Investigation of the 240Pu (n, f) reaction at the n_TOF/EAR2 facility in the 9 meV–6 MeV range

PHYSICAL REVIEW C 102, 014616 (2020) Investigation of the 240Pu(n,f) reaction at the n_TOF/EAR2 facility in the 9 meV–6 MeV range A. Stamatopoulos ,1,*A. Tsinganis,1,2N. Colonna,3M. Kokkoris,1R. Vlastou,1M. Diakaki,4,1P. Žugec,5P. Schillebeeckx,6 F. Gunsing,4,2M. Sabaté-Gilarte,2,7M. Barbagallo,3O. Aberle,2J. Andrzejewski,8L. Audouin,9V. Bécares,10 M. Bacak,11 J. Balibrea,10 S. Barros,12 F. Beˇ cvᡠr,13 C. Beinrucker,14 F. Belloni,4E. Berthoumieux,4J. Billowes,15 D. Bosnar,5M. Brugger,2 M. Caamaño,16 S. Lo Meo,17,18 F. Calviño,19 M. Calviani,2D. Cano-Ott,10 F. Cerutti,2E. Chiaveri,2G. Cortés,19 M. A. Cortés-Giraldo,7L. Cosentino,20 L. A. Damone,3,21 K. Deo,22 C. Domingo-Pardo,23 R. Dressler,24 E. Dupont,4 I. Durán,16 B. Fernández-Domínguez,16 A. Ferrari,2P. Ferreira,12 P. Finocchiaro,20 R. J. W. Frost,15 V. Furman,25 K. Göbel,14 A. R. García,10 I. Gheorghe,26 T. Glodariu,26,†I. F. Gonçalves,12 E. González-Romero,10 A. Goverdovski,27 E. Griesmayer,11 C. Guerrero,7H. Harada,28 T. Heftrich,14 S. Heinitz,24 A. Hernández-Prieto,2,19 J. Heyse,6D. G. Jenkins,29 E. Jericha,11 F. Käppeler,30 Y. Kadi,2T. Katabuchi,31 P. Kavrigin,11 V. Ketlerov,27 V. Khryachkov,27 A. Kimura,28 N. Kivel,24 I. Knapova,13 M. Krtiˇ cka,13 E. Leal-Cidoncha,16 C. Lederer,14,32 H. Leeb,11 J. Lerendegui-Marco,7M. Licata,18,33 R. Losito,2D. Macina,2 J. Marganiec,8T. Martínez,10 C. Massimi,18,33 P. Mastinu,34 M. Mastromarco,3F. Matteucci,35,36 E. Mendoza,10 A. Mengoni,17 P. M. Milazzo,35 F. Mingrone,18 M. Mirea,26 S. Montesano,2A. Musumarra,20,37 R. Nolte,38 F. R. Palomo-Pinto,7 C. Paradela,16 N. Patronis,39 A. Pavlik,40 J. Perkowski,8A. Plompen,6J. I. Porras,2,41 J. Praena,7J. M. Quesada,7 T. Rauscher,42,43 R. Reifarth,14 A. Riego-Perez,19 M. Robles,16 C. Rubbia,2J. A. Ryan,15 A. Saxena,22 S. Schmidt,14 D. Schumann,24 P. Sedyshev,25 A. G. Smith,15 S. V. Suryanarayana,22 G. Tagliente,3J. L. Tain,23 A. Tarifeño-Saldivia,23 L. Tassan-Got,9S. Valenta,13 G. Vannini,18,33 V. Variale,3P. Vaz,12 A. Ventura,18 V. Vlachoudis,2A. Wallner,44 S. Warren,15 M. Weigand,14 C. Weiss,2,11 and T. Wright15 (n_TOF Collaboration) 1National Technical University of Athens, Athens, Greece 2European Organization for Nuclear Research (CERN), Geneva, Switzerland 3Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Bari, Italy 4CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 5Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 6European Commission, Joint Research Centre, Retieseweg 111, B-2440 Geel, Belgium 7Universidad de Sevilla, Sevilla, Spain 8University of Lodz, Lodz, Poland 9Institut de Physique Nucléaire, CNRS-IN2P3, Université Paris-Sud, Université Paris-Saclay, F-91406 Orsay Cedex, France 10Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 11Technische Universität Wien, Wien, Austria 12Instituto Superior Técnico, Lisbon, Portugal 13Charles University, Prague, Czech Republic 14Goethe University, Frankfurt, Germany 15University of Manchester, Manchester, United Kingdom 16University of Santiago de Compostela, Santiago de Compostela, Spain 17Agenzia Nazionale per le Nuove Tecnologie (ENEA), Bologna, Italy 18Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Bologna, Italy 19Universitat Politècnica de Catalunya, Barcelona, Spain 20INFN Laboratori Nazionali del Sud, Catania, Italy 21Dipartimento di Fisica, Università degli Studi di Bari, Bari, Italy 22Bhabha Atomic Research Centre (BARC), Mumbai, India 23Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Valencia, Spain 24Paul Scherrer Institut (PSI), Villingen, Switzerland 25Joint Institute for Nuclear Research (JINR), Dubna, Russia 26Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 27Institute of Physics and Power Engineering (IPPE), Obninsk, Russia 28Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan 29University of York, York, United Kingdom 30Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 31Tokyo Institute of Technology, Tokyo, Japan 32School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 33Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna, Italy 34Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Legnaro, Italy 35Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Trieste, Italy 2469-9985/2020/102(1)/014616(23) 014616-1 Published by the American Physical Society A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) 36Dipartimento di Astronomia, Università di Trieste, Trieste, Italy 37Dipartimento di Fisica e Astronomia, Università di Catania, Catania, Italy 38Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 39University of Ioannina, Ioannina, Greece 40University of Vienna, Faculty of Physics, Vienna, Austria 41University of Granada, Granada, Spain 42Centre for Astrophysics Research, University of Hertfordshire, Hatfield, United Kingdom 43Department of Physics, University of Basel, Basel, Switzerland 44Australian National University, Canberra, Australia (Received 18 January 2020; accepted 11 May 2020; published 21 July 2020) Background: Nuclear waste management is considered amongst the major challenges in the field of nuclear energy. A possible means of addressing this issue is waste transmutation in advanced nuclear systems, whose operation requires a fast neutron spectrum. In this regard, the accurate knowledge of neutron-induced reaction cross sections of several (minor) actinide isotopes is essential for design optimization and improvement of safety margins of such systems. One such case is 240Pu, due to its accumulation in spent nuclear fuel of thermal reactors and its usage in fast reactor fuel. The measurement of the 240Pu(n,f) cross section was previously attempted at the CERN n_TOF facility EAR1 measuring station using the time-of-flight technique. Due to the low amount of available material and the given flux at EAR1, the measurement had to last several months to achieve a sufficient statistical accuracy. This long duration led to detector deterioration due to the prolonged exposure to the high α activity of the fission foils, therefore the measurement could not be successfully completed. Purpose: It is aimed to determine whether it is feasible to study neutron-induced fission at n_TOF/EAR2 and providedataonthe240Pu(n,f) reaction in energy regions requested for applications. Methods: The study of the 240Pu(n,f) reaction was made at a new experimental area (EAR2) with a shorter flight path which delivered on average 30 times higher flux at fast neutron energies. This enabled the measurement to be performed much faster, thus limiting the exposure of the detectors to the intrinsic activity of the fission foils. The experimental setup was based on microbulk Micromegas detectors and the time-of-flight data were analyzed with an optimized pulse-shape analysis algorithm. Special attention was dedicated to the estimation of the non-negligible counting loss corrections with the development of a new methodology, and other corrections were estimated via Monte Carlo simulations of the experimental setup. Results: This new measurement of the 240Pu(n,f) cross section yielded data from 9 meV up to 6 MeV incident neutron energy and fission resonance kernels were extracted up to 10 keV. Conclusions: Neutron-induced fission of high activity samples can be successfully studied at the n_TOF/EAR2 facility at CERN covering a wide range of neutron energies, from thermal to a few MeV. DOI: 10.1103/PhysRevC.102.014616 I. INTRODUCTION A. Motivation A significant fraction of electricity production (25% in Europe [1]) is based on nuclear sources; however, this results in the accumulation of long-lived radioactive waste. A possible means of disposing this waste is through its transmutation in advanced nuclear systems, such as Gen-IV reactors [2,3] and accelerator driven systems [4,5], which will be operated with a fast neutron spectrum. The consumption of known uranium resources by 2050 [6] should also be considered in the design of future power plants since it constrains the *[email protected] †Deceased. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. nuclear fuel possibilities. The accurate knowledge of neutroninduced reactions is therefore essential for feasibility studies and optimum operation of such systems. At the same time, the improvement of safety margins of thermal reactors which are currently in operation is considered equally important, therefore the accurate knowledge of cross sections on fertile isotopes is also required. In this respect, the Nuclear Energy Agency (NEA) [7] has introduced the High Priority Request List (HPRL) [8] in which data on a plethora of reactions and derived quantities are requested. 