Measurement and analysis of the 241Am(n,γ ) cross section with liquid scintillator detectors using time-of-flight spectroscopy at the n_TOF facility at CERN

PHYSICAL REVIEW C 89, 044609 (2014) Measurement and analysis of the 241Am(n,γ) cross section with liquid scintillator detectors using time-of-flight spectroscopy at the n_TOF facility at CERN K. Fraval,1F. Gunsing,1,*S. Altstadt,2J. Andrzejewski,3L. Audouin,4M. Barbagallo,5V. B ´ ecares,6F. Beˇ cv´ aˇ r,7F. Belloni,1 E. Berthoumieux,1,8J. Billowes,9V. Boccone,8D. Bosnar,10 M. Brugger,8M. Calviani,8F. Calvi ˜ no,11 D. Cano-Ott,6 C. Carrapic¸o,12 F. Cerutti,8E. Chiaveri,1,8M. Chin,8N. Colonna,5G. Cort´ es,11 M. A. Cort´ es-Giraldo,13 M. Diakaki,14 C. Domingo-Pardo,15 I. Duran,16 R. Dressler,17 N. Dzysiuk,18 C. Eleftheriadis,19 A. Ferrari,8S. Ganesan,20 A. R. Garc´ ıa,6 G. Giubrone,15 M. B. G´ omez-Hornillos,11 I. F. Gonc¸alves,12 E. Gonz´ alez-Romero,6E. Griesmayer,21 C. Guerrero,8 P. Gurusamy,20 A. Hern´ andez-Prieto,8,11 D. G. Jenkins,22 E. Jericha,21 Y. Kadi,8F. K ¨ appeler,23 D. Karadimos,14 N. Kivel,17 P. Koehler,24 M. Kokkoris,14 M. Krtiˇ cka,7J. Kroll,7C. Lampoudis,1C. Langer,2E. Leal-Cidoncha,16 C. Lederer,2,25 H. Leeb,21 L. S. Leong,4R. Losito,8A. Mallick,20 A. Manousos,19 J. Marganiec,3T. Mart´ ınez,6C. Massimi,26 P. F. Mastinu,18 M. Mastromarco,5M. Meaze,5E. Mendoza,6A. Mengoni,27 P. M. Milazzo,28 F. Mingrone,26 M. Mirea,29 W. Mondalaers,30 C. Paradela,16 A. Pavlik,25 J. Perkowski,3A. Plompen,30 J. Praena,13 J. M. Quesada,13 T. Rauscher,31 R. Reifarth,2A. Riego,11 M. S. Robles,16 F. Roman,8,29 C. Rubbia,8,32 M. Sabat´ e-Gilarte,13 R. Sarmento,12 A. Saxena,20 P. Schillebeeckx,30 S. Schmidt,2 D. Schumann,17 G. Tagliente,5J. L. Tain,15 D. Tarr´ ıo,16 L. Tassan-Got,4A. Tsinganis,8S. Valenta,7G. Vannini,26 V. Variale,5 P. Vaz,12 A. Ventura,27 R. Versaci,8M. J. Vermeulen,22 V. Vlachoudis,8R. Vlastou,14 A. Wallner,25 T. Ware,9M. Weigand,2 C. Weiß,21 T. Wright,9and P. ˇ Zugec10 (n_TOF Collaboration)† 1CEA Saclay - Irfu, Gif-sur-Yvette, France 2Johann-Wolfgang-Goethe Universit¨ at, Frankfurt, Germany 3Uniwersytet Ł´ odzki, Lodz, Poland 4Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France 5Istituto Nazionale di Fisica Nucleare, Bari, Italy 6Centro de Investigaciones Energeticas Medioambientales y Tecnol´ ogicas (CIEMAT), Madrid, Spain 7Charles University, Prague, Czech Republic 8European Organization for Nuclear Research (CERN), Geneva, Switzerland 9University of Manchester, Oxford Road, Manchester, United Kingdom 10Department of Physics, Faculty of Science, University of Zagreb, Croatia 11Universitat Politecnica de Catalunya, Barcelona, Spain 12Instituto Tecnol´ ogico e Nuclear, Instituto Superior T´ ecnico, Universidade T´ ecnica de Lisboa, Lisboa, Portugal 13Universidad de Sevilla, Spain 14National Technical University of Athens (NTUA), Greece 15Instituto de F´ ısica Corpuscular, CSIC-Universidad de Valencia, Spain 16Universidade de Santiago de Compostela, Spain 17Paul Scherrer Institut, Villigen PSI, Switzerland 18Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy 19Aristotle University of Thessaloniki, Thessaloniki, Greece 20Bhabha Atomic Research Centre (BARC), Mumbai, India 21Atominstitut, Technische Universit¨ at Wien, Austria 22University of York, Heslington, York, United Kingdom 23Karlsruhe Institute of Technology, Campus Nord, Institut f¨ ur Kernphysik, Karlsruhe, Germany 24Department of Physics, University of Oslo, N-0316 Oslo, Norway 25University of Vienna, Faculty of Physics, Austria 26Dipartimento di Fisica e Astronomia, Universit` a di Bologna, and Sezione INFN di Bologna, Italy 27Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy 28Istituto Nazionale di Fisica Nucleare, Trieste, Italy 29Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN HH, Bucharest - Magurele, Romania 30European Commission JRC, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium 31Department of Physics and Astronomy - University of Basel, Basel, Switzerland 32Laboratori Nazionali del Gran Sasso dell’INFN, Assergi (AQ), Italy (Received 21 November 2013; published 23 April 2014) The 241Am(n,γ) cross section has been measured at the n_TOF facility at CERN using deuterated benzene liquid scintillators, commonly known as C6D6detectors, and time-of-flight spectrometry. The results in the resolved resonance range bring new constraints to evaluations below 150 eV, and the energy upper limit was extended *Corresponding author: [email protected] †www.cern.ch/ntof 0556-2813/2014/89(4)/044609(14) 044609-1 ©2014 American Physical Society K. FRAVA L et al. PHYSICAL REVIEW C 89, 044609 (2014) from 150 to 320 eV with a total of 172 new resonances not present in current evaluations. The thermal capture cross section was found to be σth =678 ±68 b, which is in good agreement with evaluations and most previous measurements. The capture cross section in the unresolved resonance region was extracted in the remaining energy range up to 150 keV, and found to be larger than current evaluations and previous measurements. DOI: 10.1103/PhysRevC.89.044609 PACS number(s): 25.40.Lw,25.40.Ny,28.20.Fc INTRODUCTION Nuclear power