neutron captures, and hence on the available neutron densities, these processes are referred to as slow (s) or rapid (r). In the r-process, neutron densities as high as 10 18 –10 22 cm −3 are attained, triggering the production of very neutron-rich nuclei in an extremely short time, by means of neutron capture sequences much faster than the β -decays. The physical conditions of the r-process are met in explosive scenarios, such as neutron star mergers or core-collapse supernovae, which have typical time scales of the order of a few seconds. The s-process mainly takes place in the late evolutionary phases of low-mass stars, in particular during their thermally pulsing asymptotic giant branch (TP-AGB) phase. During that evolutionary stage, a succession of burning and mixing episodes leads to the production of neutrons through the reactions 13 C( α ,n) 16 O and 22 Ne( α ,n) 25 Mg. While the former reaction Universe 2021,7, 200 3 of 11 is the major neutron source in low-mass AGBs, the latter significantly contributes to heavyelement nucleosynthesis in more massive AGBs (5–6 M sun ) (see, e.g., [ 3 , 4 ]). Typical neutron densities in AGB stars are ≈ 10 7 cm −3 : the relatively long time between two consecutive captures allows the unstable nuclei to eventually β -decay; the resulting s-process path then follows the valley of stability up to the synthesis of lead and bismuth. In the last few decades, accurate knowledge of the s-process site led to a considerable effort in modeling the evolution of the AGB stars and evaluating the contribution of all the nuclear reactions involved in the nucleosynthesis of heavy elements (see, e.g., [ 5 – 8 ]). These models allowed studying the AGB chemical evolution and their role as polluters of the galactic medium. Clearly, high-quality nuclear data are required in order to determine the final abundances of all the elements produced in stellar interiors. This holds in particular for neutron capture cross-sections. The comparison between the observed abundances and the ones predicted by stellar models represents an essential tool to test the robustness of the models, in particular for those elements that are synthesized mainly via the s-process. Such a kind of comparison was carried out by Straniero et al. [ 9 ], considering the globular clusters M4 [ 10 ] and M22 [ 11 ]. Thanks to the presence of many stars, which allow calculating the average distributions and the clear determination of the r-process contribution, these clusters represent an ideal site to test the robustness of s-process predictions. Figure 1shows the comparison between stellar models’ predictions and the chemical composition of M22 stars, in the usual spectroscopic notation 1 . In this case, AGB stars in the mass range 3–6 M sun contribute to the s-process production. In general, a good agreement is observed for most of the elements, but a large discrepancy is present in the case of Ce (Z = 58). For cerium, belonging to the second s-process peak (Ba-La-Pr-Nd), the theoretical expectation is ≈ 30% lower than the values observed in the M22 cluster, while nearby elements astonishingly agree with the theory. Being a neutron-magic nucleus, 140 Ce represents a very interesting isotope, since its very small capture cross-section acts as a bottleneck for the s-process, greatly enhancing its abundance with respect to the nearby non-magic isotopes. Since the majority of natural cerium is made of 140 Ce (89%), its destruction channel, i.e., the capture of a neutron, largely determines the cerium abundance on the stellar surface. The reaction 140 Ce(n, γ ) lacks accurate experimental data, while its production channel, the neutron capture on 139 La, has already been investigated with high accuracy at n_TOF [ 12 ]. Therefore, a nuclear origin of the discrepancy needed to be further investigated, since a reduction of the 140 Ce(n, γ ) cross-section could justify the observed overestimation. Considering the potential contributions from AGBs with different initial masses, the evaluation of a variation of the neutron capture rate on the whole energy spectrum will be performed when the stellar neutron capture rate is available. The main nuclear capture quantities that serve as the input for s-process nucleosynthesis models are the MACS, i.e., the convolution of the capture cross-sections and the Maxwellian energy distribution of neutrons for a given temperature kT. The results presented in [ 9 ] were obtained by using the MACS provided by the database KADoNiS0.3 [ 13 ], which is a (on-line) database for cross-sections relevant to the s- and p-processes. The MACS reported in KADoNiS comes from activation measurement [ 14 ] of a natural cerium sample using a quasi-stellar neutron spectrum of kT = 25 keV. Theoretical models, based on the Hauser–Feshbach (HF) theory, permitted extrapolating the experimental MACS for other values of kT. The HF calculations of the 30 keV MACS reported in [ 14 ] overestimated the experimental MACS by 30% in the case of 140 Ce. The disagreement is