Measurement of capture cross sections of 90,91,92,94,96Zr isotopes at n_TOF

Measurement of Capture Cross Sections of 90,91,92,94,96Zr Isotopes at n_TOF C. Moreau, K. Fujii, G. Tagliente, 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. Carillo de Albornoz, P. Cennini, V. Chepel, E. Chiaveri, N. Colonna, G. Cortes, D. Cortina, A. Couture, J. Cox, S. David, R. Dolfini, C. Domingo‐Pardo, W. Dridi, I. Duran, M. Embid‐Segura, L. Ferrant, A. Ferrari, L. Fitzpatrick, H. Frais‐Koelbl, W. Furman, R. Gallino, I. Goncalves, E. Gonzalez‐Romero, A. Goverdovski, F. Gramegna, E. Griesmayer, C. Guerrero, 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, M. Mosconi, F. Neves, H. Oberhummer, S. O’Brien, M. Oshima, J. Pancin, C. Papachristodoulou, C. Papadopoulos, T. Papaevangelou, 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, J. L. Tain, 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, and K. Wisshak Citation: AIP Conference Proceedings 769, 880 (2005); doi: 10.1063/1.1945146 View online: http://dx.doi.org/10.1063/1.1945146 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/769?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Measurement of 139La(n,γ) Cross Section at n_TOF AIP Conf. 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Downloaded to IP: 193.136.215.14 On: Wed, 02 Sep 2015 12:45:10 Measurement of Capture Cross Sections of 90,91,92,94,96Zr Isotopes at n_TOF C.Moreau14∗, K.Fujii14, G.Tagliente13, U.Abbondanno14, G.Aerts7, H.Álvarez24, F.Alvarez-Velarde20, S.Andriamonje7, J.Andrzejewski33, P.Assimakopoulos9, L.Audouin5, G.Badurek1, P.Baumann6, F.Bečvář 31, J.Benlliure24, E.Berthoumieux7, F.Calviño25, D.Cano-Ott20, R.Capote23, A.Carillo de Albornoz30, P.Cennini4, V.Chepel17, E.Chiaveri4, N.Colonna13, G.Cortes25, D.Cortina24, A.Couture29, J.Cox29, S.David5, R.Dolfini15, C.Domingo-Pardo21, W.Dridi7, I.Duran24, M.Embid-Segura20, L.Ferrant5, A.Ferrari4, L.Fitzpatrick4, H.Frais-Koelbl3, W.Furman18, R.Gallino36, I.Goncalves30, E.Gonzalez-Romero20, A.Goverdovski19, F.Gramegna12, E.Griesmayer3, C.Guerrero20, F.Gunsing7, B.Haas32, R.Haight27, M.Heil8, A.Herrera-Martinez4, M.Igashira37, S.Isaev5, E.Jericha1, Y.Kadi4, F.Käppeler8, D.Karamanis9, D.Karadimos9, M.Kerveno6, V.Ketlerov19, P.Koehler28, V.Konovalov18, E.Kossionides39, M.Krtička31, C.Lamboudis10, H.Leeb1, A.Lindote17, I.Lopes17, M.Lozano23, S.Lukic6, J.Marganiec33, L.Marques30, S.Marrone13, P.Mastinu12, A.Mengoni4, P.M.Milazzo14, M.Mosconi8, F.Neves17, H.Oberhummer1, S.O'Brien29, M.Oshima38, J.Pancin7, C.Papachristodoulou9, C.Papadopoulos40, T. Papaevangelou4, C.Paradela24, N.Patronis9, A.Pavlik2, P.Pavlopoulos34, L.Perrot7, R.Plag8, A.Plompen16, A.Plukis7, A.Poch25, C.Pretel25, J.Quesada23, T.Rauscher26, R.Reifarth27, M.Rosetti11, C.Rubbia15, G.Rudolf6, P.Rullhusen16, J.Salgado30, L.Sarchiapone4, C.Stephan5, J.L.Tain21, L.Tassan-Got5, L.Tavora30, R.Terlizzi13, G.Vannini35, P.Vaz30, A.Ventura11, D.Villamarin20, M.C.Vincente20, V.Vlachoudis4, R.Vlastou40, F.Voss8, H.Wendler4, M.Wiescher29, and K.Wisshak8 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. The neutron capture cross sections for Zr isotopes have important implications in nuclear technologies as well in the field of nuclear astrophysics. The innovative features of the n_TOF CERN facility allowed for an accurate determination of the capture cross sections for these isotopes. The preliminary results of the measurements show capture-resonance strengths systematically smaller than in previously available data. ∗ Corresponding author: [email protected] 880 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 193.136.215.14 On: Wed, 02 Sep 2015 12:45:10 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 Since the baryonic freeze-out in the Big Bang, stars have been identified as the sites for nucleosynthesis. Beyond Iron, the only method of production of isotopes is via neutron capture, through two dominant processes, which differ by the time scale on which they occur: the r (rapid) process, related to extremely hot (T>109 K), neutron rich (nn >>1020 cm-3) environments; the s (slow) process, which operates at lower temperatures (T≈108 K) and densities (nn≈108 cm-3) [1]. Realistic stellar models have been developed, which can account for the actual stellar conditions under which the s process is taking place [2]. However, the reliability of the stellar s process models can only be tested by new developments in neutron-capture cross-section measurements, with uncertainties of only a few %. This improved accuracy is a pre-requisite for the application in Nuclear Astrophysics. Moreover, recent developments have emphasized the importance of neutron-capture nucleosynthesis for probing the deep interior of Red Giant stars and for following the enrichment of heavy elements during galactic evolution. These data, required in the energy range from 0.3 to 300 keV, are largely missing, particularly in the mass region A≤100, where cross sections are small and dominated by single resonances [3]. The Zr isotopes represent an important example to illustrate this situation. 