using fission technology has to cope with the issue of radioactive waste, mostly coming from irradiated construction materials in the reactor and spent fuel. The latter carries the major disadvantage of having very long-lived and highly radioactive isotopes, consisting of fission products and actinides. For example, the spent fuel of a pressurized water reactor (PWR) using MOX fuel with 8.2% content of Pu, after a burn-up of 43.5 GW·d/t and four years of cooling has a 0.6% mass content of minor actinides, among which is 50% of 241Am. Isotopes of Am and Np are responsible for the totality of the long-term radiotoxicity for times greater than 105 years [1–3]. The nucleus 241Am with spin parity Jπ=5/2− has a half life of 432 y, and 237Np a half life of 2.14 ×106 years. Also, a high burn-up of fuel in existing nuclear reactors inevitably leads to the accumulation of more minor actinides. Transmutation or incineration of actinides is a possibility to be considered, either in conventional uranium-plutonium fuelcycle nuclear reactors [4,5] or in future thorium-uranium based reactors [6]. Experimental nuclear research is needed to bring accurate cross section data on nuclei of interest. This work, as a part of the EC-FP7 ANDES project [7], is concerned with the 241Am(n,γ) reaction, which is on the High Priority List of the Nuclear Energy Agency [8–10]. Current available experimental data on the 241Am neutron cross section include activation and pile oscillation measurements to retrieve the thermal capture cross section, and time-of-flight experiments which allow resonance analysis. A measurement of the thermal cross section using pile oscillations was performed by Pomerance et al. in 1955 [11]. Most thermal data are obtained using the activation technique, including the work of Bak et al. [12], Dovbenko et al. [13], Harbour et al. [14], Gavrilov et al. [15], Shinohara et al. [16], Maidana et al. [17], Fioni et al. [4], Nakamura et al. [18], Bringer et al. [19], Belgya et al. [20], and Genreith et al. [21]. These data are summarized in Fig. 11. It should be noted that the activation technique requires the use of the Westcott convention, which assumes a shape for the neutron flux and energy dependence of the cross section. The presence of large resonances at very low energy in 241Am complicates this analysis step, and corrections are needed. This may partly explain the difficulties and discrepancies in the past measurements. Older time-of-flight measurements are the transmission data sets from Derrien and Lucas [22] and from Kalebin et al. [23,24], as well as the capture data from Weston and Todd [25] and from Van Praet et al. [26]. More recently two time-of-flight measurements of 241Am have been published. The neutron capture measurement of neutron energy (eV) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 capture cross section (b) 0 1 2 3 4 5 6 3 10× Kalebin Weston Jandel Lampoudis JEFF-3.1.2 ENDF/B-VII.1 JENDL-4.0/ FIG. 1. (Color online) The 241Am(n,γ) cross section at 300 K for the first three resonances for different measurements and evaluations, all calculated from resonance parameters. Jandel et al. [27] was performed in Los Alamos National Laboratory, using the 4πcalorimeter of BaF2crystals DANCE. The cross section was measured using a 219-μg241Am target at a 20.2-m distance from the neutron source. The analysis was performed from 0.02 eV to 320 keV, and included resolved resonance shape analysis from 0.02 to 12 eV. The work of Lampoudis et al. [28] was performed at the GELINA facility in Geel, Belgium. It used a 325-mg 241Am content and included both transmission and capture measurements. The transmission measurement was performed at a flight path of 26.45 m and the capture experiment at a flight path of 12.5 m, using two liquid scintillator detectors based on deuterated benzene (C62H6), in the following denoted as C6D6. A resonance shape analysis was performed up to 110 eV. The capture data were normalized to the transmissions data, expected to give an absolute normalization. In evaluated data libraries, ENDF/B-VII.1 [29] adopted the resonance parameters evaluated by JENDL-4.0 [30], which took into account the work of Jandel et al. [27], whereas the parameters of JEFF-3.1.2 [31] are based on data from Refs. [22,23,25,26,32–36]. Figure 1shows the discrepancies between experimental and evaluated data sets in the 241Am neutron capture cross section for the first three resonances, of which the parameters are given in Table I. In order to improve the accuracy on the neutron induced cross sections of 241Am, a new neutron capture measurement has been performed at the neutron time-of-flight facility n_TOF at CERN and is reported in this paper. 