Neutron capture cross sections for the Re/Os clock

Neutron Capture Cross Sections for the Re/Os Clock M. Mosconi8,A. Mengoni4,M.Heil 8,F.Käppeler 8,G.Aerts 7, R. Terlizzi13, U. Abbondanno14, H. Álvarez24, F. Alvarez-Velarde20, S. Andriamonje7,J.Andrzejewski 33, P. Assimakopoulos 9, L. Audouin 5G. Badurek1, P. Baumann6,F.Beˇcváˇr31, J. Benlliure24, E. Berthoumieux7, F. Calviño25, D. Cano-Ott20,R.Capote 23, A. Carrillo de Albornoz30, P. Cennini4, V. Chepel17,E.Chiaveri 4, N. Colonna13,G.Cortes 25, D. Cortina24, A. Couture29,J.Cox 29, S. David5,R.Dolfini 15, C. Domingo-Pardo21,W.Dridi 7,I.Duran 24, M. Embid-Segura20,L.Ferrant 5, A. Ferrari4, L. Fitzpatrick4, R. Ferreira-Marques17, H. Frais-Kölbl3,K.Fujii 13,W.Furman 18, C. Guerrero20 I. Goncalves30, R. Gallino36, E. Gonzalez-Romero20, A. Goverdovski19, F. Gramegna12,E.Griesmayer 3, F. Gunsing7,B.Haas 32, R. Haight27, A. Herrera-Martinez4,M.Igashira 37,S.Isaev 5,E.Jericha 1,Y.Kadi 4, 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, P.M. Milazzo14, C. Moreau14,F.Neves 17, H. Oberhummer1,S.O’Brien 29,M.Oshima 38, J. Pancin7, C. Papachristodoulou9, C. Papadopoulos40, C. Paradela24,N.Patronis 9,A.Pavlik 2, P. Pavlopoulos34, L. Perrot7, R. Plag8,A.Plompen 16, A. Plukis7,A.Poch 25,C.Pretel 25, J. Quesada23,T.Rauscher 26, R. Reifarth27, M. Rosetti11, C. Rubbia15, G. Rudolf6, P. Rullhusen16, J. Salgado30, L. Sarchiapone4, C. Stephan5, G. Tagliente13,J.L.Tain 21, L. Tassan-Got5,L.Tavora 30, G. Vannini35,P.Vaz 30,A.Ventura 11, D. Villamarin20,M.C.Vincente 20, V. Vlachoudis4,R.Vlastou 40 F. Voss8, H. Wendler4,M.Wiescher 29, K. Wisshak8 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. The radioactive decay of 187Re  187Os (t1  2= 43 Gyr) is suited for dating the onset of heavy-element nucleosynthesis. The radiogenic contribution to the 187Os abundance is the difference between the natural abundance and the corresponding s-process component. This component can be obtained via the well-established σ N systematics using the neighboring s-only isotope 186Os, provided the neutron-capture cross sections of both isotopes are known with sufficient accuracy.We report on a new setof experiments performed with a C6D6detector array at the n_TOF neutron spallation facility of CERN. The capture cross sections of 186Os, 187Os, and 188Os have been measured in the neutron-energy range between 1 eV and 1 MeV, and Maxwellian-averaged cross sections were deduced for the relevant thermal energies from kT=5 keV to 100 keV. 1335 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 Clayton proposed the Re/Os cosmochronometer forty years ago [1] a considerable effort has been devoted to determining with the required accuracy the basic nuclear physics quantities hampering its complete utilization. Not long ago, the temperature dependence of the half-life of 187Re was deduced by the measurement of the half-life of fully stripped 187Re performed at GSI [2]. This result provided thenecessary experimental information on what was considered the largest source of uncertainty of the clock [3]. The effect of the shorter 187Re half-life under stellar conditions has been analyzed by Takahashi [4] with the main conclusion being that the clock is still reliable. r,s r,s r,s r,s r,s r,sss s process WW Re Re Re Re Os Os 186 187 188 189 Os Os 185 186 187 188 186185184 W187W r process r process FIGURE 1. Canonical s-process (thick solid line) and rprocess paths (gray arrows) in the mass region around A  187. Branchings in the s-process path are denoted by dashed lines. Stable or long-lived nuclei are denotedby boldboxes,including their nucleosynthetic origin. The 187Re β -decay is marked as a dotted arrow. The effect of the s-process branchings (see Fig. 1) has been investigated [5] and the branching at 185W was analysed on the basis of a new evaluation of the 185W(n, γ ) cross section obtained by the measurement of the inverse reaction at S-DALINAC [6] resulting in a discrepancy of 20% between the calculated and observed abundance of 186Os. This result indicates the possibility of an error in the tabulated osmium cross sections. Measurements of neutron-capture cross sections of 186Os and 187Os have been performed in the past by Winters et al. [7] and Browne et al. [8] in limited energy ranges,1, which makes it difficult to calculate the Maxwellian-averaged cross sections at the energies predicted by the recent s-process models. In fact, a thermal energy of kT=8 keV is estimated