Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis

Neutron Capture Cross Section of Unstable 63Ni: Implications for Stellar Nucleosynthesis C. Lederer, 1,2 C. Massimi, 3 S. Altstadt, 2 J. Andrzejewski, 4 L. Audouin, 5 M. Barbagallo, 6 V. Be ´cares, 7 F. Bec ˇva ´r ˇ, 8 F. Belloni, 9 E. Berthoumieux, 9,10 J. Billowes, 11 V. Boccone, 10 D. Bosnar, 12 M. Brugger, 10 M. Calviani, 10 F. Calvin ˜o, 13 D. Cano-Ott, 7 C. Carrapic¸o, 14 F. Cerutti, 10 E. Chiaveri, 9,10 M. Chin, 10 N. Colonna, 6 G. Corte ´s, 13 M. A. Corte ´s-Giraldo, 15 M. Diakaki, 16 C. Domingo-Pardo, 17 I. Duran, 18 R. Dressler, 19 N. Dzysiuk, 20 C. Eleftheriadis, 21 A. Ferrari, 10 K. Fraval, 9 S. Ganesan, 22 A. R. Garcı ´a, 7 G. Giubrone, 17 M. B. Go ´mez-Hornillos, 13 I. F. Gonc¸alves, 14 E. Gonza ´lez-Romero, 7 E. Griesmayer, 23 C. Guerrero, 10 F. Gunsing, 9 P. Gurusamy, 22 D. G. Jenkins, 24 E. Jericha, 23 Y. Kadi, 10 F. Ka ¨ppeler, 25 D. Karadimos, 16 N. Kivel, 19 P. Koehler, 26 M. Kokkoris, 16 G. Korschinek, 27 M. Krtic ˇka, 8 J. Kroll, 8 C. Langer, 2 H. Leeb, 23 L. S. Leong, 5 R. Losito, 10 A. Manousos, 21 J. Marganiec, 4 T. Martı ´nez, 7 P. F. Mastinu, 20 M. Mastromarco, 6 M. Meaze, 6 E. Mendoza, 7 A. Mengoni, 28 P. M. Milazzo, 29 F. Mingrone, 3 M. Mirea, 30 W. Mondelaers, 31 C. Paradela, 18 A. Pavlik, 1 J. Perkowski, 4 M. Pignatari, 32 A. Plompen, 31 J. Praena, 15 J. M. Quesada, 15 T. Rauscher, 32,33 R. Reifarth, 2 A. Riego, 13 F. Roman, 10,30 C. Rubbia, 10,34 R. Sarmento, 14 P. Schillebeeckx, 31 S. Schmidt, 2 D. Schumann, 19 G. Tagliente, 6 J. L. Tain, 17 D. Tarrı ´o, 18 L. Tassan-Got, 5 A. Tsinganis, 10,16 S. Valenta, 8 G. Vannini, 3 V. Variale, 6 P. Vaz, 14 A. Ventura, 28 R. Versaci, 10 M. J. Vermeulen, 24 V. Vlachoudis, 10 R. Vlastou, 16 A. Wallner, 1,35 T. Ware, 11 M. Weigand, 2 C. Weiß, 23 T. J. Wright, 11 and P. Z ˇugec 12 (n_TOF Collaboration) 1 University of Vienna, Faculty of Physics, 1090 Vienna, Austria 2 Johann-Wolfgang-Goethe Universita ¨t, 60438 Frankfurt, Germany 3 Dipartimento di Fisica, Universita `di Bologna, and Sezione INFN di Bologna, 40100 Bologna, Italy 4 Uniwersytet Ło ´dzki, 90131 Lodz, Poland 5 Centre National de la Recherche Scientifique/IN2P3-IPN, 91406 Orsay, France 6 Istituto Nazionale di Fisica Nucleare, 70125 Bari, Italy 7 Centro de Investigaciones Energeticas Medioambientales y Tecnolo ´gicas (CIEMAT), 28040 Madrid, Spain 8 Charles University, CZ-180 00 Prague, Czech Republic 9 Commissariat a `l’E ´nergie Atomique (CEA) Saclay-Irfu, 91191 Gif-sur-Yvette, France 10 European Organization for Nuclear Research (CERN), CH-1211 Geneva, Switzerland 11 University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom 12 Department of Physics, Faculty of Science, University of Zagreb, 10002 Zagreb, Croatia 13 Universitat Politecnica de Catalunya, 08028 Barcelona, Spain 14 IST/ITN, Instituto Superior Te ´cnico, Universidade Te ´cnica Lisboa, Estrada Nacional 10, 2695-066 Bobadela, Portugal 15 Universidad de Sevilla, 41080 Sevilla, Spain 16 National Technical University of Athens (NTUA), 15780 Athens, Greece 17 Instituto de Fı ´sica Corpuscular, CSIC-Universidad de Valencia, 46071 Valencia, Spain 18 Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain 19 Paul Scherrer Institut, 5232 Villigen PSI, Switzerland 20 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, 35020 Legnaro, Italy 21 Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 22 Bhabha Atomic Research Centre (BARC), Trombay, 400085 Mumbai, India 23 Atominstitut, Technische Universita ¨t Wien, 1040 Vienna, Austria 24 University of York, Heslington, York YO10 5DD, United Kingdom 25 Karlsruhe Institute of Technology, Campus Nord, Institut fu ¨r Kernphysik, 76021 Karlsruhe, Germany 26 Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37831, USA 27 Technical