240Pu(n,f) is among these reactions since 2008 [9] and up to present the requested accuracies [10] have not been met. 240Pu is a long-lived fertile plutonium isotope and is produced in conventional reactors from neutron capture on 239Pu, therefore it plays an important role in the U/Pu cycle affecting the breeding process. In addition, about ≈60 kg of 240Pu are annually discharged per reactor unit [11], which is a significant quantity to be used as fuel in future fast reactors. Finally, the intermediate structures that can be observed in the (n,f) cross section in the resolved resonance region can provide constraints on phenomenological fission models 014616-2 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) through the characterization of resonance properties. At the same time, resonance structures appear in the cross section in the hundreds of keV region near the threshold fission, as an effect of vibrational states in the second well of the double-humped fission barrier, which require a combination of high flux and resolution to be observed and can contribute to the understanding of the fission mechanism. B. Previous measurements Due to the importance of the 240Pu(n,f) reaction, many data sets exist in the EXFOR database [12] covering incident neutron energies from 25.3 meV up to 200 MeV. More specifically, the cross section was measured at the thermal point by Pratt et al. (σth =3700(8000) mb, [13]) and Eastwood et al. (σth =30(45) mb, [14]), and both results were uncertain and discrepant by more than two orders of magnitude. In addition, spectrum and Maxwellian average cross section at the thermal point were reported by Bigham [15] and Hulet et al. [16], respectively. The first resonance in the 240Pu +nsystem is observed 1.05 eV above the neutron separation energy. For neutroninduced fission, only a single data set exists in this region, reported by Leonard et al. [17], which was obtained with poor neutron energy resolution. Up to 5 keV, several measurements have been performed; however only the data by Weston et al. [18]havethelevelof resolution and statistics required to perform resonance analyses, according to the extensive argumentation of Bouland et al. [19]. Between 5 and 50 keV, the data reported by Weston [18] and by Budtz-Jorgensen and Knitter [20] show overlapping class-II resonance structures which are quite discrepant. For instance the structures seen at En≈13.5keV(Fig.19) and 20 keV are discrepant by 40% and 30%, respectively. Above 50 keV up to the vicinity of the fission threshold, a plethora of measurements has been performed. The three latest ones were reported by Salvador-Castineira et al. [21], Tovesson et al. [22], and Laptev et al. [23] and discrepancies that reach up to 15% were observed. In addition, the latest time-of-flight data by Tovesson et al. [22] are of insufficient resolution to observe structures attributed to vibrational phenomena. Finally, in the first chance fission plateau up to 6 MeV, several measurements have been performed as well. Concerning the three latest ones, the data by Tovesson et al. [22] are systematically higher by about 6% compared to the corresponding ones by Salvador-Castineira et al. [21] and Laptev et al. [23], which justifies the need for additional measurements in this region as well. C. The need for a second experimental area at n_TOF A study of the 240Pu(n,f) reaction was attempted at n_TOF in 2010 at the horizontal 185-m-long flight path, commonly referred to as EAR1, using the time-of-flight technique to determine the incident neutron energy [24] and Micromegas fission fragment detectors. The moderate neutron flux delivered at EAR1, inevitably led to a lengthy measurement to achieve sufficient statistical accuracy in the MeV region. The detectors were therefore exposed for several months to the high intrinsic αactivity of the samples, which caused them to deteriorate and eventually rendered the study incomplete. To further expand the measuring capabilities of n_TOF and to perform studies of important reactions where samples with either high activity, low mass, or small cross section are needed, a second experimental beam line (EAR2) was commissioned in 2014 [25]. The present measurement [26,27], where high activity samples were used, along with the 7Be(n,α) one [28], in which the short half-life of 7Be (t1/2= 53.2 d) limits the study of its low cross section, exemplify the capabilities of EAR2, which are a result of the high instantaneous flux and good resolution (see Sec. II A). Taking advantage of these characteristics, a new study of the 240Pu(n,f) reaction was successfully performed in EAR2. This experimental campaign was the first performed in EAR2 and the derived cross section spanned across nine orders of magnitude in incident neutron energy, ranging from 9 meV up to 6 MeV. The results that will be presented illustrate the potential of EAR2 in completing challenging fission studies, which was also demonstrated by subsequent measurements [29–31]. II. EXPERIMENTAL DETAILS A. Neutron source Neutrons at n_TOF are produced by spallation with a 20 GeV/cpulsed proton beam that impinges on a lead block. The spallation target assembly consisted of a cylindrical lead block, 40 cm in length and 60 cm in diameter, which was surrounded by a thin layer of water for cooling and moderation purposes, thus the neutron spectrum delivered in EAR2 covered a broad energy range from thermal energies up to 100 MeV [32]. The proton beam is delivered by CERN’s Proton Synchrotron (PS) at a low frequency which does not exceed 0.8 Hz and has a spread of 7 ns RMS. The beam intensity was 6.6×1012 protons/bunch on average and was constant within 2%. The experimental area rests at the end of a 18.4-m-long beamline from the center of the spallation target, which is kept under a 10−2mbar vacuum. The beam was shaped by means of a 3-m-long neutron collimator with an aperture of 2.2 cm, which consisted of 2 m Fe and 1 m polyethylene enriched with boron. The proximity of EAR2 to the target yielded a 30 times higher flux than the one of EAR1, while neutrons needed an approximately 10 times shorter time of flight to reach the experimental area. These attributes resulted in a considerably improved background suppression, as shown in Fig. 1, and mitigated the effects of the strong αactivity which occurred in EAR1. B. Fission foils Three high purity 240Pu samples in the form of 240PuO2, with a total activity of 19.22 MBq, were originally prepared at EC-JRC-Geel [33] for the measurement in EAR1 but were also used in the EAR2 experimental campaign. The plutonium material was deposited through molecular plating 014616-3 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) FIG. 1. Amplitude spectra recorded in EAR1 and EAR2 for a 240Pu sample. The α-particle background in EAR2 is appreciably suppressed while the fission rate is significantly higher. on 0.25-mm-thick and 5-cm-diameter aluminium backings, whereas the deposits themselves had a diameter of 3 cm. It needs to be noted that the small difference in the diameters did not affect the analysis and the results, as shown in Ref. [34]. Two additional samples were used as reference foils: (a) a 235Usample with a 40.5 Bq activity and (b) a 238Usample with 9.4 Bq activity. The 235Udeposit had a diameter of 2.9cm and was in the chemical form of UF4.The238Usample had a diameter of 3 cm and was made of U(OH)6material. Both samples were manufactured by means of molecular plating and had aluminium backings similar to the plutonium ones. The main characteristics of the fission foils used in the measurement can be seen in Table I. C. Detectors To detect the fission fragments a setup based on the compact and neutron-transparent microbulk Micromegas detector was used [35]. The gas volume of the detector was divided intworegionsbyathin(5μm) copper micromesh: (a) the drift region (6 mm), between the cathode and the micromesh and (b) the narrow amplification gap (50 μm) between the micromesh and the 5-μm-thick copper anode. In this configuration, the fission foil was positioned so that the deposit faced the drift region and its backing served as the cathode. An electric field of the order of 50 kV/cm was applied in the amplification gap, which is sufficient to cause avalanche multiplication resulting in a high detector gain. What is remarkable in this detector is the fact that its gain is intrinsic and depends only on the applied electric field, hence enhancing the ratio of signal to electronic background. This is important in cases where the electronic noise is high and the signal must be individually amplified. All detector-sample sets were stacked in a cylindrical aluminium chamber which was equipped with 50-μm-thick kapton windows. The spacing between the detector-sample sets was 2 cm. The chamber was filled with a circulating gas mixture of Ar : CF4:iC 4H10 at 88 : 10 : 2 volume fraction, at atmospheric pressure and room temperature. The low amount of material present in the Micromegas minimized the production of charged particles from neutron interactions with the detector itself, which was confirmed by an empty cathode-detector set, placed behind the 238Usample, as schematically shown in Fig. 2. In addition to the fission detectors, a setup based on silicon detectors was used to monitor the neutron beam, based on the detection of α-particles and tritons produced from the 6Li(n,t) reaction. Details on the monitor setup, which is referred to as “SiMon2,” can be found in Ref. [36]. D. Data acquisition Data were digitized through the use of 8-bit flash analogto-digital converters (ADCs) that were operated at a 500 MHz sampling rate. The