New Measurement of the Capture Cross Section of Bismuth and Lead Isotopes

AIP Conference Proceedings 769, 1521 (2005); https://doi.org/10.1063/1.1945294 769, 1521 © 2005 American Institute of Physics. New Measurement of the Capture Cross Section of Bismuth and Lead Isotopes Cite as: AIP Conference Proceedings 769, 1521 (2005); https://doi.org/10.1063/1.1945294 Published Online: 13 June 2005 C. Domingo-Pardo, J. L. Tain, U. Abbondanno, G. Aerts, H. Álvarez, F. Alvarez-Velarde, S. Andriamonje, J. Andrzejewski, P. Assimakopoulos, L. Audouin, G. Badurek, P. Baumann, F. Bečvář, J. Benlliure, E. Berthoumieux, F. Calviño, D. Cano-Ott, R. Capote, A. Carrillo de Albornoz, P. Cennini, V. Chepel, E. Chiaveri, N. Colonna, G. Cortes, D. Cortina, A. Couture, J. Cox, S. David, R. Dolfini, W. Dridi, I. Duran, M. Embid-Segura, L. Ferrant, A. Ferrari, L. Fitzpatrick, R. Ferreira-Marques, H. Frais-Koelbl, K. Fujii, W. Furman, C. Guerrero, I. Goncalves, R. Gallino, E. Gonzalez-Romero, A. Goverdovski, F. Gramegna, E. Griesmayer, F. Gunsing, B. Haas, R. Haight, M. Heil, A. Herrera-Martinez, M. Igashira, S. Isaev, E. Jericha, Y. Kadi, F. Käppeler, D. Karamanis, D. Karadimos, M. Kerveno, V. Ketlerov, P. Koehler, V. Konovalov, E. Kossionides, M. Krtička, C. Lamboudis, H. Leeb, A. Lindote, I. Lopes, M. Lozano, S. Lukic, J. Marganiec, L. Marques, S. Marrone, P. Mastinu, A. Mengoni, P. M. Milazzo, C. Moreau, M. Mosconi, F. Neves, H. Oberhummer, S. O’Brien, M. Oshima, J. Pancin, C. Papachristodoulou, C. Papadopoulos, C. Paradela, N. Patronis, A. Pavlik, P. Pavlopoulos, L. Perrot, R. Plag, A. Plompen, A. Plukis, A. Poch, C. Pretel, J. Quesada, T. Rauscher, R. Reifarth, M. Rosetti, C. Rubbia, G. Rudolf, P. Rullhusen, J. Salgado, L. Sarchiapone, C. Stephan, G. Tagliente, L. Tassan-Got, L. Tavora, R. Terlizzi, G. Vannini, P. Vaz, A. Ventura, D. Villamarin, M. C. Vincente, V. Vlachoudis, R. Vlastou, F. Voss, H. Wendler, M. Wiescher, K. Wisshak, and The n_TOF Collaboration ARTICLES YOU MAY BE INTERESTED IN Measurements at n_TOF of the Neutron Capture Cross Section of Minor Actinides Relevant to the Nuclear Waste Transmutation AIP Conference Proceedings 769, 1442 (2005); https://doi.org/10.1063/1.1945275 The n_TOF Facility at CERN: Performances and First Physics Results AIP Conference Proceedings 769, 724 (2005); https://doi.org/10.1063/1.1945110 Measurement of the 232Th Neutron Capture Cross Section at the CERN n_TOF Facility AIP Conference Proceedings 769, 1470 (2005); https://doi.org/10.1063/1.1945282 New Measurement of the Capture Cross Section of Bismuth and Lead Isotopes C. Domingo-Pardo21 1 ,J.L.Tain 21, U. Abbondanno14,G.Aerts 7,H.Álvarez 24, F. Alvarez-Velarde20, S. Andriamonje7,J.Andrzejewski 33, P. Assimakopoulos 9, L. Audouin 5G. Badurek1,P.Baumann 6,F.Beˇcváˇr31, J. Benlliure24, E. Berthoumieux7,F.Calviño 25, D. Cano-Ott20, R. Capote23, A. Carrillo de Albornoz30, P. Cennini4, V. Chepel17,E.Chiaveri 4, N. Colonna13,G.Cortes 25,D.Cortina 24, A. Couture29,J.Cox 29,S.David 5,R.Dolfini 15, W. Dridi7,I.Duran 24, M. Embid-Segura20,L.Ferrant 5, A. Ferrari4, L. Fitzpatrick4,R.Ferreira-Marques 17, H. Frais-Koelbl3,K.Fujii 13 W. Furman18, C. Guerrero20 I. Goncalves30, R. Gallino36, E. Gonzalez-Romero20, A. Goverdovski19,F.Gramegna 12,E.Griesmayer 3, F. Gunsing7, B. Haas32, R. Haight27,M.Heil 8, A. Herrera-Martinez4,M.Igashira 37,S.Isaev 5,E.Jericha 1,Y.Kadi 4,F.Käppeler 8, D. Karamanis9,D.Karadimos 9, M. Kerveno,6, V. Ketlerov19,P.Koehler 28, V. Konovalov18, E. Kossionides39,M.Krtiˇcka31, C. Lamboudis10, H. Leeb1, A. Lindote17, I. Lopes17,M.Lozano 23,S.Lukic 6,J.Marganiec 33,L.Marques 30,S.Marrone 13, P. Mastinu12, A. Mengoni4, P.M. Milazzo14, C. Moreau14, M. Mosconi8,F.Neves 17, H. Oberhummer1, S. O’Brien29, M. Oshima38,J.Pancin 7, C. Papachristodoulou9, C. Papadopoulos40, C. Paradela24,N.Patronis 9, A. Pavlik2, P. Pavlopoulos34,L.Perrot 7,R.Plag 8,A.Plompen 16,A.Plukis 7,A.Poch 25,C.Pretel 25,J.Quesada 23,T.Rauscher 26, R. Reifarth27,M.Rosetti 11, C. Rubbia15, G. Rudolf6,P.Rullhusen 16,J.Salgado 30, L. Sarchiapone4, C. Stephan5, G. Tagliente13,L.Tassan-Got 5,L.Tavora 