University of Munich, 80333 Munich, Germany 28 Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (ENEA), 40129 Bologna, Italy 29 Istituto Nazionale di Fisica Nucleare, 34127 Trieste, Italy 30 Horia Hulubei National Institute of Physics and Nuclear Engineering, P.O. Box MG-6, 077125 Bucharest Magurele, Romania 31 European Commission JRC, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium 32 Department of Physics-University of Basel, 4056 Basel, Switzerland 33 Institute of Nuclear Research (ATOMKI), H-4001 Debrecen, POB 51, Hungary 34 Laboratori Nazionali del Gran Sasso dell’INFN, 67010 Assergi (AQ), Italy 35 Research School of Physics and Engineering, Australian National University, ACT 0200, Australia (Received 8 October 2012; published 8 January 2013) The 63Niðn; Þcross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) PRL 110, 022501 (2013) PHYSICAL REVIEW LETTERS week ending 11 JANUARY 2013 0031-9007=13=110(2)=022501(5) 022501-1 Ó2013 American Physical Society resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from kT ¼5–100 keV with uncertainties around 20%. Stellar model calculations for a 25Mstar show that the new data have a significant effect on the s-process production of 63Cu,64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova. DOI: 10.1103/PhysRevLett.110.022501 PACS numbers: 25.40.Lw, 25.40.Ny, 26.20.Kn, 27.50.+e The weak component of the astrophysical sprocess observed in the solar system abundance distribution includes the s-process species between Fe and Sr (60 < A<90)[1]. Most of them are generated in massive stars during convective He core burning and convective C shell burning via the activation of the neutron source reaction 22Neð; nÞ25 Mg [2–6]. The long-lived radioisotope 63Ni (t1=2¼101:21:5yr[7]) is located along the neutron capture path, and in typical weak s-process conditions it may become a branching point, when the neutron capture time scale is comparable with its stellar -decay rate. In particular, at the end of He core burning the neutron source 22Neð; nÞ25Mg is activated at temperatures around 0.3 GK (GK ¼109K), corresponding to a Maxwellian neutron energy distribution for a thermal energy of kT ¼ 26 keV. At this stage, neutron densities are too weak for a subsequent neutron capture on 63Ni (with central peak neutron density on the order of 107cm3,e.g.,Refs.[5,6]) and more than 90% of the 63Ni produced decays to 63Cu. However, the s-process material is partly reprocessed during C shell burning, where the 22Neð; nÞ25Mg neutron source is reactivated at much higher temperatures of about 1 GK, corresponding to a thermal energy of kT ¼90 keV. During this second stage, neutron densities are orders of magnitude higher, reaching a maximum of 1011–12 cm3 [8]. At the 63Ni branching point, the high neutron densities favor neutron capture producing 64Ni, bypassing the production of 63Cu despite the strong temperature dependence of the 63Ni -decay rate (at C shell burning temperatures the half-life of 63Ni decreases to a few years [9]). In these conditions, the amount of 63Cu generated during He core burning is partially depleted in the C shell, but the final 63Cu abundance will increase thanks to the later radiogenic decay of the 63Ni accumulated in the C shell burning phase [8]. In Fig. 1, we show the neutron capture path in the Ni-Cu-Zn region during He core and at high neutron density during C shell burning. Up to now the stellar cross section of 63Niðn; Þ64Ni relied on calculations or extrapolations of experimental values at thermal neutron energies (0.025 eV) [10–12]. Theoretical predictions for the Maxwellian averaged cross section (MACS) at kT ¼ 30 keV are ranging from 24 to 54 mb [13–17]. The currently recommended value quoted by the compilation KADoNiS [17]is31 