044609-2 MEASUREMENT AND ANALYSIS OF THE 241Am(n, . . . PHYSICAL REVIEW C 89, 044609 (2014) I. EXPERIMENTAL SETUP A. The n_TOF facility The neutron time-of-flight facility n_TOF is located at CERN (European Organization for Nuclear Research), and uses the 20-GeV/c protons extracted from the Proton Synchrotron (PS) on a 40-cm-thick and 60-cm-diameter lead spallation target. The proton pulses from the PS have an rms width of 7 ns, a nominal intensity of 7×1012 protons per pulse, and a minimum repetition rate of 1.2 s, resulting in a very high instantaneous neutron flux. Charged particles created during the spallation process are deflected from of the beam by a dipole magnet with a bending power of 3.63 T m. The neutrons, moderated by 1 cm of cooling water and 4 cm of borated water, are collimated in a vacuum beam guide leading to the experimental area located 182.3 m away from the spallation target. The neutron fluence is measured at the entrance of the experimental area by four silicon detectors viewing an in-beam 6Li deposit [37], and two MicroMegas chambers containing 10B and 235U converters [38,39]. The combined use of these detectors allows an accurate measurement of the neutron flux from thermal energy to the GeV region. As for γdetection, the measurements carried out at n_TOF use C6D6liquid scintillators or a 4πtotal absorption calorimeter (TAC) [40], made of 40 BaF2scintillation crystals. This work was performed with two optimized Bicron C6D6detectors. More details on the facility can be found in Refs. [41] and [42]. B. Data acquisition The data acquisition at n_TOF is based on flash ADCs from Acqiris [43], using a zero suppression technique to lower data transfer rates. For the C6D6detector setup modules with eightbit amplitude resolution were used at a sampling frequency of 500 MHz. The available memory of 48 MSamples allowed us to store data up to 96 ms, or an equivalent neutron energy of 0.019 eV. However, the neutron flux measurements were performed with less memory, allowing a lowest neutron energy of 26.3 meV. The recorded signals exceeding the detection threshold of 150 keV were fitted to a reference shape in order TABLE I. A comparison of the parameters of the first three resonances at 0.306, 0.573, and 1.271 eV from measurements and evaluations. Reference 0.306 eV 0.573 eV 1.271 eV gnγgnγgnγ (meV) (meV) (meV) (meV) (meV) (meV) Derrien [22] 0.161 46.5 Kalebin [24] 0.0275 45 0.0464 43 0.165 41 Weston [25] 0.027 46.9 0.047 47.3 0.157 49.2 Jandel [27] 0.0311 44.4 0.050 43.3 0.173 45.3 Lampoudis [28] 0.0373 41.55 0.0458 42.11 0.218 41.68 JEFF-3.1.2 [31] 0.0356 43.52 0.055 40.67 0.169 48.44 JENDL-4.0 [30] 0.0329 44.4 0.0558 43.3 0.190 45.3 TABLE II. The composition of the 241Am sample. Element/isotope Mass Density (in compound) (mg) (10−4atoms/b) Al (Al2O3) 159 30.2 O(Al 2O3) 142 45.3 241Am (AmO2) 32.2 ±0.7 0.68 ±0.02 O(AmO 2) 4.28 1.36 237Np 1.1 0.024 240Pu 0.02 0.00046 to extract the amplitude and time of flight, which were stored in DST (data summary tape) files for further analysis. C. The 241Am sample The sample used for this experiment was a solution of americium oxide (241AmO2) in an aluminium oxide (Al2O3) matrix, and was part of a batch of samples produced for (n,2n) measurements [44]. After drying and calcination the material was pressed in a cylindrical pellet of 0.342 g and 12.26 mm diameter, with a 32.2±0.7-mg content of 241Am (determined by calorimetry [45]); see Table II. The pellet was encapsulated in an aluminium canning and sealed with Stycast. A schematic view of the definitive sample is shown in Fig. 2. A dummy sample without 241Am was available as well. The main impurity in the sample is 237Np, originating from the αdecay of 241Am. From the analysis of some strong resonances, visible in the spectrum, the amount was estimated to be about 2.4×10−6atoms/b. Also, some 240Pu is present in the sample, as seen in Fig. 9where the 1.05-eV resonance is clearly visible on the capture yield. Its density was estimated to be about 4.6×10−8atoms/b. Note that this is only an estimation fitted on the 1.05-eV resonance, however it does not contribute enough to the capture yield to extract a precise density. In the end 237Np and 240Pu resonances were included as well in the resonance analysis. The radioactivity of the sample was about 3 GBq at the time of the measurement, dominated by 60-keV γrays. The high activity was drawing a very high current C6D6photomultiplier and voltage divider, resulting in random periods of zero output signal lasting for several μs. This was solved by placing 2-mm lead plates in front of each detector to shield them from the 12 .26 mm 2.1 mm 7.1 mm 20 mm Al2O3matrix with AmO2 Al Stycast FIG. 2. (Color online) A schematic view of the 241Am sample used in the experiment. 044609-3 K. F RAVA L et al. PHYSICAL REVIEW C 89, 044609 (2014) -ray energy (keV)γ 12345 3 10× counts/bin 2 10 3 10 4 10 5 10 6 10 FIG. 3. (Color online) The γ-ray spectrum of the 241Am measured in two energy ranges with a HPGe setup [46]. Some characteristic γrays from αdecay or from αinduced reactions are shown as well. low energy photons and therefore lowering considerably the count rate. The αactivity of the sample generated 2235-keV photons coming from the (α,p) reaction on 27Al in the Al2O3matrix. The response to these γrays contributed to the background which is about 90% of the signal at thermal energies. In a separate measurement with HPGe detectors [46]thisγray is clearly visible, as shown in Fig. 3. This characteristic 2.235- MeV γ-ray transition was used as a