acquisition window was 16 ms wide and allowed us to reach down to thermal and cold neutron energies. Finally, an online zero-suppression algorithm was applied to minimize the amount of data recorded during the acquisition [37]. TABLE I. List of the main characteristics of the fission foils used in the experiment along with the estimated uncertainties, provided by JRC-Geel, which were determined in May 2011 for the 240Pu samples, in January 1981 for 235U, and in February 2012 for 238U. Sample Lot Reference number Mass (mg) Areal density (mg/cm2) Atomic abundance (%) 238Pu: 0.0733(29) 239Pu: 0.0144(18) TP2010-011-01 0.7163(28) 0.1017(4) 240Pu: 99.8915(18) 240Pu BC01269B TP2010-011-03 0.809(3) 0.1148(5) 241Pu: 0.00041(31) TP2010-011-04 0.763(3) 0.1083(5) 242Pu: 0.02027(41) 244Pu: 0.000046(88) Total 2.2883 0.3248 234U: 0.1698 235U: 99.475 235USP 3576 SP 3576-1 0.563(11) 0.0912(17) 236U: 0.0273 238U: 0.3277 238U2677 TP2011-008-03 0.745(15) 0.1070(22) 238U>99.9 014616-4 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) FIG. 2. Schematic view of the fission foil stack, with respect to the neutron beam direction. Apart from the fission samples, an empty cathode was placed to monitor possible proton and αrecoils from the detector itself. III. DATA REDUCTION AND ANALYSIS A. Signal processing The digitized waveforms were processed offline by a pulse shape analysis framework developed at n_TOF [38]. The signal recognition was based on a single-stage differentiation filter whereas the reconstruction of the wave forms was based on pulse shape fitting procedures. Signal processing was performed in two procedures regarding (a) the so-called γ-flash, which is a burst of photons and relativistic particles that are produced during spallation and arrive promptly at the experimental hall [39], and (b) regular fission and α-particle signals. a. γ-flash: In the present case, the baseline following the γ-flash had an oscillatory behavior that remained consistent from pulse to pulse. Since fission signals were sitting on the trailing edge of the γ-flash as well as on top of the oscillations, the subtraction of an average γ-flash shape was applied to each individual waveform, as described in detail in Ref. [38]. The calculation of the average shape was achieved from recorded wave forms which were stacked, as shown in Fig. 3. In the calculation, fission signals were not taken into account since they would have distorted the average shape. Such a procedure is important since it can extend to the highest FIG. 3. Stacked recorded waveforms in the γ-flashregionfora 240Pu sample. The solid line corresponds to the calculated average. The signals shown correspond to 1% of the statistics. A few indicative neutron energies are also shown. FIG. 4. Stacked residuals between the average γ-flash and the recorded waveforms in the γ-flash region for a 240Pu sample. The inset contains the projection of the residuals to the yaxis,upto 10 MeV neutron energy. The signals shown correspond to 1% of the statistics. reachable neutron energy and it allowed us to better discriminate low-amplitude fission signals that sit on the crest of the oscillations. This procedure was followed by the calculation of the residuals between the average γ-flash shape and each individual wave form as a means of cross-checking that the subtraction was properly applied and estimating the highest reachable energy. The individual residuals were then stacked and projected along the amplitude axis, as shown in the inset of Fig. 4. A Gaussian fit on the projected residuals indicated a mean value of 0, which verified that the subtraction was properly applied within an uncertainty of ≈5 channels (2% of the full range), up to the time of flight that corresponds to 10 MeV incident neutron energy. For smaller times the projection of the residuals significantly widened, therefore 10 MeV was considered to be the maximum highest reachable energy as far as the signal processing is concerned. b. Fission signals: A similar approach was followed concerning the fission signals. Isolated detector signals were stacked and average pulse shapes were extracted for each individual detector. These were then fed into the reconstruction routines and pulse shape fitting was applied to determine signal attributes such as the arrival time, the amplitude, etc. This information was then stored in the so-called list mode, in order to perform the offline analysis and reconstruct the reaction yield as a function of the time of flight. B. Cross section calculation The cross section was deduced with reference to 235U(n,f) in the regions 9–800 meV and 10 keV–6 MeV, using Eq. (1a). In the 800 meV–10 keV region the evaluated EAR2 flux [32] was used and the cross section was calculated using Eq. (1b): σ=C C(ref) famp f(ref) amp fimp f(ref) imp fDT f(ref) DT fabs f(ref) abs fshield f(ref) shield fSF f(ref) SF fγf f(ref) γf ×m(ref) m (ref) σ(ref),(1a) σ=Cf amp fimp fDT fabs fshield fSF fCD fγf m,(1b) 014616-5 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) FIG. 5. Typical 2D distribution of the reconstructed time-offlight and amplitude signals for a 240Pu sample. Residuals from the γ-flash subtraction and signals from the αactivity are illustrated in the bottom left and right parts of the figure, respectively. Resonances are also visible. A few indicative neutron energies are shown. where (1) Crefers to the fission counts. (2) famp is the correction factor for the rejected fission signals below the amplitude threshold which was applied to reject α-particles and noise (see Sec. III B 2). (3) fimp corrects for the parasitic counts that contributed to the recorded yield and were attributed to fission reactions from contaminants or impurities in the fission foils. (4) fDT is a correction factor applied for counting losses due to dead time, pile-up, and insufficient signal reconstruction effects. (5) fabs takes into account the self-absorption of fission fragments within the fission foils. (6) fshield is the correction factor for the neutron selfshielding of the various layers in the detector-sample stacks. (7) fSF accounts for the contribution of spontaneous fission events. (8) fγfis the correction factor due to parasitic counts that contributed to the recorded fission yield from photofission reactions. (9) mis the mass term and corresponds to the areal density of the fission foil (Table I). (10) is the neutron fluence incident at the corresponding foil. The terms that include the superscript “(ref)” refer to the reference sample. 1. Fission counts The number of fission events as a function of the time of flight was determined from the signal processing described in Sec. III A. A typical distribution of the reconstructed time of flight vs amplitude can be seen in Fig. 5,fora240Pu sample. The reconstructed signals were then thoroughly checked in order to reject noise (i.e., saturated signals from sparks in the gas, falsely reconstructed signals, etc.) and to apply the proper thresholds to reject nonfission events (i.e., α-particles). In the FIG. 6. Statistical uncertainties, after applying the corrections, in the 100 keV–6 MeV high-energy region concerning the lightest 240Pu sample. Up to 1 MeV an isolethargic binning of 100 bins per decade was used whereas in the MeV region a custom binning that is shown in Appendix Bwas adopted. latter case the appropriate correction factors were applied to the fission yield, as will be described later in the text. The statistical uncertainties after the application of the correction factors were of the order of 10% in the thermal region and vary between 6–60% and 5–30% in the resolved and unresolved resonance regions, respectively. These high statistical uncertainties were observed in the valleys between resonances where the reaction rate was quite low. At higher neutron energies the statistical uncertainties did not exceed 8%, as shown in Fig. 6. 2. Amplitude threshold A typical fission amplitude spectrum, such as the one reconstructed in the present case and shown in Fig. 7, consists mainly of two parts: (a) the fission fragments and (b) the α-particles from the intrinsic radioactivity of the fission foil. FIG. 7. Comparison between the experimental and simulated amplitude spectra from a 240Pu sample. For the low amplitude region, a beam-off spectrum was added to the simulated one. The reproduction of the experimental points is quite satisfactory. The shaded area represents the fraction of the rejected FF for an amplitude threshold equal to 30 channels. 