90Zr, 91Zr, 92Zr, and 94Zr are predominantly of s-process origin. However, the small cross section of the neutron magic 90Zr isotope acts as a bottleneck for the reaction flow towards heavier elements. Moreover, even if the heaviest isotope, 96Zr, is traditionally considered to be an r-only isotope, a non-negligible s-process admixture must be considered in their formation [4]. Apart from the impact on problems of Nuclear Astrophysics, the (n, γ) cross sections of the Zr isotopes are of interest for technological reasons also. Zr constitutes, in fact, an important component in alloys used as structural material and in fuel assemblies in nuclear reactors. In particular the NEA “High Priority Nuclear Data Request List” asks for a fine (n, γ) cross section determination from thermal to 1-MeV neutron energy [5]. Existing data in the range between 0.1 and 500 keV exhibit uncertainties larger than 10% and discrepancies up to a factor 2 between different measurements. EXPERIMENTAL SET-UP The measurements were performed over the energy range between 1 eV to 1 MeV at the spallation neutron facility n_TOF at CERN [6]. The low repetition frequency of the proton beam driver, the extremely high instantaneous neutron flux, the low background conditions in the experimental area, together with the low neutron sensitivity of the capture detector make this facility unique for neutron-induced reaction crosssection measurements. The neutrons are generated by spallation reactions induced by a pulsed beam of 20-GeV protons on a massive Pb target. Neutrons are slowed down in the lead and moderated in a surrounding 5-cm-thick layer of cooling water. An evacuated neutron flight path with collimators leads to the measuring station at a distance of 185 m from the spallation target. Background due to fast charged particles is suppressed by means of a sweeping magnet, heavy concrete walls, and a 3.5-m-thick iron shielding [7]. The experimental set-up consists of two C6D6 (deuterated benzene) detectors with minimized neutron sensitivity [8], placed perpendicular to the neutron beam at a distance of about 3 cm from the beam axis. The background due to in-beam γ-rays was reduced by placing the detectors 9.2 cm upstream of the sample position. The light output of the detectors was calibrated by means of 137Cs, 60Co, and Pu/C γ-ray sources. The calibrated neutron time of flight was used to determine the neutron energy. The detector signals were recorded with fast flash ADC using the standard n_TOF data acquisition system [9]. All zirconium samples were prepared from oxide powder, which was pressed to pellets 22 mm in diameter and encapsulated in a thin aluminum can. Additional Au and Pb samples of the same diameter were used for flux and background measurements. The relative neutron flux was measured 1.5 m upstream of the capture samples with a low mass flux monitor consisting of a thin layer of 6Li surrounded by four silicon detectors outside the neutron beam [10]. DATA ANALYSIS The quantity determined in a neutron capture experiment is the capture yield, i.e., the fraction of incident neutrons undergoing (n, γ) interaction on a defined sample. The capture yield is directly linked to the capture and total cross sections. The data analysis was then based on an accurate measurement of the capture yield. 