6mb. Because such calculations are vulnerable to large systematic uncertainties, measurements have been attempted at Los Alamos National Laboratory [18] and at CERN. In this Letter we report on the first experimental results for the 63Ni cross section at stellar energies obtained at the n_TOF facility at CERN. The measurement was performed at the neutron time-of- flight facility n_TOF located at CERN. Neutrons are produced via spallation reactions of 20 GeV=c protons from the Proton Synchrotron with a massive Pb target. With the high intensity of the pulsed proton beam, the repetition rate of 0.4 Hz, a short proton pulse width of 6 ns, and a neutron flight path of 185 m, the n_TOF facility is unique for the combination of a high neutron energy resolution and a high instantaneous neutron flux. A more detailed description of the facility can be found in Ref. [19] and references therein. The 63Ni sample was produced by irradiating highly enriched 62Ni in a thermal reactor [12,20,21]. Since the irradiation took place more than 20 years ago, the original 63Ni fraction had partially decayed to 63Cu. To avoid background due to 63Cuðn; Þ64Cu reactions, this 63Cu impurity has been chemically separated prior to the (n,) measurement. The originally metallic target was dissolved in concentrated nitric acid and the copper fraction was precipitated as CuS using gaseous H2S. The remaining solution was treated with NaOH to precipitate NiðOHÞ2, which was calcinated at 800Cto form NiO. By means of mass spectrometry, the 63Ni=62Ni ratio in the sample was determined to 0:123 0:001, and the contribution from other Ni isotopes was found to be 1%. In total, 1156 mg NiO powder were encapsulated in a thin-walled cylinder made of PEEK (polyether ether ketone, net mass 180 mg) to produce a sample 20 mm in diameter and 2.2 mm in thickness. The neutron capture yield was measured as a function of neutron energy by detecting the prompt capture rays with a pair of liquid C6D6scintillation detectors. These detectors are optimized to exhibit a very low sensitivity to neutrons, thus minimizing the background produced by neutrons scattered on the sample [22]. The dependence of the detection efficiency on -ray energy and the effect of the -ray FIG. 1. The s-process reaction path in the Ni-Cu-Zn region during He core burning (dashed lines) and C shell burning (solid lines). PRL 110, 022501 (2013) PHYSICAL REVIEW LETTERS week ending 11 JANUARY 2013 022501-2 threshold of 250 keV were corrected using the pulse height weighting technique [23,24]. By application of a pulseheight dependent weight on the deposited energy, the detection efficiency becomes a linear function of the excitation energy of the compound nucleus, "kEc. Choosing k¼1MeV 1, the capture yield can be obtained as Y¼NCw Ec ;(1) where Cware the weighted, background-subtracted counts, denotes the relative neutron flux, and Nis a normalization factor for the absolute capture yield. The normalization factor was determined via the saturated resonance technique [25] in an additional run with a Au sample of the same size as the Ni sample. The Au sample was chosen such that the gold resonance at 4.9 eV is saturated, which means that all neutrons of that energy are absorbed in the sample, thus providing a measure for the absolute neutron flux at 4.9 eV. The energy dependence of the neutron flux was measured relative to the standard cross sections 10Bðn; Þand 6Liðn; Þup to 150 keV and 235Uðn; fÞat higher energies. Because the size of the neutron beam changes slightly with neutron energy, the normalization factor N, related to the fraction of the neutron beam intercepted by the sample, changes as well. This effect was taken into account by simulations of the neutron beam profile [19]. The experimental background