014616-6 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) TABLE II. List of the correction factors that were applied to the fission yields along with the corresponding uncertainties (when estimated). In cases of energy dependent correction factors, a reference to a figure is given. When a single correction factor is given, it corresponds to all fission foils, unless a hyphen is used in the corresponding row. Correction factor fabs fSF,fCD fγf Sample famp fimp fDT (%) fshield (%) (%) ratio 235U1.040(2) – 1.000 240Pu-04 1.070(4) 0.996 240Pu-01 1.115(10) Fig. 8Fig. 10 <0.100(1) <0.40(2) <0.2 Fig. 12 0.992 240Pu-03 1.090(9) 0.988 238U1.020(3) – 0.984 To reject the α-counts, an amplitude threshold was introduced in the analysis based on beam-off runs to locate the high amplitude tail of the α-particle spectrum. However, a fraction of fission counts was inevitably rejected as well, whose estimation was based on Monte Carlo simulations by coupling the GEF [40] and FLUKA [41] codes. Fission fragment (FF) distributions were generated in GEF and were then used as a source term in FLUKA. Fission fragments were produced within the sample and propagated towards the gas in order to estimate the deposited energy. The simulated energy deposition was convoluted with an appropriate response function of the detection/read-out system and was finally calibrated in order to be compared to the experimental amplitude spectrum. The α-particles were not simulated since only a small part of the tailing edge was recorded; however, in order to benchmark the simulations, beam-off spectra, that practically consisted only of α-counts, were used. More specifically, the simulated spectra, which contained only FF, were summed with beam-off amplitude distributions and were then compared to experimental beam-on spectra, which consisted of both FF and α-counts. As characteristically shown for a 240Pu sample in Fig. 7, a quite satisfactory agreement was achieved. The famp correction factor can then be estimated from the simulations as the fraction of the integral beneath the corresponding amplitude threshold (shaded area, Fig. 7). The aforementioned procedure was performed individually for the 240Pu, 235U, and 238Usamples and correction factors in the 2–11.5% range were determined, as shown in Table II. To estimate the uncertainty of the simulations, the uranium samples were used. The low activity of these samples (a few tens of Bq) and the narrow acquisition window (16 ms) made the detection of α-particles highly improbable. In this respect, the simulated and experimental fraction of the rejected FF was compared and an agreement within 3% was achieved, which was considered to be the upper bound of systematic uncertainty of this correction factor. In the simulations, apart from the energy deposition in the gas, several other effects on the correction factor were studied, such as (a) the chemical composition of the samples, which might deviate from the nominal one due to the preparation method [42] and/or environmental conditions (i.e., moisture), and (b) the FF angular distribution which might be important above 1 MeV. In the former case the chemical composition was varied [e.g., in the 238Usample from U(OH)6to U(OH)10] while in the latter one FF were propagated unidirectionally towards the gas from 0◦to 89◦with respect to the neutron beam. In both studies the effect on famp was less than 3% and 1%, respectively. More information can be found in Ref. [34]. 3. Impurities It was previously mentioned that in the 240Pu samples impurities with a total abundance of 0.1% were present (Table I). Despite this small fraction, their contribution to the fission yield was high in the thermal and resolved resonance regions, attributed mainly to the fissile 239Pu. The estimation of the fimp correction factor was based on “weighting” the ENDF/BVIII.0 evaluated (n,f) cross section σ(i)of each isotope found in the samples with its reported atomic abundance f(i) abun,as seen in Eq. (2): σ(i) w=f(i) abunσ(i).(2) Then fimp was calculated, pointwise with respect to the neutron energy, from the ratio of Eq. (3), where the sum in the denominator includes the isotopes reported in Table Ias FIG. 8. The fimp correction factor (top panel) applied to 240Pu with respect to the neutron energy. The bottom panel shows the total estimated uncertainty, which was obtained from the diagonal elements of the covariance matrix. 014616-7 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) well as the 236Udaughter nucleus1from the αdecay of 240Pu: fimp =σ240Pu w iσ(i) w ;(3) see also Fig. 8. The uncertainty in the correction was determined by means of the covariance matrix provided by EC-JRC-Geel. As far as the ENDF/B-VIII.0 cross sections were concerned, the main contribution to the uncertainty was the 239Pu(n,f) cross section, since it was the contaminant that mainly contributed to the fission yield. The ENDF/B-VIII.0 239Pu(n,f) cross section was evaluated with a 1.4% uncertainty above 2.5keV, therefore it was considered negligible compared to the uncertainties of the atomic abundances. Below 2.5 keV, the ENDF/B-VIII.0 library reports uncertainties of the order of a few percent (<4% at a 2 bins/decade binning) which, although non-negligible, was not included in the covariance matrix because its component relies on evaluations which can change in the future; therefore only experimental components were propagated. In the case of the uranium samples, the corresponding correction was negligible. 4. Counting losses Below the fission threshold, up to about 1 MeV, the recorded fission rate did not exceed 1 MHz for the plutonium and uranium samples. The analytical correction formulas proposed by Coates [43] and Moore [44] were applied to the recorded fission counts which practically yielded identical corrections. Correction factors less than 0.5% and 25% were estimated in the 9 meV–300 keV and 300 keV–1 MeV regions respectively, concerning 240Pu. For 235U,a0.6% correction was estimated at 56 meV, where the fission rate peaked in the thermal region. An average 1% correction was applied up to 20 keV while, up to 1 MeV, the estimated counting losses progressively reached 16%. The corresponding correction for 238Uwas practically negligible. Above 1 MeV, the expected instantaneous counting rate reached several MHz and resulted in significant pile-up that was observed in the reconstructed counting spectra. Indeed, between 850 keV and 10 MeV (Figs. 3and 5) signals with systematically higher amplitudes were reconstructed, which is attributed to pile-up effects. The analytical methods used below 1 MeV were not able to provide realistic corrections, therefore a new methodology was developed [45] to treat such cases based on two approaches: (a) exponential decay fits in experimental waiting time distributions as shown in Fig. 9and (b) correction functions predicted from detector emulation devices. It has to be mentioned that this methodology can also account for an insufficient signal reconstruction, which can occur at high counting rates. It was demonstrated that both approaches provide compatible corrections for counting rates up to 2 MHz; however, the uncertainty of method (a) is higher. In the present measurement, the fission rate in 240Pu was 1About 0.04% of the initial 240Pu had decayed to 236Uafter 3.5 y from the sample characterization when the measurement took place. FIG. 9. Exponential fits in waiting time distributions are a useful experimental tool in estimating counting losses by calculating the integral below the extrapolated fitting function [46]. higher than 2 MHz, therefore fDT was estimated by means of fitting waiting time distributions, yielding a correction factor that varied from 1.44 up to 2.26 with 10% uncertainty. For the uranium samples the correction function described in Ref. [45] was used. The correction factors that were calculated with a 3% uncertainty did not exceed 1.62 and 1.31 for 235Uand 238U, respectively. Finally, in Fig. 10 the correction factors are shown that were applied to the recorded fission yield. It has to be noted that above 6 MeV the waiting time distributions lacked sufficient statistical accuracy, which was a limiting factor for the highest reachable neutron energy. In addition, concerning the 01 and 03 targets, the signal reconstruction above 4 MeV was not possible since the γflash subtraction could not be applied at higher energies. In addition, above 3 MeV the trends in the correction factors shown in Fig. 10 are attributed to counting losses not only due to pile-up effects, but also those due to inefficient signal reconstruction. FIG. 10. Estimated correction factors for counting losses. Below 1 MeV the methodology proposed by Coates [43] and Moore [44] was applied, while above 1 MeV the correction was based on Ref. [45]. Average correction factors are shown per 0.5MeV,above 1MeV. 014616-8 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) FIG. 11. The neutron self-shielding correction was based on the Beer-Lambert law and ENDF/B-VIII.0 (n,tot) cross sections for the materials seen in the figure. 