related to the uncertainty and incorrectness of the average resonance parameters used in the calculations. Although there exist estimates based on experimental data of resonance spacing (D0) and radiation width ( Γγ ) for s-wave resonances, any estimate of the average Γγ for p-wave (and higher l) resonances is missing (and is thus based only on calculations). Furthermore, since the large part of the 140 Ce captures take place at kT ≈ 8 keV, its abundance results in being even more sensitive to the extrapolation of the MACS towards lower energies, especially because relatively few resonances are present in this energy range. Universe 2021,7, 200 4 of 11 Figure 1. Average abundances observed in the globular cluster M22 (red points) compared to the theoretical expectations deduced from stellar models (from [9]). Very few experimental energy-dependent cross-sections of 140 Ce are available, and the cross-section reported by the nuclear libraries largely relies on transmission experiments, usually performed with natural cerium samples, hence with large effects due to the presence of 142 Ce (which acts as a contaminant). The only capture measurement with an energy resolution sufficient to effectively resolve individual resonance was performed by Musgrove et al. [ 15 ], where C 6 F 6 detectors were employed, which are known to be not particularly suited for capture measurement on isotopes with very high scattering cross-sections [16], such as 140Ce. The lack of highly reliable experimental data makes the evaluation process very challenging, so that it is not surprising that the MACS evaluated with the resonance parameters of the major nuclear libraries (as ENDF, JEFF, and JENDL) led to very different results, as shown in Figure 2. In particular, the MACS calculated with the ENDF/B- VIII [ 17 ], JENDL-4.0 [ 18 ], and JEFF3.3 [ 19 ] resonance parameters largely disagree, and all are systematically lower than KADoNiS0.3 and the most recent KADoNiS1.0 [ 20 ]. The accuracy of the MACS can be significantly improved by adding new experimental data. In particular, more strict constraints on the statistical model parameters are required to reduce the uncertainty of the theoretical calculations. In order to clarify the discrepancy that emerged in [ 9 ] and to produce a more accurate evaluation of the MACS, a new measurement of the 140Ce(n,γ) cross-section was performed at the n_TOF facility in 2018. Universe 2021,7, 200 5 of 11 Figure 2. The MACS calculated with the resonance parameters provided by major libraries compared with the values reported in KADoNiS0.3 and KADoNiS1.0. 2. Experimental Apparatus and Data Analysis The n_TOF facility represents a unique site where high-precision measurements of neutron-induced cross-sections can be performed. Since its operational start in 2001, the n_TOF collaboration [ 21 ] has largely contributed to the improvement of nuclear data of interest for the nuclear astrophysics community, in particular performing many accurate neutron capture cross-section measurements (see, e.g., the two recent works [ 22 , 23 ]). The n_TOF white pulsed neutron beam is produced by spallation reactions on a cylindrical lead target induced by a 20 GeV/c proton beam accelerated with the CERN Proton Synchrotron (PS). The kinetic energy of the neutrons reaching the two experimental areas is measured with the time-of-flight technique. The 140 Ce(n, γ ) measurement required the high-energy resolution of the beam present in Experimental Area 1 (EAR1), thanks to its long flight path of ≈ 185 m. In EAR1, it is possible to reach a resolution from 5 × 10 −4 at 1 keV to 3×10−3 at 100 keV [ 24 ], which is essential to effectively resolve the resonances in the region of interest. The sample employed was composed of 12.318 grams of CeO 2 powder, enriched to 99.4% of 140 Ce, with only 0.6% of 142 Ce as a relevant contamination (natural cerium presents 11% of 142 Ce). The sample was produced at Paul Sherrer Insitut (PSI) via the sintering process. The CeO 2 powder was pressed and enclosed in a PEEK (polyether ether ketone) cylindrical capsule of 1 mm in thickness and heated at 100 ◦ C for 4 h in a glove box with a controlled O 2 and H 2 O atmosphere (concentration lower than 1 ppm). A 197 Au sample, having a diameter almost identical to the cerium one, was used to normalize the cerium data and to exactly determine the flight path length. The latter was obtained from fitting the gold capture resonances in the energy interval from 100 eV to 2 keV. In order to evaluate the different sources of background data, an empty sample was used to measure the component related to the beam, while a lead sample was employed to measure the sample-related background. Finally, the detectors were calibrated, acquiring data on a weekly basis with four γsources: 137Cs, 137Y, Am-Be, and Cm-C. The neutron captures were observed by detecting the γ -rays produced by the decay of the compound nucleus 141 Ce. The experimental apparatus was made of four deuterated benzene (C 6 D 6 ) liquid scintillator