30, R. Terlizzi13, G. Vannini35,P.Vaz 30,A.Ventura 11, D. Villamarin20, M. C. Vincente20, V. Vlachoudis4,R.Vlastou 40 F. Voss8,H.Wendler 4,M.Wiescher 29,K.Wisshak 8 The n_TOF Collaboration 1Atominstitut der Österreichischen Universitäten, Technische Universität Wien, Austria  2Institut für Isotopenforschung und Kernphysik, Universität Wien, Austria  3Fachhochschule Wiener Neustadt, Wiener Neustadt, Austria  4CERN, Geneva, Switzerland  5Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France  6Centre National de la Recherche Scientifique/IN2P3 - IReS, Strasbourg, France  7CEA/Saclay - DSM, Gif-sur-Yvette, France  8Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany  9University of Ioannina, Greece  10Aristotle University of Thessaloniki, Greece  11ENEA, Bologna, Italy  12Laboratori Nazionali di Legnaro, Italy  13Istituto Nazionale di Fisica Nucleare, Bari, Italy  14Istituto Nazionale di Fisica Nucleare, Trieste, Italy  15Università degli Studi Pavia, Pavia, Italy  16CEC-JRC-IRMM, Geel, Belgium  17LIP - Coimbra & Departamento de Fisica da Universidade de Coimbra, Portugal  18Joint Institute for Nuclear Research, Frank Laboratory of Neutron Physics, Dubna, Russia  19Institute of Physics and Power Engineering, Kaluga region, Obninsk, Russia  20Centro de Investigaciones Energeticas Medioambientales y Technologicas, Madrid, Spain  21Consejo Superior de Investigaciones Cientificas - University of Valencia, Spain  22Universidad Politecnica de Madrid, Spain  23Universidad de Sevilla, Spain  24Universidade de Santiago de Compostela, Spain  25Universitat Politecnica de Catalunya, Barcelona, Spain  26Department of Physics and Astronomy - University of Basel, Basel, Switzerland  27Los Alamos National Laboratory, New Mexico, USA  28Oak Ridge National Laboratory, Physics Division, Oak Ridge, USA  29University of Notre Dame, Notre Dame, USA  30Instituto Tecnológico e Nuclear, Lisbon, Portugal  31Charles University, Prague, Czech Republic  32Centre National de la Recherche Scientifique/IN2P3 - CENBG, Bordeaux, France  33University of Lodz, Lodz, Poland  34Pôle Universitaire Léonard de Vinci, Paris La Défense, France  35Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy  36Dipartimento di Fisica Generale, Università di Torino and Sezione INFN di Torino, I-10125 Torino, Italy  37Tokyo Institute of Technology, Tokyo, Japan  38Japan Atomic Energy Research Institute, Tokai-mura, Japan  39NCSR, Athens, Greece  40National Technical University of Athens, Greece Abstract. All the stable lead isotopes and 209Bi have been recently measured at n_TOF [1] (CERN) in the range from 1 eV up to 1 MeV, using an optimized experimental setup. A set of two homemade C6D6detectors with carbon-fibre canning was designed to greatly reduce the sensitivity to scattered neutrons. Surrounding elements like the sample holder and the sample exchangerhave been made also of carbon fibre. The pulse-height weighting technique has been used in order to determine the (n, γ ) cross sections. Concerning this technique, special care has been taken in the treatment of several sources of error leading to a total systematic uncertainty of less than 3%. Results on the analysed 209Bi and 207Pb capture data and a comparison with previous experiments and evaluations are presented. 1521 CP769, International Conference on Nuclear Data for Science and Technology, edited by R. C. Haight, M. B. Chadwick, T. Kawano, and P. Talou © 2005 American Institute of Physics 0-7354-0254-X/05/$22.50 INTRODUCTION Lead and bismuth capture data are important in the field of energy production and nuclear-waste transmutation, in order to evaluate the neutron balance in the core of an Accelerator Driven System (ADS) with a Pb-Bi spallation target and also to account for the synthesis of 210mBi through capture reactions, which determines the longterm radioactivity of the spallation source. In astrophysics, 208Pb and 209Bi represent the end point of the s-process, which by means of (n, γ ) reactions builds around half of the isotopic abundances in the mass region A  56. Accurate capture data on these end-point