reference point for a dynamical energy calibration (see Sec. II A). D. The C6D6detectors TwoBicronC 6D6liquid scintillator detectors were used for the γ-ray detection. The main advantages of these detectors are a very good time resolution, with pulse widths of the order of 10 ns, and a low sensitivity to scattered neutrons. On the other hand, these detectors have low efficiency and poor energy resolution. Figure 4showsanMCNP[47] geometry of the sample and detectors implemented for this analysis. FIG. 4. (Color online) A schematic view of the detector and sample setup used for this experiment. The distance between the two deuterated benzene, C6D6, detectors was 6.1 cm. II. DATA PROCESSING A. Amplitude-energy calibration Three standard calibration sources were used for the amplitude to energy calibration: 137Cs providing a 662-keV γray, 88Y providing two simultaneous γrays of 898 and 1836 keV, and an Am-Be source providing a 4.4-MeV γray from the (α,n) reaction on 9Be. To obtain the calibration function for each detector an MCNP simulation of the detectors’ response was performed, and was adjusted to match the measured response data, with the free parameters being the resolution and calibration functions both chosen as second order polynomials, as applied in Ref. [48]. The measured and simulated response and the resulting calibration are shown in Fig. 5. However the amplitude response was not stable over time, and a dynamical adjustment was applied using the 2235-keV γray from the 27Al(α,p)30Si* reaction, which was clearly visible in every measurement run. The procedure was very similar to the one used with the standard calibration sources, the C6D6response to the 2235-keV photon was simulated and fit to the experimental data (see Fig. 5). In practice, the calibration function fwas used on a corrected amplitude k×Ain order to give the correct energy f(k×A)=E. The correction factor kwas determined by linearly scaling the observed amplitude response of the 2235-keV photon to the amplitude response at the time of the calibration. This gives an average correction factor kof respectively 0.90 and 0.97 for the two detectors, with a standard deviation of 0.04. Figure 6shows the deposited energy spectra for the Am sample with and without beam (where the 2.235-MeV photon energy deposition is clearly seen), together with the 250-μm-thick gold sample spectrum. The spectra were processed with a software threshold of 300 keV, larger than the threshold of approximately 150 keV of the data acquisition. B. Weighting function procedure The pulse height weighting technique (PHWT) was used to determine the efficiency for a capture reaction cindependent FIG. 5. (Color online) The simulated detector responses and measured spectra from calibration sources. The resulting energy calibration is shown in the inset. The highlighted regions correspond to the intervals where the calibrations sources were fitted. 044609-4 MEASUREMENT AND ANALYSIS OF THE 241Am(n, . . . PHYSICAL REVIEW C 89, 044609 (2014) deposited energy (MeV) 12345 normalized counts/bin 2 10 3 10 4 10 5 10 Am sample - beam on Am sample - beam off Au sample - beam off FIG. 6. (Color online) The deposited energy spectra for the Am sample with and without beam, and the gold sample, corresponding to all neutron energies with times of flight between 104and 108ns. The beam-off spectrum is normalized to acquisition time, and the beam-on is normalized to neutrons received relative to the Am sample spectrum. from the details of the gamma-ray cascade spectrum following neutron capture. This method, explained in more detail in for example [49,50], exploits the low efficiency ifor one γray Eiof the cascade of the total energy Sn+En, corresponding to the sum of the neutron binding energy Snand the neutron kinetic energy En, such that c(En)=1− i (1 −i)≈ i i.(1) The main difficulty resides in the random character of the γcascade, since many de-excitation paths to the ground state are possible, resulting in many different values for the total efficiency c(En). The idea to circumvent this issue is to use a weighting function W(E) in order to make the efficiencies iproportional to the γenergy Eiby modifying the detector response function Ri(E) so that the γ-ray efficiency i=Ri(E)dE becomes proportional to its energy i=Eup Elow W(E)Ri(E)dE =k×Ei,(2) and therefore the cascade efficiency c(En)≈ i i= i Ei=Sn+En,(3) when the proportionality constant kis set to 1 per energy unit. The weighting function W(E) was taken as a fourth order polynomial 4 0aiEiof which the coefficients aiwere determined by likelihood maximization using simulated detector response functions to several monoenergetic γrays. The detailed geometry in these simulations includes the sample, canning, detectors, and immediate surroundings as shown in Fig. 4. The effective threshold used for event selection was taken as the lower limit Elow in Eq. (2), while the upper limit Eup was chosen slightly above the excitation energy Sn+En. The possible influence of the difference between the Am and Au γspectra under the threshold on the normalization is discussed in Sec. III C. C. Deadtime