was determined in dedicated runs with an empty PEEK container, with a 62Ni sample of the same diameter, and in runs without neutron beam. Additionally, the neutron capture yield has been measured with a set of neutron filters located about 50 m upstream of the sample. These W, Mo, and Al filters are thick enough to exhibit black resonances at certain energies, so that all neutrons in these windows are completely removed from the beam and do not reach the sample. Accordingly, the level in the corresponding dips in these spectra is expected to represent the experimental background. When the run with filters was repeated using only the empty container, the same background level was observed in the filter dips, thus confirming the background measured with the empty sample container. Figure 2shows a comparison of the capture yield of the 63Ni sample, the empty PEEK container, and the 62Ni sample. The background from the radioactivity of the 63Ni sample and from ambient radiation, which was obtained from a measurement without neutron beam, is given as well. Between 100 eV and 2 keV four resonances are visible in the spectrum of the 63Ni sample, which are obviously not correlated with the 62Ni content. The first of these resonances (marked by an arrow) can be attributed to a resonance in the 59Niðn; Þreaction [26], expected at that neutron energy and compatible with the measured 0.03% impurity of 59Ni in our sample. The other three resonances are clearly attributable to the 63Niðn; Þchannel. This holds also for several other resonances up to neutron energies of 55 keV, for which the capture kernels A¼1 222Zþ1 1 ðEÞdE ¼gs n nþ ;(2) characterizing the strength of the resonance, could be deduced by a resonance shape analysis with the R-matrix code SAMMY [27] (Table I). The capture kernel is determined by the spin statistical factor gs, the neutron width n, and the radiative width . For two resonances the orbital angular momentum ‘, derived from the shape of the resonance, is also given. The neutron energy interval between 2 and 8 keV is dominated by the strong resonance in 62Niðn; Þat 4.6 keV; therefore, smaller resonances in 63Niðn; Þmight be invisible due to this background. In summary, 12 levels in 64Ni were observed for the first time. As a consequence of the small sample mass, the signal- to-background ratio starts to deteriorate already above 10 keV. Accordingly, it is increasingly difficult to identify resonances with confidence at higher energies. Thus, MACSs were calculated using resonance parameters only below 10 keV, whereas averaged cross section data have been determined from 10 keV to 200 keV. These data were Neutron Energy (eV) -1 10 110 2 10 3 10 4 10 5 10 Capture Yield -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 Ni sample 63 Ni sample 62 PEEK container Beam Off FIG. 2 (color online). Capture yield of the 63Ni sample (black) compared to the empty PEEK container and to the spectrum obtained with a pure 62Ni sample. The background from the activity of the sample and from ambient radiation is almost negligible at keV energies. The capture yield of the 62Ni sample was scaled for the 62Ni mass present in the 63Ni sample. The first resonance at 203 eV (marked by an arrow) is assigned to a small impurity of 59Ni in the sample. TABLE I. Resonance energies Er(laboratory energy) and capture kernels Afor the 63Niðn; Þreaction. For resonances marked with an asterisk the orbital angular momemtum ‘¼0 could be deduced from the resonance shape. Er(eV) A(meV) Er(eV) A(meV) 397:96 0:04 5:70:4 9776 3 100 10 587:25 0:09340 20 13984 3 131 45 1366 1810 40 17127 4 108 59 8634 2459 19561 6 130 20 8981 35010 32330 10 500 200 9154 4439 54750 30 700 200 PRL 110, 022501 (2013) PHYSICAL REVIEW LETTERS week ending 11 JANUARY 2013 