for the 13C( α ,n)16Ore- action, which is considered to be the main neutron source for the s-process in thermally pulsing low-mass AGB stars. To evaluate the Maxwellian-averaged cross section 1The range of Browne measurements is from 1 keV to 148 keV. The Winters measurementcovered a range from 2.5 keV to 2.5 MeV. in this wider range of relevant thermal energies, the cross section between 0.1 keV and 500 keV has to be known. Moreover, the (n, γ ) cross section of 187Os measured in the laboratory is not sufficient to determine the stellar cross section because at the typical energies of the s-process a large fraction of the 187Os nuclei are excited to the first level at 9.75 keV above the ground state [9]. To allow theoretical calculations of the (n, γ ) cross section of the excited 187Os nucleus, additional indirect experimental data are required. The measurement of the inelastic scattering cross section of 187Os can help to evaluate the weight of the scattering channels of the compound nucleus. Measurements of this cross section were already performed at 34 keV [10] and 60 keV [11] but the uncertainties of these measurements are considered too large to allow a good theoretical determination of the Maxwellian-averaged cross section at stellar conditions [12]. A measurement of the inelastic-scattering cross section is planned at the 3.7-MV Van de Graaff accelerator of Forschungszentrum Karlsruhe. The measurement of neutron-capture cross sections performed at n_TOF and reported here is an attempt to improve the present uncertainties and to obtain consistent data over the entire neutron energy range for the isotopes of interest. EXPERIMENTAL METHOD The measurements of the 186  187  188Os capture cross sections have been performed at the n_TOF facility at CERN [13]. Neutrons are produced by spallation reactions of 20-GeV protons on a lead target. Protons are accelerated by the CERN Proton Synchrotron (PS) in bunches of up to 7  1012 particles. The typical repetition rate of the proton beam of the PS accelerator for n_TOF is 0.4 Hz. A flight path of 185 m is used to obtain good energy resolution via the time-of-flight (TOF) technique. A flux of  106neutrons per bunch is obtained in the experimental area. A set of collimators is used to reduce the neutron beam diameter to 4 cm at the sample position. In-beam photons are produced by the spallation reactions, leading to a prompt “ γ -flash” that is used to identify the starting time t0of each pulse. In addition,thermal neutroncapture on the materials surroundingthe lead target, and especially in the cooling water are responsible for an additional γ -ray production that takes place between 1 µ s and a few 100 µ safterthet 0and interferes with neutron-induced events in the experimental area in the energy range of astrophysical interest. This effect, which results in a background component for capture events, has to be carefully analysed, e.g., by means of neutron filters. The neutron filters are mounted 50 m up- 1336 stream of the experimental area and can be inserted into or removed from the beam at any time. The material and the thickness of each filter have been chosen in order to ensure that the respective strongest resonances are becoming “black.” Experimental Setup and Procedure Enriched 186  187  188Os samples have been provided by Oak Ridge National Laboratory in the form of metallic powder and in amounts of 2 g for each sample. The main contaminant in the 187Os sample (enriched to 70%) and in the 186Os sample (enriched to 80%) is 188Os. Therefore the 188Os cross section has been additionally measured to improve the isotopic correction2.Thesample material was sealed in cylindrical Al cans, 1.5 cm in diameter. A thin foil of 6Li, viewed by four silicon detectors located outside the beam served as a neutron monitor. The capture γ -ray detection system is based on two C6D6 scintillators,specifically designed for the n_TOF capture setup. The two detectors were mounted perpendicular to the neutron beam line and 9.8 cm upstream with respect to the sample position in order to reduce the effect of inbeam γ -ray scattering. The three osmium samples were mounted on a sample changer together with an empty canning for measuring the background, a lead sample for evaluating the effect the in-beam γ -rays, and a carbon sample to evaluate the effect of scattered neutrons. To determine carefully the neutron flux intercepted by the samples, a gold sample