correction From the time distribution between consecutive events in the same detector, a deadtime of approximately τ=25 ns was observed. Therefore in the analysis a slightly higher deadtime of τ=30 ns was applied, which was assumed to be nonextending. This hypothesis was tested by comparing the deadtime correction for 30-ns and 1-μs effective deadtime, and no systematic difference was obtained after correction. The deadtime correction factor fτ(T) as a function of time of flight Tis obtained from the observed total count rate Ctot(T) without applying any event selection conditions by fτ(T)=1−1 T0 T  t=T−τ Ctot(t)−1 ,(4) where T0is the number of time-of-flight bunches. In the end, the deadtime correction is at maximum 0.4% in the peaks of the large resonances at low energy. III. ANALYSIS OF THE DATA A. The capture yield The observable accessible with this experiment is the capture yield Y(En), which is the fraction of incident neutrons that induce an (n,γ) reaction, and is related to the observed spectra in the following way: Y(En)=N×CW(En)−BW(En) φ(En)×(En),(5) where CW(En) and BW(En) are, respectively, weighted counts and background, φ(En) is the number of neutrons in the sample, (En) is the detection efficiency, which for weighted counts is equal to the excitation energy (Sn+En) according to Eq. (3) and Nstands for the absolute efficiency normalization. The capture yield without multiple scattering corrections is related to the (n,γ) and total reaction cross sections σn,γ (En) and σt(En)by Y(En)=(1 −e−n×σt(En))×σn,γ (En) σt(En),(6) where nis the sample density in atoms/b. The first factor is usually called self-shielding, and expresses the attenuation of neutron density along the sample. The second factor is the probability of the (n,γ) reaction occurring given that the neutron has interacted. B. Background estimation A number of background contributions to the observed counts were identified and they are shown in Fig. 7together with the measured spectrum of the 241Am sample. These contributions were due to the radioactivity of the sample, neutron capture and neutron scattering on the aluminium canning and on the Al2O3matrix, in-beam photons, and ambient radiation. The sample’s activity was measured with a beam-off run, and normalized to acquisition time. The aluminium canning was placed alone in the beam for estimating capture 044609-5 K. F RAVA L et al. PHYSICAL REVIEW C 89, 044609 (2014) neutron energy (eV) -1 10 1 10 2 10 3 10 4 10 5 10 count rate (dc/dlnE) 3 10 4 10 5 10 6 10 7 10 8 10 Am sample fitted activity (Am, beam off) + SmOcanning + Al canning only ambient background in-beam photon background FIG. 7. (Color online) The measured 241Am spectrum together with the various measured and calculated scaled background components. The count rate is expressed in a width-independent logarithmic equidistant binning. and neutron scattering sensitivity, and this spectrum was normalized to the number of received neutrons. The estimation of the background component due to the Al2O3matrix using a measurement with the dummy sample could not be used at thermal energies. A contamination by Sm was found in the dummy spectrum only, visible by its high thermal cross section and characteristic resonances, and estimated at about 3 ×10−9atoms/b. After examination by x-ray photography, it appeared that the dummy sample was broken in pieces and glued together, explaining the presence of Sm as a component in the used glue. Since the dummy sample could therefore not be used at low energies, the capture contribution of Al2O3was instead simulated with aSAMMY [51] calculation and used as a background in the resolved resonance region. The uncertainty of the nuclear data on O and Al isotopes was neglected. Sensitivity to scattered neutrons is a possible source of remaining background in the resolved resonance region, but it is negligible on the resonance peaks. In the unresolved resonance range, the measurement of the dummy Al2O3sample was used, because the small amount of Sm contributes to the count rate only significantly at low neutron energy. The background from the 2.2-MeV in-beam photons from thermal neutron capture on hydrogen from water surrounding the spallation target has been a major source of background at n_TOF in the past. However the recent use of borated water as a moderator has strongly reduced this component, because of the very high 10B(n,α) cross section at thermal neutron energies. This reaction releases a 480-keV photon with a 0.94 probability, but the corresponding Compton edge of 310 keV means that most of the energy deposit of these photons is below the applied threshold. Runs were also taken with in-beam neutron filters of W, Al, and Co, removing all neutrons from the beam at large (black) resonances at 4.2, 18.8, 132 eV and 34.8 keV, giving access to the remaining background. When comparing the spectrum of the 241Am sample with the neutron filters, and the sum of all background components except in-beam photons, the count rates were identical within the uncertainties at the black resonance energies meaning that the contribution from the in-beam photons could be neglected for the present configuration of the moderator with borated water. This was also confirmed by the expected count rate for this background, shown in Fig. 7, which was calculated from the analytical form of this background, fitted to a measured spectrum of Pb after applying capture corrections and normalization. Finally, the ambient background (no beam, no sample) was measured and normalized to the acquisition time. The total background BW(En)inEq.