022501-3 obtained by subtraction of the yield measured with the 62Ni sample after it had been properly scaled for the 62Ni content of the 63Ni sample. The background due to oxygen is negligibly small because of its very small (n,) cross section. The MACSs for thermal energies from kT ¼ 5–100 keV are listed in Table II, together with the theoretical predictions in the KADoNiS compilation [17]. Our results are approximately a factor of 2 higher than the calculated cross section. The total systematic uncertainties in our results of 20% are mainly due to subtraction of the background and the effect of sample impurities— particularly in the region between 2 and 8 keV, where the spectrum is dominated by the 4.6 keV resonance in 62Niðn; Þ. Comparably minor contributions to the systematic uncertainty are caused by the neutron flux (3%–5%), the pulse height weighting technique (2%), the flux normalization (1%), and the 63Ni=62Ni ratio (1.6%). The impact of our new results was investigated for the sprocess in a full stellar model for a 25Mstar with an initial metal content Z¼0:02. The complete nucleosynthesis was followed with the postprocessing NuGrid code MPPNP [28]. The stellar rates were obtained by combining the measured 63gsNiðn; Þ64Ni cross sections and theoretically predicted contributions to the stellar rate due to 63Niðn; Þ64Ni reactions as described in Ref. [29]. While the contribution of 63gsNiðn; Þ64Ni reactions to the stellar rate is still around 90% at He core burning temperatures, it drops to around 40% at the higher temperature in the C shell burning phase. Because of the larger uncertainties of the 63Niðn; Þ64Ni cross sections, the uncertainty of the stellar rate increases with temperature. Apart from the reaction cross sections, the final abundance pattern is also affected by the temperature dependence of the radioactive decay rates under stellar conditions. In the investigated mass region the concerned rates for the  decay of 63Ni and the þ=decays of 64Cu have been adopted from Ref. [9]. By variation within reasonable limits [8], it was found that the decay rates of both isotopes have a comparably small effect on the investigated abundances, because the reaction flow in the 63Ni branching is governed by the neutron density conditions, which lead either to much lower or much higher (n,) rates during core He and C shell burning, respectively. The calculated abundance distribution from Fe to Zr shown in Fig. 3represents the sabundances after core He and C shell burning, i.e., prior to the supernova explosion, at a point where the nucleosynthesis yields are well characterized by the model [8]. The distribution is compared in Fig. 3to the one obtained with the neutron capture rates of the KADoNiS evaluation [17]. The ratio of the two distributions in the lower panel of Fig. 3shows that the new 63Ni cross section affects only a few isotopes between Ni and Zn. An enhancement of about 20% is found for 64Ni, while 63Cu is depleted by about 15%. As the 65Cu yields remain essentially unchanged, the isotopic ratio 63Cu=65Cu is correspondingly reduced at the end of C shell burning. 64Zn is depleted as well (by about 30%), because 63Cu and 64Zn are populated by the nucleosynthesis channel following the branch 62Niðn;Þ63NiðÞ63Cuðn;Þ64CuðÞ64Zn. However, the s-process contribution to 64Zn remains TABLE II. Maxwellian averaged cross sections (in mb) of 63Niðn; Þcompared to previously recommended values based on theoretical predictions [17]. The respective contributions from resonances below 10 keV (RC) are listed separately. Uncertainties are 1. kT KADoNiS This work (keV) RC Total 5 112 174 6 224 8stat 45syst 10 66 51 2 129:57:125:9 15 50 24 1 101:36:920:3 20 41 14 185:56:417:1 25 35 9:30:474:95:915:0 30 31 66:60:366:75:413:3 