detectors [ 25 ] encapsulated in a cylindrical case made of carbon fiber, to guarantee a very low neutron sensitivity. A relatively high neutron sensitivity is one of the difficulties encountered with C 6 F 6 detectors employed in the measurement by [ 15 ]. The detectors were placed at ≈ 10 cm from the center of the sample holder at angles of 125 ◦ with respect to the neutron beam. The adopted configuration minimized the effect of the anisotropic emission of γ -rays; moreover, the upstream position Universe 2021,7, 200 6 of 11 with respect to the sample position reduced the background due to the in-beam γ -rays, which were scattered by the sample. In order to monitor the neutron flux, the Silicon Monitor (SiMon) detector [ 26 ] was installed upstream with respect to the capture apparatus. The data analysis employed the total energy detection principle in combination with the pulse height weighting technique [ 27 , 28 ] to eliminate the dependence of the detection efficiency from the γ cascade path following the neutron capture. This is achieved if the detection efficiency of an i-th γ ray, emitted during the γ cascade, is proportional to its energy Ei γ . In such a case, the detection of the full cascade becomes proportional to its total energy Etot γ: εcascade = m ∑ i=1 εi(Ei γ) = m ∑ i=1 k×Ei γ=k×Etot γ(1) In the case of C 6 D 6 detectors, the proportionality between the deposited energy and the γ -rays detection efficiency is achieved through an off-line weighting function applied to the detector signals. These functions were calculated by simulating the full experimental apparatus with a Monte Carlo simulation, performed with the Geant4 [ 29 ] code. After the weighting procedure, the experimental capture yield can be written as: Yexp(En) = NCw(En)−Bw(En) ε(En)φ(En)(2) where Cw are the weighted count rates with the sample, Bw is the weighted background, φ the neutron flux on the target, and N a normalization factor. The latter includes many geometrical factors, such as the sample area, the beam interception factor (BIF), and the different solid angle of the flux monitor and the capture setup. The neutron flux measured with SiMon was compared with the official n_TOF flux [ 30 ], evaluated with different detectors and standard reactions, resulting in an excellent agreement in the energy interval of interest. Therefore, in order to calculate the 140 Ce capture yield, the official neutron flux was used, which is known with an uncertainty better than 1% below 3 keV and within 4–5% up to 100 keV. The gold sample background was evaluated according to the method described in [ 28 ], using the data collected with the empty sample and with the lead sample, properly scaled. As an example, Figure 3shows the gold neutron energy spectrum for one of the detectors, compared with the background and the beam-off component. The latter corresponds to the ambient background, and it is almost negligible for E n > 10 eV. As expected, from 20 to 50 eV, the gold spectrum is almost equal to the background because of the very small 197 Au capture cross-section, confirming the correctness of the procedure adopted for the background evaluation. The same method could not be applied for cerium, since the component of the background depending on the sample was dominant, due to the very high areal density of the cerium sample (1.291×10−2atoms/barn) and high neutron scattering cross-section. At the present stage of analysis, the cerium background is considered to be linear in the vicinity of each fitted resonance. A more rigorous approach, by means of a Monte Carlo simulation, is currently under study. Universe 2021,7, 200 7 of 11 Figure 3. Energy spectrum measured with the gold sample (red), compared with the total evaluated background (blue) and the ambient background (green). The normalization factor N was calculated by applying the saturated resonance method [ 31 ] to the opaque gold capture resonance at 4.9 eV. The resulting value (N = 0.7127 ± 0.0014) was substantially in agreement with the beam interception factor reported in [ 24 ] for the 2 cm-diameter samples. The cerium data were suitably corrected to take into account the slightly smaller diameter of the sample (1.95 cm). The gold data allowed verifying the robustness of the analysis in the energy interval where the 140 Ce capture resonances are located. Figure 4shows that the experimental results on average agreed with the 197 Au(n, γ ) cross-section evaluated by the ENDF/B-VIII library, in the neutron energy range from 1 keV to 100 keV. Figure 4. Experimental 197 Au(n, γ ) cross-section (black dots) compared to the evaluation of ENDF/B- VIII (red dashed line). A preliminary analysis of the cerium capture yield was carried out with the Bayesian R-matrix code SAMMY [ 32 ]. This code can manage the self-shielding and multiple interactions of neutrons in the sample and other experimental effects such as Doppler and resolution broadening by including the resolution function, which is a property of all neutron time-of-flight