881 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 193.136.215.14 On: Wed, 02 Sep 2015 12:45:10 One has to consider that due to the small solid angle coverage and the low intrinsic efficiency of the C6D6 detectors, which results in an overall efficiency of ≈10%, only one γ-ray per event is detected from the de-excitation cascade following neutron capture. For an accurate cross-section determination, the efficiency of the set-up has to be made independently on the detail of the de-excitation cascade, in particular of the γ-ray multiplicity. To this end, a pulse height weighting function (PHWF) was used [11]. It consists of suitably modifying via software the detector response so that the efficiency εγ is proportional to the photon energy Eγ. Under these conditions the efficiency for detecting a cascade becomes proportional to the known cascade energy Ec and independent of the cascade path. The proportionality of the efficiency with the γ-ray energy is achieved by modifying the detector energy response distribution R(E) with a pulse-height (deposited energy) dependent weighting function (W(E)), applied to the recorded spectrum. A check on the reliability of the weighting function technique has been performed by measuring isotopes with well-known capture cross sections (natFe, natAg, 197Au) [12]. As a further step, ambient and sample related backgrounds were subtracted by means of the spectra measured with an empty Al can (the Zr samples were lodged inside this support) and with the Pb sample. For the five measured isotopes, only the 90, 92, and 96 are completely analyzed; 94 is well in progress and 91 has still to be analyzed. Figure 1 shows the capture yield for 92Zr with the overall background. FIGURE 1. Capture yield of the 92Zr(n, γ)93Zr reaction (black) and of the overall background (gray). The absolute normalization of the capture yields has been made via the spectrum measured with the Au sample. In particular, the normalization factor was determined with accuracy better than 1.5% by analyzing the saturated standard 4.9-eV resonance. Once extracted, the experimental capture yields and the analysis of the resonances were done by means of the multi-level analysis code SAMMY [13]. The Energy E, the Γγ and Γn widths are extracted from the fit on the n_TOF data given by SAMMY. The neutron capture strengths gΓγΓn / ΓTot , where g is the spin statistical factor and ΓTot the total width are therefore calculated [14]. Figure 2 shows the comparison between the neutron capture strengths for the 90Zr, 92Zr, and 96Zr (n, γ) measurements with those determined in previous experiments [14]. FIGURE 2. Ratio between the n_TOF capture measured strengths and previously observed values [14]. On this plot, the errors are proceeding from three independent sources: flux determination (1.5%), pulse height weighting function (2%), and SAMMY fit, which varies with the resonance and increases with the energy (5-10%). Except for a very few resonances, the n_TOF results are systematically lower, for all the measured isotopes. Figure 3 shows the distribution of the ratio between the n_TOF and the available previously measured data concerning the neutron capture strengths. 882 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 193.136.215.14 On: Wed, 02 Sep 2015 12:45:10 FIGURE 3. Distribution of the ratio between the n_TOF capture measured strengths and previous observed values, for different Zr isotopes shown in Fig. 2 [14]. CONCLUSIONS The capture cross sections of 90,91,92,94,96Zr isotopes have been measured at the CERN n_TOF facility in the energy range from 1 eV to 1 MeV. The innovative features of this facility allow performance of highaccuracy measurements. The obtained results concerning the capture strengths are systematically lower than those presented in previous experiment; this fact could be related to the lower neutron sensitivity of the experimental set-up used at n_TOF. REFERENCES 1. Wallerstein G. et al., Rev. Mod. Phys. 69, 995 (1997). 2. Käppeler F., Prog. Nucl. Part. Phys. 43, 419 (1999). 3. Bao Z. et al., Atomic Data Nucl. Data Tables 76, 70 (2000). 4. Toukan K.A., Debus K., Käppeler F. and Reffo G., Phys. Rev. C51, 1540 (1995). 5. The NEA High Priority Nuclear Data Request List, OECD-NEA, Nuclear Science Committee (www.nea.fr/html/dbdata/hprl/hprlmain.cgi). 6. Borcea C. et al., Nucl. Inst. Methods A513, 523 (2003). 7. Abbondanno U. et al., Report CERN-SL-2002-053 ECT, CERN, (2003). 8. Plag R. et al., Nucl. Instrum. Methods A496, 425 (2003). 9. Abbondanno U. et al., to be published on Nucl. Instrum. Methods A. 10. Marrone S. et al., Nucl. Instrum. Methods A517, 389 (2004). 11. Corvi F., Fioni G., Gasperini F., Smith P.B., Nucl. Sci. Eng. 107, 272 (1991). 12. Abbondanno U. et al., Nucl. Instrum. Methods A521, 454 (2004). 13. Larson N., report ORNL/TM-9179/R5, Oak Ridge National Laboratory (2000). 14. For access to neutron evaluated and experimental data, visit the website www.nndc.bnl.gov/index.jsp. 883 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 193.136.215.14 On: Wed, 02 Sep 2015 12:45:10