40 25 3:80:254:54:610:9 50 20 2:40:145:63:99:1 60 17 1:70:138:83:47:8 80 13 0:97 0:05 29:12:75:8 100 10 0:63 0:03 22:52:14:5 FIG. 3 (color online). (a) Final isotopic s-process distributions using the new measured 63Ni MACS (red circles) and the MACS quoted by KADoNiS (blue squares) [17]. The distribution is normalized to solar system abundances. (b) Ratio of the two distributions in (a), zoomed in the Ni-Cu-Zn mass region. Isotopes of the same element are connected by solid lines. PRL 110, 022501 (2013) PHYSICAL REVIEW LETTERS week ending 11 JANUARY 2013 022501-4 marginal as this isotope results predominantly from later explosive nucleosynthesis during core collapse supernovae [30]. Also the propagation effect of the new MACS of 63Ni on heavier s-process species is rather small, of the order of a few percent. Although the s-process component at the end of convective C shell burning is well defined by these calculations, the abundances in the Ni-Cu-Zn region may be affected by following burning stages (for instance, the possible merging of shells [31]) and by the subsequent supernova explosion before enriching the interstellar medium. Given the complexity of this scenario, the final abundances are yet subject to considerable uncertainty as emphasized by several sensitivity studies [8,31,32]. Nevertheless, the present results represent a fundamental improvement in constraining the weak s-process component from the convective He core burning and convective C shell burning phases. A better knowledge of the preexplosive weak s-process component will allow to also better define the following explosive contribution to the copper inventory, once robust theoretical predictions are compared with spectroscopic observations. Another relevant observational constraint is given by the copper isotopic ratio in the Solar System, where the sprocess in massive stars provides the dominant contribution ([8], and references therein). In summary, we measured the energy-dependent 63Niðn; Þcross section at the n_TOF facility providing the first experimental results for MACSs at stellar neutron energies. The MACSs ranging from kT ¼5–100 keV exhibit total uncertainties of 20%–22% and are about a factor of 2 higher than the theoretical prediction of the KADoNiS compilation. Our results improve one of the main nuclear uncertainties affecting theoretical predictions for the abundances of 63Cu,64Ni, and 64Zn in s-process rich ejecta of core collapse supernovae. Furthermore, these results are a fundamental step to constrain the contribution from explosive nucleosynthesis to these species. The authors would like to thank H. Danninger and C. Gierl of the Technical University of Vienna for their help preparing the 62Ni sample. This work was partly supported by the Austrian Science Fund (FWF), Projects No. P20434 and No. I428. M. P. also acknowledges the support from the Ambizione grant of the Swiss NSF, the NSF Grants No. PHY 02-16783 and No. PHY 09-22648 (Joint Institute for Nuclear Astrophysics, JINA), EU Grant No. MIRG-CT- 2006-046520, and EuroGenesis. T. R. acknowledges support from EuroGenesis, the FP7 ENSAR/THEXO project, and a ‘‘Distinguished Guest Scientist Fellowship’’ from the Hungarian Academy of Sciences. [1] F. Ka ¨ppeler, H. Beer, and K. Wisshak, Rep. Prog. Phys. 52, 945 (1989). [2] J. G. Peters, Astrophys. J. 154, 225 (1968). [3] R. G. Couch, A. B. Schmiedekamp, and W. D. Arnett, Astrophys. J. 190, 95 (1974). [4] S. A. Lamb, W. M. Howard, J. W. Truran, and I. Iben, Astrophys. J. 217, 213 (1977). [5] C. M. Raiteri, M. Busso, G. Picchio, and R. Gallino, Astrophys. J. 371, 665 (1991). [6] C. M. Raiteri, M. Busso, G. Picchio, R. Gallino, and L. Pulone, Astrophys. J. 367, 228 (1991). [7] R. 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