5. Miscellaneous corrections The remaining correction factors were either estimated to be negligible or did not require a complicated analysis; however a brief discussion of the their calculation will follow. a. Self-absorption of fission fragments. Emitted fission fragments deposit an amount of their kinetic energy in the sample. A fraction of those might then produce a signal below the detection threshold, thus the fission yield is underestimated. To estimate the amount of these fission fragments, the Monte Carlo simulations described in Sec. III B 2 were used. A fraction that did not exceed 0.1% was estimated with an uncertainty that is defined by the uncertainty of the reported masses and has negligible contribution to the final cross section uncertainty. Nevertheless, at high neutron energies the fission fragment angular distribution (FFAD) might have an effect on the self-absorption and thus on the detection efficiency, as demonstrated in Refs. [47–49]. In the present case, the Monte Carlo simulations described in Sec. III B 2 were used and the fission fragments were propagated towards the gas at angles ranging from 0◦to 90◦. The simulations showed that the effect on the correction can be neglected. b. Neutron beam attenuation. The neutron beam attenuation in the detector stack layers (Fig. 11), was taken into account using Beer-Lambert’s attenuation law and ENDF/BVIII.0 (n,tot) cross sections (σtot). According to the configuration shown in Fig. 11, the beam with an I0intensity, that exits 235U, suffered successive losses when crossing a layer with natoms/cm2, described by the ratio seen in Eq. (4), where idenotes each layer from the exit of 235Uup to the corresponding fission foil: fshield f(ref) shield =exp i niσtot,i.(4) The neutron transport in the gas was neglected due to its negligible mass, therefore it is not visible in Fig. 11, and Kapton was assumed to be pure 12C, which accounts for 70% of Kapton [50]. The estimated correction factors can be seen in Fig. 12.It has to be noted that the correction in 238Uwas not applied below 1 MeV due to the absence of statistics. In addition, the uncertainty of this correction depends mainly on the FIG. 12. Correction factors for neutron beam attenuation that were applied to 240Pu and 238U. uncertainty of the evaluated cross sections and was estimated to be less than 2%, since the number of atoms was known with an accuracy better than 1%. c. Spontaneous fission. To estimate the contribution of spontaneous fission and cluster decay, the beam-off spectra were used. It was experimentally shown that per proton bunch (Fig. 13) less than 0.4% of the recorded counts were attributed to spontaneous fission and cluster decay events. The uncertainty in this case was estimated to be 5% based on the statistical uncertainty of the recorded spontaneous fission events in the longest beam-off run, which corresponded to 50000 proton bunches. It has to be mentioned that the branching ratio of cluster decay is appreciably smaller than spontaneous fission, therefore it was neglected in the correction. d. Photofission. To estimate the contribution of photofission events, Monte Carlo simulations were used. More specifically, the simulated photon fluence from the spallation process was used, along with the ENDF/B-VIII.0 (γ, f) cross sections, in order to calculate the expected reaction rate. Photofission events were estimated to contribute less than 0.2% in the worst case. FIG. 13. Comparison between beam-on and -off spectra recorded from the most massive 240Pu sample. The contribution of spontaneous fission was considered negligible. Spectra are normalized to the number of triggers for a direct comparison. 014616-9 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) in overall agreement with the evaluation by Bouland et al. [19], including fission and neutron widths. On top of that, new and/or more accurate resonance parameters could be proposed. The resulting fission kernels which were extracted with a statistical accuracy better than 30% are listed in Table III,in comparison to the ones proposed by Bouland et al. VI. CONCLUSION The second experimental area (EAR2, 19 m flight path) was commissioned in 2014 [25] in order to expand the measuring capabilities of CERN’s n_TOF facility in studying reactions where high activity and/or low mass samples are involved. In this respect, the first experiment that was performed was the study of the 240Pu(n,f) cross section, which could not be completed in a previous measurement in the existing experimental area (EAR1, 185 m flight path) due to the detector deterioration induced by the long exposure to the activity of the fission foils [24]. The present measurement was successfully completed and yielded a cross section in a broad energy range from 9 meV up to 6 MeV incident neutron energy, covering almost nine orders of magnitude. This experimental campaign demonstrated the capabilities of EAR2 for measurements especially at neutron energies below the fission threshold, where the limited amount of fission material makes the study of resonances and thermal cross sections challenging. The high instantaneous neutron flux, which was delivered in a short time interval, compensated for this experimental limitation, thus appreciably reducing the intrinsic background from the αactivity and providing a sufficient fission rate to observe resonance structures. These structures were analysed by means of SAMMY fits [63], incorporating the R-matrix formalism. A total of 25 resonance kernels are reported although the experiment was not initially designed for sub-barrier fission. The majority of fission kernels is in agreement with evaluations [19], while three new values could be determined and recommended. In the near-threshold region, resonance structures were also observed which correspond to overlapping class-II states, but they could not be analyzed using the available statistical model codes. Above the fission threshold, the high instantaneous fission rate resulted in appreciably large counting losses, which were estimated by means of a dedicated methodology that was applied to the fission counts [45]. The derived cross section is in agreement with the latest data set by Salvador-Castineira et al. [21] and the time-of-flight data by Laptev et al. [23] but is systematically smaller than the latest time-of-flight measurement by Tovesson et al. [22] and the ENDF/B-VIII.0 and JEFF-3.3 evaluations. An overall agreement was observed with the CENDL-3.1 and JENDL-4.0 evaluation libraries. The present measurement is expected to provide additional material for the evaluated libraries while emphasizing the need for an additional study in the resolved resonance region. The further upgrade of the n_TOF spallation target is expected to offer an increased neutron flux and a significantly better resolution. Finally, due to the substantially higher instantaneous flux especially near thermal energies, EAR2 is expected to facilitate the measurement of new fission cross section data concerning actinides, which are important both in nuclear energy applications and fundamental research. ACKNOWLEDGMENT Some of the authors would like to acknowledge the support by the Croatian Science Foundation under the project 8570. APPENDIX A: REICH-MOORE RESONANCE PARAMETERS The resonance parameters that reproduce the reported cross sections are given in Table IV. Each file line corresponds to the parameters of one resonance. From left to right the columns contain the energy, radiation, neutron, and fission widths of each resonance. The first five fictitious resonances were adopted from Bouland et al. [19] and were used to simulate the contributions of external resonances. The sign in the fission widths is used to indicate the definite amplitude of fission. TABLE IV. Resonance parameters that were used to parametrize the 240Pu(n,f) cross section. The resonances were considered s waves, therefore the resonance spins are J=1/2. Energy γnf (eV) (meV) (meV) (meV) −4.070 ×1033.18 ×1013.55 ×1043.37 ×10−3 −1.300 ×1033.18 ×1013.52 ×103−4.31 ×10−2 −3.050 ×1023.18 ×1012.14 ×1024.00 ×10−2 −7.010 ×1013.18 ×1013.09 ×102−4.00 ×10−2 −3.000 ×1003.91 ×1011.31 ×1001.00 ×10−3 1.058 ×1002.91 ×1012.45 ×1007.65 ×10−3 2.043 ×1012.70 ×1012.75 ×100−2.90 ×10−1 3.835 ×1012.40 ×1011.96 ×1011.74 ×10−2 4.175 ×1012.55 ×1011.74 ×1017.11 ×10−3 6.664 ×1013.30 ×1015.55 ×1013.27 ×10−2 7.277 ×1012.64 ×1012.17 ×1019.78 ×10−2 9.078 ×1013.08 ×1011.33 ×101−1.01 ×10−2 9.249 ×1012.83 ×1013.00 ×100−6.32 ×10−2 1.050 ×1022.85 ×1014.62 ×101−5.10 ×10−3 1.217 ×1023.36 ×1011.49 ×1018.70 ×10−2 1.257 ×1023.18 ×1011.20 ×10−1−2.00 ×10−2 1.308 ×1023.09 ×1011.79 ×10−12.41 ×10−1 1.351 ×1023.29 ×1011.83 ×1014.83 ×10−2 1.520 ×1023.75 ×1011.35 ×1013.77 ×10−1 1.627 ×1022.91 ×1018.48 ×1001.58 ×100 1.698 ×1023.10 ×1011.32 ×101−1.37 ×10−1 1.858 ×1023.10 ×1011.58 ×1018.95 ×10−3 1.920 ×1023.06 ×1012.85 ×10−1−1.28 ×10−1 1.956 ×1023.18 ×1011.60 ×10−11.20 ×10−1 1.974 ×1023.18 ×1011.60 ×10−1−1.20 ×10−1 1.997 ×1022.86 ×1019.70 ×10−11.37 ×10−1 2.389 ×1022.87 ×1011.19 ×1011.35 ×10−1 2.605 ×1023.28 ×1012.23 ×101−1.19 ×10−1 014616-16 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 2.869 ×1023.20 ×1011.35 ×102−3.69 ×10−1 3.049 ×1023.39 ×1017.37 ×1002.12 ×10−1 3.136 ×1023.18 ×1011.20 ×10−1−2.50 ×10−1 3.181 ×1023.22 ×1015.23 ×1003.21 ×10−1 3.207 ×1023.49 ×1011.89 ×101−3.26 ×10−2 3.327 ×1023.18 ×1011.30 ×10−12.49 ×10−2 3.383 ×1023.14 ×1015.94 ×100−4.57 ×10−3 3.459 ×1023.39 ×1011.59 ×1013.52 ×10−1 3.635 ×1023.88 ×1013.16 ×1011.37 ×10−1 3.719 ×1023.04 ×1011.33 ×101−1.35 ×10−1 3.930 ×1023.18 ×1011.50 ×10−1−1.70 ×10−2 4.050 ×1023.24 ×1011.03 ×102−4.31 ×10−1 4.189 ×1023.09 ×1015.77 ×1002.87 ×10−1 4.457 ×1023.14 ×1011.84 ×100−5.84 ×10−1 4.498 ×1023.22 ×1011.61 ×1011.47 ×10−1 4.666 ×1023.29 ×1012.65 ×1001.03 ×100 4.733 ×1023.07 ×1014.11 ×1001.00 ×100 4.938 ×1023.15 ×1015.35 ×100−5.30 ×10−1 4.989 ×1023.63 ×1011.85 ×1012.08 ×10−1 5.100 ×1023.18 ×1014.14 ×10−16.40 ×10−2 5.125 ×1023.18 ×1015.17 ×10−1−4.47 ×10−2 5.145 ×1023.36 ×1012.09 ×101−2.06 ×10−1 5.263 ×1023.18 ×1019.61 ×10−11.00 ×100 5.308 ×1023.18 ×1016.77 ×10−12.92 ×100 5.463 ×1023.99 ×1013.11 ×101−9.97 ×10−2 5.534 ×1023.48 ×1011.79 ×1013.95 ×10−1 5.665 ×1023.38 ×1013.14 ×101−2.79 ×10−1 5.844 ×1023.18 ×1011.15 ×1003.61 ×100 5.966 ×1023.72 ×1015.42 ×1011.22 ×10−1 6.080 ×1022.91 ×1012.22 ×101−9.02 ×10−2 6.322 ×1023.24 ×1011.35 ×101−4.07 ×10−1 6.376 ×1023.06 ×1011.19 ×101−1.16 ×10−1 6.498 ×1023.18 ×1011.20 ×1002.20 ×100 6.657 ×1022.74 ×1012.03 ×102−3.59 ×10−1 6.789 ×1023.20 ×1012.54 ×101−1.31 ×100 7.121 ×1023.18 ×1011.33 ×1003.26 ×10−1 7.433 ×1023.18 ×1011.01 ×1005.60 ×10−1 7.503 ×1023.25 ×1016.95 ×101−1.36 ×101 7.589 ×1023.20 ×1015.82 ×1001.68 ×10−1 7.783 ×1023.18 ×1011.12 ×1005.85 ×10−1 7.829 ×1023.12 ×1013.33 ×100−1.86 ×103 7.905 ×1022.32 ×1012.52 ×101−1.34 ×101 8.103 ×1023.73 ×1012.20 ×1021.55 ×101 8.200 ×1022.98 ×1011.11 ×1026.46 ×10−1 8.333 ×1023.18 ×1011.02 ×100−3.50 ×100 8.456 ×1023.36 ×1019.48 ×1001.24 ×10−1 8.550 ×1023.47 ×1014.71 ×101−3.33 ×10−1 8.680 ×1023.18 ×1011.02 ×1001.42 ×100 8.764 ×1023.29 ×1011.45 ×1017.68 ×10−1 8.917 ×1023.23 ×1019.47 ×101−9.35 ×10−1 9.000 ×1023.18 ×1011.00 ×100−1.20 ×101 9.040 ×1023.48 ×1012.21 ×101−7.32 ×10−1 9.089 ×1023.22 ×1017.79 ×1013.24 ×10−2 9.152 ×1023.48 ×1013.59 ×101−3.40 ×10−1 9.435 ×1023.27 ×1011.23 ×102−2.98 ×10−1 9.584 ×1023.10 ×1017.39 ×1017.04 ×10−2 9.700 ×1023.18 ×1011.00 ×1005.00 ×100 TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 9.713 ×1022.99 ×1017.98 ×1016.00 ×10−2 9.792 ×1023.18 ×1017.20 ×100−4.37 ×10−1 9.830 ×1023.18 ×1011.00 ×1004.80 ×101 9.919 ×1023.18 ×1013.00 ×10−12.67 ×104 1.002 ×1032.98 ×1019.73 ×101−1.56 ×100 1.012 ×1033.18 ×1012.00 ×1008.11 ×100 1.024 ×1033.18 ×1015.23 ×1008.05 ×10−1 1.029 ×1033.18 ×1012.00 ×1004.53 ×100 1.037 ×1033.18 ×1012.00 ×100−2.17 ×100 1.042 ×1032.97 ×1011.21 ×101−1.70 ×10−1 1.046 ×1033.18 ×1013.94 ×1002.47 ×100 1.051 ×1033.18 ×1012.00 ×1007.49 ×100 1.072 ×1032.91 ×1011.09 ×102−2.72 ×10−1 1.077 ×1033.18 ×1011.70 ×100−1.85 ×100 1.086 ×1033.18 ×1012.00 ×1002.21 ×100 1.100 ×1033.41 ×1018.00 ×101−3.04 ×10−1 1.116 ×1033.18 ×1012.57 ×100−5.47 ×10−1 1.129 ×1033.09 ×1014.98 ×1016.72 ×10−1 1.134 ×1033.18 ×1016.97 ×1003.62 ×10−1 1.143 ×1033.10 ×1014.22 ×101−4.22 ×10−1 1.160 ×1033.29 ×1012.38 ×101−6.87 ×10−1 1.176 ×1033.18 ×1011.50 ×1004.12 ×100 1.186 ×1033.21 ×1011.59 ×1021.11 ×10−1 1.191 ×1033.18 ×1011.14 ×102−1.46 ×10−1 1.201 ×1033.18 ×1012.00 ×1001.40 ×100 1.209 ×1033.17 ×1016.25 ×101−3.50 ×10−1 1.228 ×1033.18 ×1011.04 ×1019.40 ×10−1 1.237 ×1033.18 ×1011.12 ×1017.82 ×10−1 1.256 ×1033.12 ×1017.99 ×101−4.52 ×100 1.281 ×1033.18 ×1014.20 ×100−1.01 ×100 1.301 ×1033.06 ×1012.49 ×102−2.67 ×10−1 1.328 ×1033.27 ×1013.68 ×1025.07 ×10−1 1.345 ×1033.18 ×1012.49 ×1011.09 ×10−1 1.351 ×1033.18 ×1017.74 ×100−2.72 ×10−2 1.363 ×1033.18 ×1017.31 ×1002.78 ×10−1 1.377 ×1033.12 ×1016.61 ×101−1.13 ×10−1 1.389 ×1033.18 ×1011.47 ×1016.30 ×100 1.402 ×1033.10 ×1019.58 ×100−2.09 ×103 1.408 ×1033.18 ×1019.91 ×100−8.52 ×101 1.426 ×1032.99 ×1013.91 ×1015.49 ×100 1.429 ×1033.18 ×1011.57 ×101−1.02 ×100 1.442 ×1033.18 ×1012.00 ×1006.74 ×100 1.450 ×1033.18 ×1012.69 ×101−1.49 ×100 1.451 ×1033.15 ×1012.74 ×101−2.74 ×100 1.463 ×1033.18 ×1012.18 ×1013.72 ×10−1 1.466 ×1033.18 ×1012.00 ×100−2.73 ×100 1.475 ×1033.18 ×1012.00 ×100−4.67 ×100 1.481 ×1033.18 ×1019.76 ×1002.01 ×100 1.498 ×1033.18 ×1012.00 ×1004.27 ×100 1.503 ×1033.18 ×1014.00 ×100−1.11 ×10−1 1.529 ×1033.18 ×1015.00 ×1003.25 ×100 1.540 ×1033.23 ×1011.02 ×102−1.60 ×10−1 1.549 ×1033.17 ×1011.62 ×1024.11 ×10−1 1.555 ×1033.18 ×1012.50 ×100−3.64 ×100 1.564 ×1033.04 ×1011.18 ×102−1.20 ×10−1 1.575 ×1033.16 ×1011.26 ×102−5.10 ×100 1.582 ×1033.18 ×1013.00 ×1001.10 ×10−1 014616-17 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 1.600 ×1033.18 ×1012.00 ×100−1.01 ×10−1 1.610 ×1033.18 ×1013.60 ×1017.25 ×10−1 1.621 ×1033.18 ×1012.80 ×101−3.70 ×10−1 1.629 ×1033.18 ×1015.00 ×1008.37 ×10−1 1.643 ×1033.17 ×1011.11 ×1029.52 ×10−1 1.663 ×1033.22 ×1016.91 ×101−7.91 ×10−1 1.667 ×1033.18 ×1016.00 ×1001.12 ×10−1 1.688 ×1033.18 ×1013.53 ×101−1.89 ×100 1.707 ×1033.18 ×1014.50 ×1001.43 ×100 1.724 ×1033.14 ×1018.44 ×1011.79 ×100 1.749 ×1033.18 ×1013.00 ×100−9.90 ×10−2 1.742 ×1033.18 ×1012.48 ×1017.81 ×10−1 1.764 ×1033.18 ×1015.55 ×101−2.68 ×10−1 1.772 ×1033.18 ×1019.73 ×1009.92 ×10−2 1.779 ×1033.07 ×1014.87 ×102−4.53 ×10−2 1.789 ×1033.18 ×1015.00 ×1008.02 ×10−1 1.811 ×1033.18 ×1015.00 ×1007.41 ×10−1 1.842 ×1033.31 ×1011.28 ×102−1.10 ×101 1.853 ×1033.18 ×1013.39 ×101−1.26 ×100 1.862 ×1033.18 ×1014.00 ×100−1.01 ×10−1 1.873 ×1033.07 ×1018.07 ×1014.14 ×100 1.886 ×1033.18 ×1015.00 ×100−2.28 ×100 1.902 ×1033.18 ×1012.18 ×1023.71 ×100 1.917 ×1033.06 ×1013.52 ×1018.70 ×101 1.939 ×1033.10 ×1011.31 ×100−1.81 ×103 1.943 ×1033.18 ×1017.93 ×1001.74 ×101 1.948 ×1033.18 ×1018.58 ×1011.12 ×101 1.955 ×1033.08 ×1012.76 ×102−2.12 ×101 1.974 ×1033.18 ×1017.16 ×1011.76 ×100 1.991 ×1033.07 ×1011.18 ×102−4.79 ×10−2 1.999 ×1033.18 ×1015.40 ×1004.76 ×10−2 2.017 ×1033.15 ×1015.50 ×101−3.98 ×10−1 2.023 ×1032.87 ×1016.02 ×1011.83 ×100 2.033 ×1033.23 ×1011.11 ×1021.46 ×101 2.038 ×1033.18 ×1015.00 ×1001.16 ×10−1 2.054 ×1032.84 ×1017.25 ×101−5.76 ×100 2.061 ×1033.10 ×1015.00 ×1008.57 ×10−2 2.083 ×1033.09 ×1019.91 ×101−1.53 ×10−1 2.097 ×1033.18 ×1011.00 ×1016.94 ×10−1 2.111 ×1033.18 ×1011.39 ×101−2.40 ×100 2.127 ×1033.18 ×1016.00 ×100−7.72 ×10−1 2.142 ×1033.18 ×1018.00 ×100−8.85 ×10−1 2.155 ×1033.18 ×1011.41 ×1011.36 ×100 2.177 ×1033.18 ×1011.00 ×1012.64 ×100 2.182 ×1033.01 ×1018.96 ×1011.20 ×10−1 2.198 ×1033.07 ×1011.40 ×102−5.09 ×10−1 2.223 ×1033.18 ×1011.20 ×101−1.40 ×10−1 2.230 ×1033.18 ×1019.00 ×1001.17 ×10−1 2.241 ×1033.18 ×1013.41 ×101−9.16 ×10−1 2.257 ×1033.10 ×1011.37 ×1024.21 ×10−1 2.263 ×1033.18 ×1011.00 ×101−1.17 ×10−1 2.268 ×1033.18 ×1018.00 ×1001.04 ×10−1 2.278 ×1033.16 ×1013.98 ×1024.62 ×10−1 2.283 ×1033.10 ×1012.79 ×1017.64 ×10−1 2.291 ×1033.09 ×1012.18 ×102−2.36 ×10−1 2.303 ×1033.18 ×1011.70 ×101−1.00 ×10−1 2.318 ×1033.18 ×1011.00 ×101−4.83 ×100 TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 2.334 ×1033.18 ×1013.78 ×1015.53 ×10−1 2.351 ×1033.18 ×1013.85 ×1011.29 ×10−1 2.360 ×1033.18 ×1011.20 ×101−1.27 ×10−1 2.366 ×1033.05 ×1012.43 ×1023.84 ×10−1 2.373 ×1033.18 ×1019.65 ×100−1.03 ×10−1 2.386 ×1033.18 ×1011.83 ×1011.34 ×100 2.405 ×1033.18 ×1012.50 ×101−6.17 ×10−2 2.416 ×1033.18 ×1016.84 ×1015.86 ×10−1 2.425 ×1033.18 ×1015.00 ×1001.04 ×10−1 2.434 ×1033.04 ×1012.15 ×1023.00 ×10−1 2.459 ×1033.18 ×1012.63 ×101−4.30 ×10−1 2.470 ×1033.18 ×1014.89 ×101−2.10 ×10−1 2.477 ×1033.18 ×1011.00 ×101−5.15 ×100 2.484 ×1033.18 ×1012.14 ×1013.39 ×10−1 2.512 ×1033.18 ×1011.00 ×101−1.13 ×10−1 2.521 ×1033.38 ×1011.14 ×1023.50 ×10−1 2.531 ×1033.18 ×1011.50 ×101−1.04 ×10−1 2.538 ×1033.23 ×1012.87 ×1022.10 ×10−1 2.543 ×1033.18 ×1017.00 ×10−19.88 ×10−2 2.549 ×1033.26 ×1018.56 ×101−6.55 ×10−1 2.563 ×1033.18 ×1017.00 ×10−1−1.00 ×10−1 2.575 ×1033.64 ×1014.68 ×101−4.84 ×10−1 2.578 ×1033.18 ×1011.00 ×1019.50 ×10−2 2.595 ×1033.18 ×1011.00 ×101−1.12 ×100 2.602 ×1033.18 ×1011.00 ×1016.67 ×100 2.627 ×1033.18 ×1011.50 ×101−8.15 ×10−2 2.633 ×1033.18 ×1011.00 ×1019.23 ×10−2 2.645 ×1033.16 ×1014.30 ×102−4.59 ×100 2.652 ×1033.18 ×1013.83 ×1011.36 ×101 2.670 ×1033.18 ×1011.00 ×101−1.02 ×101 2.698 ×1033.18 ×1013.26 ×1021.20 ×102 2.700 ×1033.18 ×1011.50 ×1017.56 ×101 2.706 ×1033.18 ×1011.00 ×101−1.97 ×101 2.718 ×1033.18 ×1014.04 ×1011.97 ×100 2.729 ×1033.18 ×1011.00 ×101−1.02 ×10−1 2.739 ×1033.18 ×1011.82 ×1026.71 ×10−1 2.754 ×1032.91 ×1011.14 ×1028.33 ×100 2.764 ×1033.18 ×1011.00 ×1019.80 ×10−2 2.817 ×1033.18 ×1014.43 ×101−1.60 ×100 2.844 ×1033.18 ×1011.72 ×102−1.28 ×10−1 2.858 ×1033.18 ×1012.87 ×1011.52 ×100 2.882 ×1033.18 ×1013.20 ×101−3.50 ×10−1 2.896 ×1033.18 ×1016.39 ×1011.60 ×10−1 2.905 ×1033.18 ×1011.23 ×1026.10 ×10−1 2.924 ×1033.18 ×1011.80 ×101−1.00 ×10−1 2.938 ×1033.18 ×1011.53 ×102−4.00 ×10−1 2.969 ×1033.18 ×1019.87 ×101−3.60 ×10−1 2.980 ×1033.18 ×1011.12 ×1025.00 ×10−2 2.987 ×1033.18 ×1011.09 ×101−9.60 ×10−1 2.994 ×1033.18 ×1016.12 ×1013.25 ×10−1 3.004 ×1033.18 ×1018.39 ×1015.65 ×10−1 3.018 ×1033.18 ×1011.27 ×102−1.93 ×10−1 3.029 ×1033.18 ×1012.01 ×1012.17 ×100 3.040 ×1033.18 ×1011.00 ×101−2.32 ×10−1 3.048 ×1033.18 ×1011.00 ×1013.71 ×10−1 3.055 ×1033.18 ×1014.90 ×101−5.81 ×100 3.070 ×1033.18 ×1011.37 ×1012.76 ×101 014616-18 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 3.078 ×1033.18 ×1011.33 ×1023.82 ×100 3.088 ×1033.18 ×1013.35 ×101−7.94 ×10−1 3.092 ×1033.18 ×1011.00 ×101−2.59 ×100 3.106 ×1033.18 ×1016.00 ×100−1.27 ×101 3.113 ×1033.18 ×1013.97 ×1018.34 ×10−1 3.140 ×1033.18 ×1014.00 ×100−4.21 ×100 3.173 ×1033.18 ×1012.39 ×1021.56 ×100 3.185 ×1033.18 ×1018.00 ×100−3.07 ×10−1 3.192 ×1033.18 ×1013.60 ×1024.41 ×10−1 3.209 ×1033.18 ×1011.50 ×1013.18 ×10−1 3.238 ×1033.18 ×1017.40 ×101−7.59 ×10−1 3.258 ×1033.18 ×1016.00 ×100−3.11 ×10−1 3.266 ×1033.18 ×1012.60 ×1011.24 ×10−1 3.269 ×1033.18 ×1011.09 ×1021.72 ×10−1 3.291 ×1033.18 ×1011.00 ×101−1.81 ×100 3.305 ×1033.18 ×1011.20 ×101−1.01 ×100 3.317 ×1033.18 ×1011.50 ×1012.99 ×10−1 3.332 ×1033.18 ×1011.48 ×101−1.65 ×100 3.340 ×1033.18 ×1011.40 ×1012.86 ×100 3.346 ×1033.18 ×1015.00 ×1006.25 ×100 3.360 ×1033.18 ×1011.30 ×101−7.34 ×100 3.382 ×1033.18 ×1011.50 ×101−3.09 ×10−1 3.382 ×1033.18 ×1011.60 ×1012.74 ×103 3.389 ×1033.18 ×1011.50 ×1013.00 ×10−1 3.423 ×1033.18 ×1013.51 ×1010.00 ×100 3.440 ×1033.18 ×1011.00 ×101−3.39 ×10−1 3.458 ×1033.18 ×1017.12 ×101−5.48 ×10−1 3.466 ×1033.18 ×1013.65 ×102−1.60 ×100 3.487 ×1033.18 ×1012.50 ×1013.47 ×10−1 3.494 ×1033.18 ×1016.59 ×101−1.22 ×100 3.500 ×1033.18 ×1011.00 ×1016.03 ×10−1 3.514 ×1033.18 ×1011.00 ×101−5.00 ×10−1 3.539 ×1033.18 ×1011.00 ×1015.00 ×10−1 3.555 ×1033.18 ×1019.06 ×1010.00 ×100 3.567 ×1033.18 ×1011.79 ×102−2.56 ×10−1 3.581 ×1033.18 ×1011.50 ×1010.00 ×100 3.595 ×1033.18 ×1014.22 ×101−3.00 ×10−1 3.610 ×1033.18 ×1017.57 ×1013.02 ×10−1 3.614 ×1033.18 ×1013.80 ×1013.65 ×10−1 3.648 ×1033.18 ×1011.00 ×1012.80 ×10−1 3.657 ×1033.18 ×1012.74 ×102−7.98 ×10−2 3.665 ×1033.18 ×1015.41 ×1012.83 ×10−1 3.682 ×1033.18 ×1011.00 ×101−9.01 ×10−1 3.702 ×1033.18 ×1015.37 ×1019.13 ×10−1 3.711 ×1033.18 ×1012.50 ×101−5.00 ×10−1 3.723 ×1033.18 ×1015.58 ×1019.40 ×10−1 3.743 ×1033.18 ×1018.00 ×1005.00 ×10−1 3.765 ×1033.18 ×1015.00 ×100−5.00 ×10−1 3.777 ×1033.18 ×1015.00 ×100−3.25 ×100 3.800 ×1033.18 ×1011.08 ×1021.14 ×100 3.823 ×1033.18 ×1018.00 ×100−4.76 ×10−1 3.833 ×1033.18 ×1014.00 ×100−4.84 ×10−1 3.844 ×1033.18 ×1018.03 ×101−9.97 ×10−2 3.853 ×1033.18 ×1011.03 ×1023.95 ×10−1 3.859 ×1033.18 ×1011.00 ×1012.70 ×100 3.872 ×1033.18 ×1014.51 ×1011.34 ×100 3.886 ×1033.18 ×1011.00 ×101−5.00 ×10−1 TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 3.901 ×1033.18 ×1012.30 ×1021.10 ×10−1 3.916 ×1033.18 ×1011.83 ×102−2.85 ×10−1 3.939 ×1033.18 ×1011.00 ×1019.34 ×10−1 3.954 ×1033.18 ×1011.09 ×102−9.12 ×100 3.960 ×1033.18 ×1011.00 ×1011.00 ×100 3.975 ×1033.18 ×1011.19 ×102−1.36 ×100 3.990 ×1033.18 ×1012.90 ×1019.02 ×10−2 4.002 ×1033.18 ×1012.50 ×101−9.96 ×100 4.022 ×1033.18 ×1013.55 ×1021.11 ×100 4.031 ×1033.18 ×1011.13 ×102−4.00 ×10−1 4.055 ×1033.18 ×1012.90 ×1013.00 ×10−1 4.073 ×1033.18 ×1017.50 ×1003.00 ×10−1 4.084 ×1033.18 ×1011.35 ×102−3.10 ×10−1 4.100 ×1033.18 ×1012.90 ×1024.69 ×10−1 4.110 ×1033.18 ×1019.00 ×1003.00 ×10−1 4.122 ×1033.18 ×1015.42 ×1021.57 ×10−1 4.135 ×1033.18 ×1016.79 ×101−3.13 ×10−1 4.143 ×1033.18 ×1015.00 ×100−3.00 ×10−1 4.149 ×1033.18 ×1012.91 ×102−2.25 ×10−1 4.160 ×1033.18 ×1019.03 ×1011.40 ×10−1 4.170 ×1033.18 ×1012.40 ×1013.00 ×10−1 4.203 ×1033.18 ×1014.61 ×102−3.31 ×10−1 4.221 ×1033.18 ×1016.89 ×1015.84 ×10−1 4.241 ×1033.18 ×1016.00 ×100−5.80 ×100 4.260 ×1033.18 ×1018.00 ×1007.84 ×100 4.271 ×1033.18 ×1011.59 ×1021.93 ×10−1 4.280 ×1033.18 ×1013.10 ×101−3.00 ×10−1 4.288 ×1033.18 ×1013.23 ×1021.52 ×10−1 4.315 ×1033.18 ×1013.50 ×101−2.98 ×10−1 4.329 ×1033.18 ×1013.19 ×102−3.96 ×10−2 4.338 ×1033.18 ×1017.50 ×1003.00 ×10−1 4.363 ×1033.18 ×1012.00 ×1015.86 ×10−1 4.376 ×1033.18 ×1018.20 ×1010.00 ×100 4.386 ×1033.18 ×1013.20 ×101−6.36 ×10−1 4.398 ×1033.18 ×1017.80 ×101−1.04 ×100 4.415 ×1033.18 ×1015.00 ×1011.30 ×101 4.422 ×1033.18 ×1016.10 ×1013.07 ×10−1 4.433 ×1033.18 ×1014.70 ×1013.05 ×100 4.447 ×1033.18 ×1011.80 ×101−3.60 ×10−1 4.459 ×1033.18 ×1011.03 ×1026.74 ×10−1 4.473 ×1033.18 ×1012.50 ×101−3.00 ×10−1 4.491 ×1033.18 ×1012.00 ×101−3.00 ×10−1 4.502 ×1033.18 ×1012.00 ×1013.00 ×10−1 4.517 ×1033.18 ×1011.00 ×101−1.88 ×100 4.538 ×1033.18 ×1012.60 ×1013.00 ×10−1 4.560 ×1033.18 ×1012.00 ×1013.00 ×10−1 4.570 ×1033.18 ×1012.35 ×102−3.60 ×10−1 4.588 ×1033.18 ×1015.50 ×102−3.09 ×10−1 4.599 ×1033.18 ×1017.54 ×101−5.61 ×10−1 4.615 ×1033.18 ×1012.65 ×102−4.36 ×100 4.646 ×1033.18 ×1011.52 ×1022.24 ×100 4.664 ×1033.18 ×1018.00 ×100−3.00 ×10−1 4.687 ×1033.18 ×1012.00 ×1013.40 ×100 4.713 ×1033.18 ×1015.60 ×1014.71 ×10−1 4.721 ×1033.18 ×1015.10 ×102−9.75 ×10−2 4.745 ×1033.18 ×1012.53 ×1023.01 ×10−1 4.755 ×1033.18 ×1015.47 ×101−1.66 ×100 014616-19 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 4.769 ×1033.18 ×1013.73 ×1011.33 ×100 4.778 ×1033.18 ×1013.42 ×1016.78 ×10−1 4.791 ×1033.18 ×1011.37 ×1029.32 ×10−1 4.800 ×1033.18 ×1012.00 ×101−4.11 ×10−1 4.812 ×1033.18 ×1011.81 ×1022.83 ×10−1 4.822 ×1033.18 ×1016.34 ×1015.58 ×100 4.843 ×1033.18 ×1011.80 ×1017.76 ×10−1 4.868 ×1033.18 ×1011.30 ×101−1.40 ×100 4.894 ×1033.18 ×1016.28 ×101−9.19 ×10−1 4.912 ×1033.18 ×1011.50 ×101−3.79 ×101 4.933 ×1033.18 ×1012.00 ×1011.90 ×101 4.949 ×1033.18 ×1015.17 ×101−8.26 ×100 4.958 ×1033.18 ×1013.20 ×1024.45 ×100 4.968 ×1033.18 ×1011.54 ×1025.92 ×100 4.974 ×1033.18 ×1017.50 ×101−3.67 ×10−1 4.994 ×1033.18 ×1019.56 ×101−1.21 ×100 5.035 ×1033.18 ×1011.50 ×1011.47 ×100 5.047 ×1033.18 ×1011.00 ×101−1.51 ×100 5.072 ×1033.18 ×1015.66 ×102−7.53 ×100 5.097 ×1033.18 ×1013.60 ×1012.34 ×100 5.111 ×1033.18 ×1018.61 ×1011.59 ×101 5.120 ×1033.18 ×1011.95 ×101−4.45 ×10−1 5.131 ×1033.18 ×1014.36 ×101−4.91 ×101 5.148 ×1033.18 ×1015.00 ×1010.00 ×100 5.161 ×1033.18 ×1014.00 ×1011.34 ×100 5.176 ×1033.18 ×1018.00 ×100−2.02 ×100 5.194 ×1033.18 ×1013.46 ×1025.56 ×10−1 5.216 ×1033.18 ×1011.62 ×102−7.15 ×10−1 5.235 ×1033.18 ×1012.40 ×1016.37 ×100 5.250 ×1033.18 ×1015.23 ×102−5.94 ×100 5.272 ×1033.18 ×1011.44 ×1022.21 ×101 5.286 ×1033.18 ×1015.30 ×1013.98 ×10−1 5.301 ×1033.18 ×1012.83 ×1023.46 ×100 5.327 ×1033.18 ×1011.78 ×102−1.28 ×101 5.353 ×1033.18 ×1011.50 ×1022.38 ×100 5.357 ×1033.18 ×1013.60 ×101−4.46 ×10−1 5.367 ×1033.18 ×1016.97 ×101−8.59 ×100 5.380 ×1033.18 ×1018.00 ×1005.99 ×10−1 5.393 ×1033.18 ×1018.46 ×1011.06 ×100 5.417 ×1033.18 ×1012.64 ×1023.21 ×10−1 5.440 ×1033.18 ×1011.20 ×101−3.75 ×100 5.456 ×1033.18 ×1018.00 ×100−4.69 ×10−1 5.465 ×1033.18 ×1014.97 ×1015.49 ×100 5.483 ×1033.18 ×1018.87 ×101−9.14 ×10−1 5.498 ×1033.18 ×1019.92 ×1015.23 ×10−1 5.511 ×1033.18 ×1013.58 ×102−4.83 ×10−1 5.523 ×1033.18 ×1011.75 ×1024.94 ×100 5.531 ×1033.18 ×1011.60 ×101−5.52 ×10−1 5.545 ×1033.18 ×1015.51 ×102−3.50 ×10−1 5.551 ×1033.18 ×1011.21 ×102−7.06 ×10−1 5.564 ×1033.18 ×1011.50 ×1017.60 ×10−1 5.574 ×1033.18 ×1017.90 ×1022.26 ×10−1 5.592 ×1033.18 ×1011.96 ×1027.61 ×10−1 5.600 ×1033.18 ×1011.41 ×102−3.32 ×10−1 5.615 ×1033.18 ×1016.20 ×1013.55 ×100 TABLE IV. (Continued.) Energy γnf (eV) (meV) (meV) (meV) 5.629 ×1033.18 ×1012.00 ×101−6.24 ×10−1 5.644 ×1033.18 ×1015.50 ×1011.26 ×100 5.667 ×1033.18 ×1014.50 ×101−7.49 ×10−1 5.682 ×1033.18 ×1011.05 ×102−7.03 ×100 5.692 ×1033.18 ×1019.10 ×1011.00 ×100 5.995 ×1033.18 ×1019.64 ×101−2.74 ×102 5.924 ×1033.18 ×1019.58 ×101−8.72 ×104 5.981 ×1033.18 ×1019.62 ×101−7.39 ×10−2 5.990 ×1033.18 ×1019.63 ×1011.70 ×10−2 6.299 ×1033.18 ×1019.88 ×101−2.38 ×100 6.427 ×1033.18 ×1019.98 ×1018.49 ×10−3 6.446 ×1033.18 ×1019.99 ×1013.22 ×10−1 6.513 ×1033.18 ×1011.00 ×1022.58 ×100 6.535 ×1033.18 ×1011.01 ×1027.01 ×100 6.551 ×1033.18 ×1011.01 ×1021.87 ×101 6.568 ×1033.18 ×1011.01 ×1022.85 ×102 7.508 ×1033.18 ×1011.08 ×1022.08 ×102 8.021 ×1033.18 ×1011.11 ×1022.98 ×100 8.064 ×1033.18 ×1011.12 ×1023.13 ×100 8.098 ×1033.18 ×1011.12 ×1021.92 ×104 8.361 ×1033.18 ×1011.14 ×1027.80 ×100 8.472 ×1033.18 ×1011.77 ×1021.60 ×101 8.708 ×1033.18 ×1011.16 ×1021.02 ×102 8.975 ×1033.18 ×1011.18 ×1025.59 ×104 1.002 ×1043.18 ×1011.25 ×1028.64 ×100 1.008 ×1043.18 ×1011.25 ×1022.69 ×102 1.015 ×1043.18 ×1011.25 ×1021.16 ×102 1.096 ×1043.18 ×1011.30 ×1026.89 ×101 1.118 ×1043.18 ×1011.32 ×1023.61 ×102 1.150 ×1043.18 ×1011.33 ×1021.15 ×103 1.166 ×1043.18 ×1011.34 ×102−4.64 ×103 1.215 ×1043.18 ×1011.37 ×1023.87 ×102 1.250 ×1043.18 ×1011.39 ×102−8.21 ×101 1.311 ×1043.18 ×1011.42 ×102−4.84 ×102 1.317 ×1043.18 ×1011.43 ×102−4.90 ×104 1.356 ×1043.18 ×1011.45 ×1021.76 ×103 1.405 ×1043.18 ×1011.48 ×1028.55 ×101 1.450 ×1043.18 ×1011.50 ×1022.39 ×102 1.447 ×1043.18 ×1011.50 ×1023.38 ×102 1.605 ×1043.18 ×1011.58 ×1026.44 ×103 1.643 ×1043.18 ×1011.60 ×102−5.70 ×102 1.748 ×1043.18 ×1011.65 ×1023.87 ×103 1.822 ×1043.18 ×1011.68 ×102−2.32 ×103 1.845 ×1043.18 ×1011.69 ×1026.22 ×102 1.921 ×1043.18 ×1011.73 ×102−1.44 ×103 APPENDIX B: CROSS SECTION IN THE 100 KEV–6 MEV REGION The derived 240Pu(n,f) cross section (σ) along with its corresponding uncertainty (δσ) is reported in Table V,inthe energy region between 100 keV and 6 MeV. 014616-20 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) TABLE V. Point-wise 240Pu(n,f) cross section above 100 keV along with the total estimated uncertainties. Energy σδσδσ (eV) (b) (b) (%) 1.01 ×1054.90 ×10−25×10−310 1.04 ×1054.89 ×10−25×10−39 1.06 ×1055.80 ×10−24×10−38 1.08 ×1056.56 ×10−25×10−37 1.11 ×1056.88 ×10−25×10−37 1.14 ×1056.91 ×10−25×10−37 1.16 ×1056.97 ×10−25×10−37 1.19 ×1054.70 ×10−24×10−39 1.22 ×1055.29 ×10−24×10−38 1.24 ×1056.19 ×10−24×10−37 1.27 ×1056.95 ×10−24×10−36 1.30 ×1057.70 ×10−24×10−36 1.33 ×1058.47 ×10−25×10−36 1.36 ×1059.09 ×10−26×10−37 1.40 ×1058.74 ×10−27×10−38 1.43 ×1056.74 ×10−27×10−310 1.46 ×1057.08 ×10−27×10−310 1.50 ×1056.19 ×10−26×10−310 1.53 ×1055.54 ×10−25×10−310 1.57 ×1056.04 ×10−26×10−310 1.60 ×1056.87 ×10−26×10−38 1.64 ×1055.71 ×10−25×10−38 1.68 ×1057.80 ×10−25×10−36 1.72 ×1056.48 ×10−24×10−37 1.76 ×1056.58 ×10−24×10−37 1.80 ×1056.43 ×10−24×10−37 1.84 ×1056.42 ×10−24×10−37 1.88 ×1058.12 ×10−25×10−36 1.93 ×1058.12 ×10−25×10−37 1.97 ×1059.02 ×10−26×10−37 2.02 ×1058.81 ×10−26×10−37 2.07 ×1057.75 ×10−25×10−37 2.11 ×1058.00 ×10−25×10−37 2.16 ×1057.92 ×10−25×10−36 2.21 ×1059.67 ×10−25×10−35 2.26 ×1058.64 ×10−25×10−36 2.32 ×1059.47 ×10−25×10−35 2.37 ×1058.84 ×10−25×10−35 2.43 ×1058.99 ×10−25×10−35 