(5) was calculated as BW=fAm fcan Ccan −Tcan Tamb Camb+TAm Tact Cact,(7) where Ccan,Camb, and Cact are the measured weighted time-of- flight spectra corresponding respectively to the canning with beam, ambient (no sample, no beam) and the activity (241Am sample, no beam), with corresponding acquisition times TAm, Tcan,Tamb,Tact, and incident integrated neutron intensities fAm and fcan. C. Normalization From the background subtracted count rate, the yield was calculated using Eq. (5). The incident neutron fluence φ(En) was derived from an evaluation based on long-term measurements with several detectors [52] and corrected for the transmission through the in-beam neutron flux detectors. This flux was normalized to the actual geometrical interception of the beam by the sample at 4.9 eV. To account for the absolute level of the neutron flux and for the absolute efficiency and geometry used for the weighting function, the normalization factor Nwas obtained by measuring a 250-μm-thick 197Au target with the same diameter and in the same setup as the 241Am sample. For this thickness the resonance at 4.9 eV is saturated in the capture yield, which reduces the impact of uncertainties on the resonance parameters during the fit of the 0.01 0.1 1 capture yield x √ E 0.11.0 neutron energy (eV) -4 -2 0 2 4 residual (a) (b) FIG. 8. (Color online) The 197Au(n,γ) capture yield and the fit of the normalization with SAMMY, using fixed resonance parameters from JEFF-3.1.2 [31]. The yaxis is multiplied by √E(eV) for a better view of the thermal region. The lower panel shows the residual of the fit, defined as the difference of the experimental yield and the fit, divided by the uncorrelated weighted uncertainty of the data points. 044609-6 MEASUREMENT AND ANALYSIS OF THE 241Am(n, . . . PHYSICAL REVIEW C 89, 044609 (2014) normalization with SAMMY, using the resonance parameters of JEFF-3.1.2 [31]. The normalization was fitted only in the small energy range from 4.55 to 5.25 eV around the saturated resonance. From this fit an uncertainty of 1% was obtained. By extrapolating the low energy part as shown in Fig. 8,the thermal capture cross section of Au, considered a standard, was reproduced within 0.7%. At 4.9 eV, this normalization accounts for both beam interception and the absolute efficiency normalization. One final correction is the energy dependence of the beam interception factor, however no experimental data nor numerical simulations provided the accuracy needed for this analysis. Hence, the energy dependence of the beam interception factor was not taken into account in this work. IV. RESULTS A. Thermal cross section From the normalized 241Am capture yield, the value of the thermal cross section was estimated. Once the resonances were adjusted individually (see Sec. IV B), the capture yield was fitted from 26.3 to 100 meV, with one bound state at the mirror energy of the first resonance, with the scattering width as a free parameter. The radiative width was taken as the average of the set of observed resonances, at 44.2 meV. Then the first resonance alone was fitted on the resonance peak. The obtained values were used as starting parameters for a fit of both the bound state and the first resonance from 0.026 meV to 0.35 eV. Figure 9shows the very low energy region of the full fit. It is interesting to note that Fig. 9exhibits the contamination of both 240Pu at 1.05 eV and 237Np at 1.48 eV. The results for the two detectors analyzed separately were giving consistent results. 10-2 10-1 capture yield x √ E 0.11.0 neutron energy (eV) -4 -2 0 2 4 residual 240Pu 237Np (a) (b) FIG. 9. (Color online) The 241Am capture yield with a 300-keV threshold is fitted up to the first three resonances to obtain the thermal cross section value. The yaxis is multiplied by √E(eV ) for convenience, to allow a better view of the thermal region. The lower panel shows the residual of the fit, defined as the difference of the experimental yield and the fit, divided by the uncorrelated weighted uncertainty of the data points. applied threshold (keV) 200 300 400 500 600 700 800 900 1000 capture yield 0.038 0.04 0.042 0.044 0.046 0.048 normalized to Au 4.9 eV resonance with 300 keV threshold Am 0.3 eV resonance FIG. 10. (Color online) Comparison of the thermal capture yield for different thresholds, normalized to the 4.9-eV Au resonance with the same threshold (in red), and normalized to the 0.3-eV 241Am resonance (in blue) which itself is normalized to Au with a threshold of 300 keV. The influence of the detection threshold on the weighting function and resulting normalization was studied by applying different thresholds ranging from 200 keV to 1 