isotopes are required to further develop nucleosynthesis stellar models and obtain heavy-element abundance predictions compatible with observations. Capture in these isotopes is difficult to measure not only due to their small cross sections but also due to the relatively high rate of scattered neutrons that can interact in the detector and surrounding material and mimic a capture signal. This has been a source of systematic uncertainty in previous experiments. EXPERIMENT Neutron-energy differential capture cross sections have been determined with high resolution at n_TOF thanks to the 185-m-long flight path and the initial sharp pulsed, 6 ns wide (RMS), 20 GeV/c proton beam that produces the neutrons by spallation on a lead block. The key improvement with respect to previous experiments is a very low neutron sensitivity, achieved mainly by means of innovative carbon-fibre homemade C6D6 detectors [2]. Other elements, like the sample holder and the sample exchanger, are made also from carbon fibre. Such a detection setup has been specially conceived for the accurate measurement of nuclides with resonances showing large scattering to capture ratios, which is in general the case of the lead and bismuth isotopes. Two carbon-fibre C6D6detectors were placed backwards at  135 Æ in order to minimize the effects of the primary angular distribution in the case of 207Pb but also to reduce detection of in-beam γ -rays, which are scattered on the sample and constitute, in these particular measurements, the main source of background. The neutron flux was monitored in parallel to the capture measurements by centering a thin 6Li foil in the beam a few meters before the capture sample under study. The outgoing 3H particles from the 6Li(n, α ) reaction were registered by means of four Si-detectors placed off the beam but covering a high solid angle. 1correspondingauthor: [email protected] The cross section was measured relative to the standard 4.9-eV 197Au resonance; therefore a thin Au-sample was regularly measured during the experiment. To carry out this normalization, the dependence of the neutron flux against the neutron energy is also needed. This has been determined not only with the Si-detectors mentioned above, but also from dedicated 235  238U fission measurements. The flux has been finally evaluated from both measurements, and its associated uncertainty can be considered on average below 1% for neutron energies up to 100 keV and increases up to  3% at 500 keV. The (n, γ ) cross section has been measured using the Pulse Height Weighting Technique (PHWT). This is based on the appropriate manipulation of the detector response in order to achieve a detection efficiency independent of the nuclear γ -cascade. As has been shown in a previous work [4], the Weighting Function depends on the exact (n, γ ) detection setup, including the sample itself. Therefore, a complete and realistic Monte Carlo simulation represents the only reliable method to determine it accurately. FIGURE 1. View of the (n, γ ) detection setup as is implemented in the code GEANT4[5] used for the MC simulations. Before starting with the measurement of the isotopes of interest, part of the n_TOF beam time was dedicated to validate and determine the systematic accuracy of the PHWT [3]. The 1.15-keV 56Fe resonance constitutes an ideal accuracy test for a radiative neutron-capture measurement. This resonance is strong and well isolated, and its capture width Γ γ is about ten times larger than itsneutron widthΓn, thus becoming the radiative-capture measurement mainly sensitive to Γn. An exhaustive modelling of the measuring setup in the MC simulation (see Fig.1), together witha detailed analysis of the different sources of systematic uncertainty like electronic threshold, summing of two or more γ - rays, and Conversion Electrons (CE), enabled us to determine the 1.15-keV 56Fe resonance cross section with an accuracy better than 2%, which is then a good measure of our experimental-technique systematic uncertainty. 