with the same diameter has been used. Additional measurements with neutron filters, 3- cm-thick aluminum and 1-mm-thick tungsten, have been performed. DATA ANALYSIS AND RESULTS The response of the scintillators has been analysed by means of the Pulse-height weighting technique [15] to make the detector efficiency proportional to the γ -ray energy, and hence independent on the multiplicity of the different capture cascades. The weighting functions were determined via Monte Carlo simulations,and were calculated independentlyby CEA/Saclay and INFN Bari using two different simulation tools. The count rate measured with the osmium samples has been corrected for “ambient” backgrounds by subtracting the spectra measured with the empty can. The neutron-scattering background 2All the stable osmium isotopes were measured by Browne from 1 keV to 148 keV [8]; only theoretical data are available at other energies [14]. obtained with a measurement of the carbon sample has been found to be negligible. The main source of background is the scattering of in-beam γ -rays. 10 -1 1 10 104105 Winters n_TOF04 N E U TR ON E N ER G Y (eV) CROSS SECTION (b) Browne PRELIMINARY 187 Os FIGURE 2. Preliminary 187Os cross section between 1 keV and 1 MeV. The uncertainties on the n_TOF data are of the order of the point size or slightly smaller. It has been evaluated considering that its shape at different neutron energies is given by the count rate measured with the lead sample. However, this rate had to be rescaled according to the different scattering properties of the osmium samples. The scaling factor was determined in simulations using GEANT 3.21, based on the photon-energy spectrum obtained by extensive simulations of the n_TOF facility [13]. 10 -1 1 104105 Winters n_TOF04 N E U TR ON E N ER G Y (eV) CROSS SECTION (b) Browne PRELIMINARY 186 Os FIGURE 3. Preliminary 186Os cross section between 1 keV and 1 MeV. The results of the simulations were validated via spectra measured with the filters in beam. By rescaling the lead spectrum measured with the filters by a factor estimated with the simulations, the measured level of the black Al and W resonances could be perfectly matched. In this way it was also verified that the shape of the gamma background was the same for lead, osmium, and gold. This test yields good agreement for all the samples in the entire TOF region where this background component is significant. Assuming that the ratio between the in-beam γ background and ambient background is similar for osmium and lead, this comparison was applied at lower neutron energies as well. Also in this region 1337 good agreement was obtained for all samples but 188Os. For this isotope, the background at low neutron energies seems to be slightly overestimated and the explanation of this result requires further investigation. 10 -1 1 104105106 Winters n_TOF04 N E U TR ON E N ER G Y (eV) CROSS SECTION (b) Browne PRELIMINARY 188 Os FIGURE 4. Preliminary 188Os cross section between 1 keV and 1 MeV. The neutron flux has been evaluated from the gold spectrum after subtraction of the different background components. In this analysis we have used the gold cross section of Macklin et al. [16] normalised to the more accurate measurement of Ratynski et al. [17]. The preliminary results for the capture cross sections of 186  187  188Os are shown in Figs. 2, 3, and 4 in comparison with the previous data. For 186  187Os theMaxwellianaveraged cross sections at 30 keV calculated with these preliminary data are in good agreement with the results of [7], but there seems to be a significant difference in the case of 188Os. CONCLUSIONS AND OUTLOOK The preliminary results of neutron-capture cross sections of 186  187  188Os measured at n_TOF exhibit good agreement with the previous results for 187Os, whereas 186Os and 188Os are showing a somewhat different energy dependence. This will be verified by a refined background analysis, in particular in the case of 188Os. In addition, the flux determination versus the Au standard will be extended toward lower energies by using the 4.9-eV resonance. The analysis will be extended to the resonance region, where resonance parameters will be extracted using the SAMMY code. ACKNOWLEDGMENTS This work is supported by National Institutions of the related n_TOF participants and partially by EC under contract FIKW-CT-2000-00107. REFERENCES 1. Clayton, D. 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