facilities, which describes the distribution of the neutron time of flight for a given kinetic energy. The resonance parameters provided by the JENDL-4.0 Universe 2021,7, 200 8 of 11 library were initially adopted as a reference, including the spin-parity assignment, which was almost identical to those of other libraries, such as ENDF/B-VIII. 3. Discussion and perspectives The preliminary 140 Ce data allowed resolving and performing the resonance shape analysis (RSA) of 81 of the 102 resonances reported by the library JENDL-4.0 below 65 keV, to determine their kernel 2 and in some cases the radiation and scattering widths ( Γγ and Γn , respectively). Between 2.5 keV, where the first 140 Ce resonance is located, and 34 keV, the data made it possible to fit the parameters of 46 resonances, while JENDL-4.0 reported 47 resonances. From 34 keV to 65 keV, JENDL-4.0 indicated the presence of 55 resonance, while the experimental data allowed fitting 35 of them; no resonances could be clearly identified above. Only one structure due to the 142 Ce contamination was observed in the experimental capture yield at 1.15 keV, far away from any 140 Ce resonance; hence, the presence of 142Ce did not represent an issue for the analysis. Figure 5shows the contribution to the MACS of a different sub-set of resonances using their parameters from the JENDL-4.0 library. It is noteworthy that the resonances with energies lower than 60 keV (red line) made the major contribution to the MACS in the temperature interval of interest for the s-process (<30 keV). One can also observe the importance of the p-waves’ contribution, which was responsible for approximately 50% of the MACS at 8 keV and even more with increasing energy. The n_TOF measurement ensured the accurate measurements of the resonances kernel; furthermore, it can increase the accuracy on their average width and spacing with respect to the values available during the evaluation of the MACS by [14]. Figure 5. Maxwellian averaged cross-section calculated with the individual resonance parameters as provided by JENDL-4.0 (orange dashed line), compared with the contribution given by different sub-sets of resonances. An example of the RSA in the case of a p-wave resonance is shown in Figure 6, where the quality of the experimental data is clearly sufficient to accurately determine the kernel and resonance energy. It is interesting that the n_TOF fit of the capture yield showed large discrepancies compared to both the JENDL-4.0 and ENDF/B-VIII libraries. As shown by Table 1 , the value of g ΓγΓn/Γ measured at n_TOF was a factor of two larger with respect to both libraries and to the values from [ 15 ]. The results showed an excellent energy resolution achievable at n_TOF; in fact, we were able to determine the resonance energy with a precision of two orders of magnitude better than [15]. Universe 2021,7, 200 9 of 11 Figure 6. Example of the RSA in the case of p-wave resonance; in this case, the n_TOF fit (red) of the capture yield shows large discrepancies, compared to both the JENDL-4.0 (green) and ENDF/B-VIII (blue) libraries. Table 1. Capture kernel measured at n_TOF compared to the values reported by the ENDF/B-VIII and JENDL-4.0 libraries and by [15]. Source Energy (eV) gΓγΓn/Γ(meV) n_TOF 5636.56 ±0.05 21.6 ±1.2 JENDL-4.0 5640 11.0 ENDF/B-VIII 5640 10.5 Musgrove et al. 5640 ±5 10 ±1 The first results demonstrated that the accurate energy-dependent cross-section of 140 Ce(n, γ ) was measured successfully at n_TOF and the resonances parameters were determined with uncertainties significantly lower than previous experiments. The combination of the C 6 D 6 detectors, with their very low neutron sensitivity, and the high energy resolution of n_TOF were decisive to measure this very low capture cross-section. The data allowed performing a reliable RSA of approximately 80 resonances, a large fraction of which are p-waves. These are of particular interest for the s-process, since they provide a larger contribution to the MACS between 8 keV and 30 keV. Direct experimental information will largely determine the MACS at 8 keV and significantly tighten the constraints of the statistical model to calculate the MACS at about 30 keV. The new neutron capture rate, once included in the nucleosynthesis stellar models, will shed more light on the discrepancy with the cerium abundance measured in the M22 globular cluster. Author Contributions: Conceptualization, S.C., C.M. and A.M. (Alberto Mengoni); formal analysis, S.A. and A.M. (Annamaria Mazzone); investigation, S.A., L.C., C.M. and A.M. (Alberto Mengoni); supervision, N.C., L.C., S.C., P.F., C.M. and A.M. (Alberto Mengoni); methodology, M.K. and S.V.; writing—original draft, S.A.; writing—review and editing, all the authors; data curation, all the authors. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The cerium oxide material for this measurement was provided by T. Katabuchi of the Tokyo Institute of Technology. Conflicts of Interest: The authors declare no conflict of interest.