1522 ANALYSIS The present work describes the results obtained for 209Bi and 207Pb. In the case of 209Bi, apart from the common experimental sources of systematical uncertainty mentioned above (threshold, γ -summing, and CE), there exists an additional effect, which is the isomeric state at 270 keV. In a similar way as in the case of the 1.15- keV 56Fe resonance described in [3], we estimated all these effects utilizing the statistical model of the nucleus. A satisfactory description of the deposited energy spectrum was obtained when compared with the experimental spectrum, as shown in the left part of Fig. 2. In 207Pb, (MeV) dep E 012345 Counts 1 10 102 103 (MeV) dep E 012345678 Counts 1 10 102 103 FIGURE 2. Left: MC simulation (black line) of the pulseheight spectrum for the 802-eV 209Bi resonance and comparison with the experimental spectrum. Right: simulation of the deposited energyspectrum for the 3-keV2  resonancein 207Pb and comparison with the experimental spectrum. capture of one neutron leads to the double magic 208Pb, which shows a very simple deexcitation pattern of only one or two transitions to the ground state depending on the spin and parity of the initial level. Therefore, the response distribution was reproduced by including those transitions in the MC simulation rather than using the statistical model. The resulting deposited energy spectrum shows good agreement with the corresponding experimental one as shown in the right part of Fig. 2. For a γ -threshold of 200 keV, the calculated yieldcorrection factors are 1.046(4), 1.137(4), and 1.016(4), respectively, for the 4.9-eV 197Au, 802-eV 209Bi, and the 3-keV 207Pb resonances. It is important to note here that an accurate determination of the final capture yield can only be made after correcting for these effects, particularly in the case of bismuth and gold, where the corrections are larger. The neutron-sensitivity correction was negligible (below 0.7%) for all the analysed resonances except for the 12.1-keV level of 209Bi. In this case the ratio Γn  Γ γ  103required a correction of 1.5%. The experimental yield, corrected for these effects, was analysed using the multilevel R-matrix code SAMMY [6]. The cross sections were parameterized using the Reich-Moore formalism and the yields calculated for a given sample composition and thickness. The main broadening in the keV region, which is due to the atomic thermal motion, is calculated from the temperature. Sample self-shielding, single-, and doubleneutron scattering in the sample are also accounted for by this code. In addition, the broadening due to the neutron-beam resolution function has been included by means of the SAMMY-RPI parameterization. When a good transmission measurement existed for the neutron width, Γn, this parameter was kept fixed together with the rest of “a priori” information mentioned above and the value of Γ γ was derived by SAMMY using the Bayes’ theorem. In case the resonance has been only observed in the capture measurement, then only the value of the radiative kernel or capture area was derived usually by fitting both Γnand Γ γ . Due to the limited statistics available, we observed 21 resonances in the case of 209Bi in the energy region between 0.8 and 23 keV. In 207Pb the number of observed resonances was 19 in the region from 3 to 317 keV. Despite the backwards position of both C6D6detectors, the in-beam γ -rays were still the main source of background, hiding most of the weaker resonances. The situation should improve very much in the future with the eventual replacement of the water moderator of the spallation target by heavy water. In this case, the in-beam γ -rays will be highly suppressed and the associated background should become practically negligible. In Fig. 3, fit examples are given for the two isotopes presented in this work. (eV) n E 700 750 800 850 900 Yield 0.005 0.01 0.015 0.02 0.025 (keV) n E 37 37.5 38 38.5 Yield 0.002 0.003 0.004 0.005 FIGURE 3. SAMMY fit of the s-wave 209Bi resonance at 802 eV (left) and the 207Pb resonanceat 37 keV (right). RESULTS In the case of 209Bi, most of the resonances are found in good agreement with the previous measurement performed at Geel [9] (see Fig. 4). Nevertheless, the systematic deviation for s-wave resonances at lower energy indicates a small overcorrection of their neutron sensitivity. It is worth noting in this case the large discrepancies found with the evaluated data files, which are based on the first capture experiment of this isotope performed at Oak Ridge [10]. This shows larger capture areas, mainly for s-wave resonances, due to the higher neutron sensitivity of their experimental setup, which required corrections as large as  50%, with correspondingly large uncertainties. 