MeV. The yield was normalized either to the first resonance of 241Am, comparing similar γ-ray cascade spectra, and to the 4.9-eV resonance of Au with a different γ-ray cascade spectrum. The two detectors always provided consistent results for a given threshold. The result of the comparison of all thresholds (with counts summed for both detectors) is shown in Fig.10.Inthis figure, the thermal capture yield at 25.3 meV with a 300-keV threshold was normalized to the 4.9-eV 197Au resonance as described in Sec. III C. The resulting uncertainty of 1% is entirely due to the fit of the Au resonance which is dominated by counting statistics. The yields for other thresholds were normalized either to the yield on the first resonance of 241Am, or normalized to 197Au extracted at the same threshold. The interest in comparing the two normalization procedures lies in the bias induced by the weighting function procedure, especially at large thresholds. Indeed, starting from a threshold of 600 keV, the two differently normalized thermal capture yields start to diverge significantly. The yields normalized to the first 241Am resonance with a 300-keV threshold are only sensitive to the background, which is dominant at thermal neutron energy. In Fig. 10,the threshold dependence of the capture yield is shown for both normalization techniques, indicating that the differences on the thermal capture yield are probably due to the effect of the weighting function on the background at thermal energies. Indeed, the background/signal ratio varies from 0.93 when using a threshold of 200 keV to 0.82 with a 1-MeV threshold. At these energies most of the background is due to the the radioactivity of the sample, and notably the response to the 2235-keV γray, as shown in Fig. 3. This shows that the background/signal ratio could fully explain the discrepancies observed at thermal energy, independent of the threshold. The final adopted value of the thermal cross section, obtained from the resonance parameters, was σγ(25.3meV)=678 ±68 b. The extracted Westcott factor is 044609-7 K. F RAVA L et al. PHYSICAL REVIEW C 89, 044609 (2014) 500 600 700 800 900 thermal capture cross section (b) 628.5 +- 35.0 Pomerance et al. (1955) [11] 740.0 +- 60.0 Bak et al. (1967) [12] 654.0 +- 104.0 Dovbenko et al. (1969) [13] 612.0 +- 25.0 Harbour et al. (1973) [14] 853.0 +- 52.0 Gavrilov et al. (1976) [15] 625.0 +- 30.0 Kalebin et al. (1976) [24] 854.0 +- 58.0 Shinohara et al. (1997) [16] 696.0 +- 48.0 Fioni et al. (2001) [4] 672.0 +- 10.0 Maidana et al. (2001) [17] 705.0 +- 23.0 Bringer et al. (2007) [19] 702.0 +- 25.0 Nakamura et al. (2007) [18] 665.0 +- 33.0 Jandel et al. (2008) [27] 603.0 +- 36.0 Belgya et al. (2012) [20] 725.4 +- 34.4 Genreith et al. (2013) [21] 711.1 +- 28.2 Genreith et al. (2013) [21] 750.0 +- 35.0 Lampoudis et al. (2013) [28] 678.0 +- 68.0 this work FIG. 11. (Color online) An overview of the experimental thermal total neutron capture cross section values as extracted from experimental data library EXFOR. The larger gray band indicates the average and standard deviation of the central values (698.5 and 72.7 b). The smaller gray band corresponds to the weighted mean and its uncertainty, excluding values without uncertainty (679.5 and 6.5 b). 0.995. The adopted uncertainty of 10% is based on the spread observed in Fig. 10 below the 500-keV threshold. In Fig. 11 this thermal value is plotted together with previously reported values. It should be mentioned that the present experiment was not optimized for the thermal region, but for the resonance region. B. Resolved resonance region (RRR) The obtained yield with 300-keV threshold was used for a resonance shape analysis using the R-matrix code SAMMY [51] in the Reich-Moore approximation. The analysis included multiple scattering and self-shielding corrections, and used the free gas model for Doppler broadening with an effective temperature of 296 K. The n_TOF phase-I resolution function as given in SAMMY [51] was used in the analysis and the neutron energy calibration was adjusted to the 4.9-eV resonance in Au using parameters from JEFF-3.1.2 [31], resulting in an effective flight path length of L=184.21 ±0.06 m. Resonances were fitted individually whenever possible, and overlapping resonances forming multiplets were fitted together. The γpartial width γwas let free for the first three large resonances, where it is expected to be sensitive to small variations of the parameters (see Table III). For the remaining set of states the value of γwas fixed to the average value γ=45.79 meV of the first three resonances. The fission widths and the spins for the existing resonances were taken from JEFF-3.1.2 [31]; for the negative resonance the same spin as the first positive resonance was chosen. For new resonances a spin of either J=2orJ=3 was arbitrarily chosen according to a 2J+1 distribution, as well as an average fission width of f=0.3597 meV. In the energy range up to 150 eV, the upper limit of all present evaluations, a number of 15 new resonances were found. From 150 to 320 eV, for which currently no resonances are present TABLE III. The resonance