2.48 ×1058.44 ×10−24×10−35 2.54 ×1058.31 ×10−24×10−35 2.60 ×1056.46 ×10−24×10−36 2.66 ×1057.65 ×10−24×10−35 2.72 ×1051.01 ×10−15×10−35 2.79 ×1051.31 ×10−16×10−35 2.85 ×1051.11 ×10−16×10−35 2.92 ×1059.86 ×10−25×10−35 2.99 ×1057.95 ×10−24×10−35 3.06 ×1057.47 ×10−24×10−35 3.13 ×1056.80 ×10−24×10−36 3.20 ×1058.64 ×10−24×10−35 3.27 ×1058.93 ×10−24×10−35 3.35 ×1051.33 ×10−16×10−34 3.43 ×1051.46 ×10−16×10−34 3.51 ×1051.68 ×10−17×10−34 TABLE V. (Continued.) Energy σδσδσ (eV) (b) (b) (%) 3.59 ×1051.59 ×10−16×10−34 3.67 ×1051.37 ×10−15×10−34 3.76 ×1051.49 ×10−15×10−34 3.85 ×1051.70 ×10−15×10−33 3.94 ×1051.77 ×10−16×10−33 4.03 ×1052.14 ×10−17×10−33 4.12 ×1052.15 ×10−17×10−33 4.22 ×1052.37 ×10−18×10−33 4.32 ×1052.52 ×10−18×10−33 4.42 ×1053.12 ×10−19×10−33 4.52 ×1053.11 ×10−18×10−33 4.62 ×1053.15 ×10−18×10−32 4.73 ×1052.97 ×10−17×10−32 4.84 ×1053.44 ×10−18×10−32 4.95 ×1053.31 ×10−17×10−32 5.07 ×1053.62 ×10−17×10−32 5.19 ×1054.17 ×10−18×10−32 5.31 ×1054.68 ×10−19×10−32 5.43 ×1054.97 ×10−11×10−22 5.56 ×1055.45 ×10−11×10−22 5.69 ×1055.67 ×10−11×10−22 5.82 ×1056.49 ×10−11×10−22 5.96 ×1056.78 ×10−11×10−22 6.10 ×1057.41 ×10−11×10−22 6.24 ×1057.32 ×10−11×10−22 6.38 ×1057.75 ×10−11×10−22 6.53 ×1058.35 ×10−11×10−22 6.68 ×1057.94 ×10−11×10−22 6.84 ×1058.31 ×10−11×10−22 7.00 ×1058.62 ×10−11×10−22 7.16 ×1058.97 ×10−12×10−22 7.33 ×1059.23 ×10−12×10−22 7.50 ×1059.74 ×10−12×10−22 7.67 ×1051.05 ×1002×10−22 7.85 ×1051.04 ×1002×10−22 8.04 ×1051.03 ×1002×10−22 8.22 ×1051.11 ×1002×10−22 8.41 ×1051.17 ×1002×10−22 8.61 ×1051.20 ×1002×10−22 8.81 ×1051.22 ×1002×10−21 9.02 ×1051.28 ×1002×10−21 9.23 ×1051.32 ×1002×10−21 9.44 ×1051.38 ×1002×10−21 9.66 ×1051.43 ×1002×10−21 9.89 ×1051.47 ×1002×10−21 1.05 ×1061.48 ×1001×10−21 1.15 ×1061.51 ×1001×10−21 1.25 ×1061.49 ×1001×10−21 1.35 ×1061.49 ×1001×10−21 1.45 ×1061.57 ×1002×10−21 1.55 ×1061.56 ×1002×10−21 1.65 ×1061.58 ×1002×10−21 1.75 ×1061.60 ×1002×10−21 1.85 ×1061.66 ×1002×10−21 1.95 ×1061.65 ×1002×10−21 2.10 ×1061.71 ×1002×10−21 014616-21 A. STAMATOPOULOS et al. PHYSICAL REVIEW C 102, 014616 (2020) TABLE V. (Continued.) Energy σδσδσ (eV) (b) (b) (%) 2.30 ×1061.70 ×1002×10−21 2.50 ×1061.60 ×1003×10−22 2.70 ×1061.72 ×1003×10−22 2.90 ×1061.73 ×1003×10−22 3.12 ×1061.71 ×1004×10−22 3.38 ×1061.71 ×1004×10−22 TABLE V. (Continued.) Energy σδσδσ (eV) (b) (b) (%) 3.62 ×1061.61 ×1004×10−22 3.88 ×1061.64 ×1008×10−25 4.25 ×1061.52 ×1008×10−25 4.75 ×1061.55 ×1008×10−25 5.25 ×1061.52 ×1008×10−25 5.75 ×1061.63 ×1001×10−18 [1] IAEA, Energy, Electricity and Nuclear Power Estimates for the Period up to 2050, Technical Report No. IAEA-RDS1-2017, 2017. [2] Generation-IV International Forum, www.gen-4.org/gif/ [3] F. Goldner and R. Versluis, Transmutation capabilities of generation 4 reactors, NEA Technical Report No. OECD-NEA39088792, 2007. [4] NEA, Technical Report, 2002, www.oecd-nea.org/ndd/reports/ 2002/nea3109.html [5] A. Stanculescu, Ann. Nucl. Energy 62, 607 (2013). [6] S. Gabriel, A. Baschwitz, G. Mathonnière, F. Fizaine, and T. Eleouet, Resour. Policy 38, 458 (2013). [7] The Nuclear Energy Agency (NEA), www.oecd-nea.org/ [8] The High Priority Request List (HPRL), www.oecd-nea.org/ dbdata/hprl/ [9] High priority request ID 37, www.oecd-nea.org/dbdata/hprl/ hprlview.pl?id=457, 2008. [10] M. Salvatores, International Evaluation Co-operation: Uncertainty and Target Accuracy Assessment for Innovative Systems Using Recent Covariance Data Evaluations, NEA-OECD Technical Report, 2008. [11] Plutonium management in the medium term, White Paper on the Physics of Plutonium Fuels and I. F. C. 1070 (WPPR), OECDNEA Technical Report, 2002. [12] N. Otuka et al.,Nucl. Data Sheets 120, 272 (2014). [13] W. W. Pratt et al., Oak Ridge National Laboratory Progress Report 2081, 1956. [14] T. A. Eastwood et al.,inProceedings of the 2nd International Conference on the Peaceful Uses of Atomic Energy, Geneva, Switzerland (International Atomic Energy Agency, Vienna, 1958), Vol 16, p. 53. [15] C. B. Bigham, Can. J. Phys. 36, 503 (1958). [16] E. K. Hulet, H. R. Bowman, M. C. Michel, and R. W. Hoff, Phys. Rev. 102, 1621 (1956). [17] B. Leonard, Jr. et al.,Bull.Am.Phys.Soc.1, 248 (1956). [18] L. W. Weston and J. H. Todd, Nucl. Sci. Eng. 88, 567 (1984). [19] O. Bouland et al.,Nucl. Sci. Eng. 127, 105 (1997). [20] C. Budtz-Jorgensen and H. H. Knitter, Nucl. Sci. Eng. 79, 380 (1981). [21] P. Salvador-Castineira et al.,Phys. Rev. C 92, 014620 (2015). [22] F. Tovesson, T. S. Hill, M. Mocko, J. D. Baker, and C. A. McGrath, Phys. Rev. C 79, 014613 (2009). [23] A. B. Laptev et al.,Nucl. Phys. A 734, E45 (2004). [24] A. Tsinganis et al.,Fission cross section measurements for 240Pu,242Pu: Deliverable 1.5 of the ANDES project (Publications Office of the European Union, Belgium 2013). [25] C. Weiss et al.,Nucl. Instrum. Methods A 799, 90 (2015). [26] A. Tsinganis, A. Stamatopoulos et al.,inProceedings of the 14th International Conference on Nuclear Reaction Mechanisms, edited by F. Cerutti, M. Chadwick, A. Ferrari, T. Kawano, and P. Schoofs (CERN, Geneva, 2015). [27] A. Stamatopoulos et al.,EPJ Web Conf. 146, 04030 (2017). [28] M. Barbagallo et al.,Phys. Rev. Lett. 117, 152701 (2016). [29] A. Stamatopoulos et al., in ND2019 Proceedings [EPJ Web Conf. (to be published)]. [30] V. Michalopoulou et al., in ND2019 Proceedings [EPJ Web Conf. (to be published)]. [31] Z. Eleme et al., in ND2019 Proceedings [EPJ Web Conf. (to be published)]. [32] M. Sabate-Gilarte et al.,Eur.Phys.J.A53, 210 (2017). [33] G. Sibbens et al.,J. Radiol. Nucl. Chem. 299, 1093 (2014). [34] A. Stamatopoulos, Study of the 240Pu(n,f)and237Np(n,f) reaction cross sections at the new experimental area (EAR2) of the CERN n_TOF facility, Doctoral thesis, Report No. CERNTHESIS-2019-260, National Technical University of Athens, 2019. [35] S. Andriamonje et al.,J. Instrum. 5, P02001 (2010). [36] L. Cosentino et al.,Rev. Sci. Instrum. 86, 073509 (2015). [37] U. Abbondanno et al.,Nucl. Instrum. Methods A 538, 692 (2005). [38] P. Žugec et al.,Nucl. Instrum. Methods A 812, 134 (2016). [39] M. Robles, Study of fast radiation-detectors based on fast halide scintillator crystals and their application to the CERN n_TOF experiment, Doctoral thesis, Report No. CERN-THESIS-2016399, University of Santiago de Compostela, 2016. [40] K.-H. Schmidt et al., General fescription of fission observables: GEF model, OECD Technical Report No. NEA/DB/DOC 1, 2014. [41] A. Ferrari et al., FLUKA: A multi-particle transport code, CERN Technical Report No. INFN/TC_05/11, CERN-2005-10, 2005. [42] G. Sibbens et al.,inProceedings of the 28th World Conference of the International International Target Development Society (INTDS2016), 13–18 November 2016, Stellenbosch, South Africa, edited by N. Kheswa and J. Greene, AIP Conf. Proc. No. 1962 (AIP, New York, 2018), p. 030007. [43] P. B. Coates, J. Phys. E 5, 148 (1972). 014616-22 INVESTIGATION OF THE 240Pu(n,f) REACTION … PHYSICAL REVIEW C 102, 014616 (2020) [44] M. Moore, Nucl. Instrum. Methods 169, 245 (1980). [45] A. Stamatopoulos et al.,Nucl. Instrum. Methods A 913,40 (2019). [46] N. Colonna et al.,Eur. Phys. J. A 56, 48 (2020). [47] E. Leal-Cidoncha, I. Durán, C. Paradela, D. Tarrío et al.,EPJ Web Conf. 111, 10002 (2016). [48] D. Tarrío, L. Leong, L. Audouin, I. Duran et al.,Nucl. Data Sheets 119, 35 (2014). [49] D. Tarrío, L. Leong, L. Audouin, I. Duran et al.,Nucl. Instrum. Methods Phys. Res., Sect. A 743, 79 (2014). [50] NIST standard reference database 124, 2017. [51] M. Sabaté Gilarte, The n_TOF-EAR2 facility at CERN: Neutron flux determination and the 33S(N,α)30Si cross section measurement; implications in BNCT, Doctoral thesis, Report No. CERN-THESIS-2014-385, CERN, Universidad de Sevilla, 2017. [52] Z. Ge, H. Wu, G. Chen, and R. Xu, EPJ Web Conf. 146, 02002 (2017). [53] JEFF-3.3: Evaluated Data Library, 2017. [54] D. Brown et al.,Nucl. Data Sheets 148, 1 (2018). [55] K. Shibata et al.,J. Nucl. Sci. Technol. 48, 1 (2011). [56] J. W. Meadows, Nucl. Sci. Eng. 79, 233 (1981). [57] V. G. Nesterov and G. N. Smirenkin, Sov. J. At. Energy 9, 511 (1961). [58] K. Kari, Ph.D. thesis, Kernforschungszentrum Karlsruhe, 1978 (unpublished), Technical Report 2673. [59] K. Kari and S. Cierjacks, in Proceedings of the Third Symposium on Neutron Capture Gamma-Ray Spectroscopy and Related Topics (Plenum Press, New York, 1978). [60] K. Kari and S. Cierjacks, in Proceedings of the Specialists Meeting on Nuclear Data of Plutonium and Americium Isotopes for Reactor Applications,editedbyR.E.Chrien(OSTI.GOV, 1978). [61] M. Cance and G. Grenier, Nuclear Data for Science and Technology, in Proceedings of the International Conference Antwerp 610 September 1982, edited by K. H. Böckhof (Springer, 1982). [62] M. Kazarinova, Yu. S. Zamyatnin, and V. Gorbachev, At. Energiya 9, 16 (1960). [63] N. M. Larson, ORNL Technical Report No. ORNL/TM9179/R8 ENDF-364/R2, 2008. [64] V. Vlachoudis (private communication). [65] R. Gwin, EXFOR entry 14249002, 1982. [66] C. Guerrero, Doctoral thesis, CIEMAT, Madrid, 2008 (unpublished), CERN-THESIS-2010-064. [67] C. Guerrero et al.,inANS Topical Meeting on Reactor Physics, PHYSOR-2006 (ANS, La Grange Park, IL, 2006), p. C031. [68] W. Kolar and K. Böckhoff, J. Nucl. Energy 22, 299 (1968). 014616-23