1523 Resonance Energy (eV) 0 5000 10000 15000 20000 (meV)Γ/ n Γ γ Γg 0 10 20 30 40 50 60 nTOF-Data MuttiPhD’97 Macklin’76 ENDF-BVI JENDL3.3 Mughabghab’84 FIGURE 4. Comparison of the Bi radiative kernel measured at n_TOF versus previous experiments and evaluated files. In the case of 207Pb, previous measurements of the resonance parameters or capture areas have been compared with n_TOF [11, 12, 13, 14] (Fig. 5). The capture areas calculated from the evaluations JENDL and ENDF and those from the compilations [7, 8] have been also included. Beyond 100 keV, again large discrepancies are Resonance Energy (eV) 104105 (meV)Γ/ n Γ γ Γg 102 103 104 nTOF-Data Raman Allen’83 Allen’73 ENDF B-VI JENDL3.2 Mughabghab Sukhoruchkin FIGURE 5. Radiative kernels measured at n_TOF for 207Pb versus previous measurements and evaluated files. found with the evaluations. In the rest of the cases the agreement is satisfactory for almost every resonance. According to Figs. 4 and 5, larger discrepancies have been found in the first case (209Bi). Thus it is interesting to discuss its impact on the calculations for stellar nucleosynthesis and abundances. The relevant quantity for that calculation is the Maxwellian Averaged Cross Section (MACS). It determines the survival of 209Bi and the production of the unstable 210Bi that α -decays, contributing to the production of 206Pb.InFig.6,the209Bi- MACS obtained for the capture areas measured at n_TOF is compared with that obtained from the two previous experiments at Geel and Oak Ridge.The effect of these new results should be checked by including them in an appropriate stellar code. Stellar Temperature kT (keV) 0 1020304050 > (mb)σ < 25keV) < n MACS(E 0 2 4 6 8 10 12 14 16 This work Mutti’97 Macklin’76 This work Mutti’97 Macklin’76 This work Mutti’97 Macklin’76 FIGURE 6. Maxwellian-averaged cross section versus the stellar temperature calculated for neutron-energy resonances below 25 keV. Another open question comes from the calculation of the thermal-capture cross section as the sum of the tails of the s-wave resonances. Computing it for the latest experiments (Geel and n_TOF) one obtains a value of σ th  24 mb, which is about 40% lower than the adopted value of 33.8(7) mb [7]. Whether this might be due to subthreshold resonances or a direct-capture component remains to be investigated. CONCLUSIONS High-resolution measurements on 204  206  207  208Pb and 209Bi have been performed at the CERN n_TOF facility. The results on 207Pb and 209Bi reported in this work reflect the low-neutron sensitivity achieved thanks to the very low-mass detection setup and elements. A systematic accuracy better than 3% was achieved by exhaustive treatment of the different sources of systematic uncertainties. The two figures comparing the radiative kernels clearly suggest updating the information existing in the evaluated libraries. REFERENCES 1. The n_TOF Collaboration, CERN/SPSC 99-8, SPSC/P 310. 2. R. Plag et al., Nuc. Instrum. Methods A 496 (2003) 425-436. 3. U. Abbondanno et al., Nucl. Instrum. Methods A, 521 (2004) 454-467. 4. J.L. Tain et al., J. Nucl. Sci. Technol., Sup. 2 (2002), p. 689. 5. S. Agostinelli et al., Nucl. Instrum. Methods A, 506, (2003), p. 250- 303. 6. N.M. Larson, ORNL/TM-9179, Oak Ridge NL, 2000. 7. S.F.Mughabghab, Neutron Cross Sections Academic press, 1984. 8. S.I. Sukhoruchkin et al., Low En. Neutron Physics, Springer, 1998. 9. P. Mutti, PhD Thesis, University of Gent, 1997. 10. R. 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