parameters for the first three resonances. Only uncertainties due to counting statistics have been used in the data, and the uncertainties resulting from the fit correspond to the square root of the diagonal terms only. The correlation factor between nand γfrom the fit was lower than 0.25. Energy J nγ (eV) ()μeV (meV) −0.306 3 275.3 ±2.0 45.79 0.306 3 49.1 ±0.1 45.35 ±0.3 0.575 2 116.7 ±0.3 44.41 ±0.5 1.272 3 290.0 ±0.9 47.16 ±0.3 in evaluations, a total number of 157 new resonances were found and analyzed. Figure 12 shows the full range of resolved resonance data together with the fitted resonances. Table IV gives the full list of resonance parameters obtained in this analysis. New resonances as compared to JEFF-3.1.2 [31]are marked with a ∗. The resonance integral I=∞ Ec σn,γ (E)×dE E(8) was calculated with Ec=0.5 eV. With the new resonance parameters this gave I=1425.0 b. It should be noted that the traditional choice of 0.5 eV for the lower bound of the integral is, in the case of 241Am, a source of uncertainty, since the first resonance is at 0.3 eV and the second one at 0.57 eV. This means that a very slight change in the resonance parameters will have a very significant effect on the resonance integral, if the lower bound is chosen at 0.5 eV. For further discussion, see Lampoudis et al. [28]. C. Statistical analysis The newly fitted resonance parameters were used to perform a statistical analysis of this new set of states. First, the resonances were all checked to be swaves, i.e., carrying =0 angular momentum in the entrance channel. This test is carried out by a statistical approach, based on Bayesian probabilities, inspired by the work of Bollinger and Thomas [53]on238U and more recently applied to 99Tc [54], 232Th [48], and 241Am [28]. The quantity of interest is the probability of a resonance being apwave, given the observed scattering width gn. It can be expressed using Bayes’ theorem, and the assumption that only sand pwaves are present in the observed set of states. The detailed mathematical treatment has been discussed by Gyulassy et al. [55], and is based on a weighted combination of Porter-Thomas distributions with one or two degrees of freedom, respectively for =0 and =1 states. Taking neutron strength functions from Ref. [56], this analysis yields no p-wave resonance in the observed set of 241Am resonances, i.e., all resonances have P(=1|gn)=0. The average spacing D0and strength function S0were extracted from the present set of s-wave resonances. To that end, the Porter-Thomas distribution was used again, expressing the number of =0 states N(xt) with x=g0 n g0 nlarger than a 044609-8 MEASUREMENT AND ANALYSIS OF THE 241Am(n, . . . PHYSICAL REVIEW C 89, 044609 (2014) -0.050 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0 5 10 15 20 (a) -0.010 0.000 0.010 0.020 0.030 0.040 0.050 20 30 40 50 60 70 80 90 (b) -0.005 0.000 0.005 0.010 0.015 0.020 0.025 0.030 100 110 120 130 140 150 160 neutron capture yield (c) -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 170 180 190 200 210 220 230 240 (d) -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 240 250 260 270 280 290 300 310 320 neutron energy (eV) (e) FIG. 12. (Color online) The resolved resonance range analyzed in this work. The blue points are the experimental data, and the red line is the fit performed with SAMMY. The first panel has 500 bins per decade, whereas the following have 5000 bins per decade. certain threshold xtas N(xt)=N0∞ xt2 πx 1 2e−x/2dx =N0[1 −erf(xt/2)] (9) with N0the expected total number of swaves in the set. Assuming that all the resonances are observed above a certain threshold xt, one can fit this function to the experimental distribution in order to get N0and g0 n(Fig. 13). From the fitted values N0and g0 nthe quantities D0and S0were extracted as D0=N0 E ,S 0=g0 n D0 .(10) In the resolved energy range up to 320 eV this procedure yields D0=0.52 ±0.03 eV and 10−4S0=1.20 ±0.01, with a correlation coefficient of −0.90. The uncertainties associated to these results come from the relative stability of the analysis with respect to variations of the gnthreshold which was varied in order to include a fraction of the observed resonances ranging from 1/2upto4/5. D. Unresolved resonance region (URR) Above 320 eV neutron energy, the resonances were overlapping and no satisfactory fit of individual resonances could be performed. Therefore this region was considered the start of the unresolved resonance range. The upper limit for this analysis was chosen at 150 keV as a consequence of the recovery time of the C6D6detectors after the gamma flash. At 150 keV, the fission cross section is 70 times lower than the capture cross section in all evaluations (around 18 mb against 1.25 b) and is expected not to contribute significantly to the background. Also, inelastic scattering is not an issue, because of the 300-keV effective threshold used for the event selection. In order to get the unresolved capture yield, the data were binned in 40 bins per energy decade to average most of the resonance structure, while still keeping the sensitivity to the global energy dependence. 044609-9