Radiative neutron capture cross-section measurement of ge isotopes at n_TOF CERN facility and its importance for stellar nucleosynthesis

ACTA PHYSICA POLONICA A No. 4 Vol. 139 (2021) Special issue, XLVI Extraordinary Congress of Polish Physicists, Warsaw, Poland, October 16–18, 2020 Radiative Neutron Capture Cross-Section Measurement of Ge Isotopes at n_TOF CERN Facility and Its Importance for Stellar Nucleosynthesis A. Gawlika,∗, C. Lederer-Woodsb, J. Andrzejewskia, J. Perkowskia, U. Battinob, P. Ferreirac, F. Gunsingd,e, S. Heinitzf, M. Krtičkag, C. Massimih,i, F. Mingroneh, R. Reifarthj, A. Tattersallb, S. Valentag, C. Weisse,k, O. Aberlee, L. Audouinl, M. Bacakk, J. Balibream, M. Barbagallon, S. Barrosc, V. Bécaresm, F. Bečvářg, C. Beinruckerj, E. Berthoumieuxd, J. Billoweso, D. Bosnarp, M. Bruggere, M. Caamañoq, F. Calviñor, M. Calvianie, D. Cano-Ottm, R. Cardellae, A. Casanovasr, D.M. Castelluccios,h, F. Ceruttie, Y.H. Chenl, E. Chiaverie, N. Colonnan, G. Cortésr, M.A. Cortés-Giraldot, L. Cosentinou, L.A. Damonen,v, M. Diakakid, M. Dietzb, C. Domingo-Pardow, R. Dresslerf, E. Dupontd, I. Duránq, B. Fernández-Domínguezq, A. Ferrarie, P. Finocchiarou, V. Furmanx, K. Göbelj, A.R. Garcíam, T. Glodariuy, I.F. Gonçalvesc, E. González-Romerom, A. Goverdovskiz, E. Griesmayerk, C. Guerrerot, H. Haradaaa, T. Heftrichj, J. Heysebb, D.G. Jenkinscc, E. Jerichak, F. Käppelerdd, Y. Kadie, T. Katabuchiee, P. Kavrigink, V. Ketlerovz, V. Khryachkovz, A. Kimuraaa, N. Kivelf, I. Knapovag, M. Kokkorisff , E. Leal-Cidonchaq, H. Leebk, J. Lerendegui-Marcot, S. Lo Meos,h, S.J. Lonsdaleb, R. Lositoe, D. Macinae, T. Martínezm, P. Mastinugg, M. Mastromarcon, F. Matteuccihh,ii, E.A. Maugerif, E. Mendozam, A. Mengonis, P.M. Milazzohh, M. Mireay, S. Montesanoe, A. Musumarrau,jj, R. Noltekk, A. Opreay, N. Patronisll, A. Pavlikmm, J.I. Porrase,nn, J. Praenat,nn, J.M. Quesadat, K. Rajeevoo, T. Rauscherpp,qq, A. Riego-Perezr, P.C. Routoo, C. Rubbiae, J.A. Ryano, M. Sabaté-Gilartee,t, A. Saxenaoo, P. Schillebeeckxbb, S. Schmidtj, D. Schumannf, P. Sedyshevx, A.G. Smitho, A. Stamatopoulosff , G. Taglienten, J.L. Tainw, A. Tarifeño-Saldiviaw, L. Tassan-Gotl, A. Tsinganisff , G. Vanninih,i, V. Varialen, P. Vazc, A. Venturah, V. Vlachoudise, R. Vlastouff , A. Wallnerrr, S. Warreno, M. Weigandj, C. Wolfj, P.J. Woodsb, T. Wrighto, P. Žugecp,e and The n_TOF Collaboration (www.cern.ch/ntof) aUniversity of Lodz, Poland bSchool of Physics and Astronomy, University of Edinburgh, United Kingdom cInstituto Superior Técnico, Lisbon, Portugal dCEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France eEuropean Organization for Nuclear Research (CERN), Switzerland fPaul Scherrer Institut (PSI), Villingen, Switzerland gCharles University, Prague, Czech Republic hIstituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy 383 The 100 years anniversary of the Polish Physical Society — the APPA Originators iDipartimento di Fisica e Astronomia, Università di Bologna, Italy jGoethe University, Frankfurt, Germany kTechnische Universität Wien, Austria lInstitut de Physique Nucléaire, CNRS-IN2P3, Univ. Paris-Sud, Université Paris-Saclay, F-91406 Orsay Cedex, France mCentro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Spain nIstituto Nazionale di Fisica Nucleare, Sezione di Bari, Italy oUniversity of Manchester, United Kingdom pDepartment of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia qUniversity of Santiago de Compostela, Spain rUniversitat Politècnica de Catalunya, Spain sAgenzia Nazionale per le Nuove Tecnologie (ENEA), Bologna, Italy tUniversidad de Sevilla, Spain uINFN, Laboratori Nazionali del Sud, Catania, Italy vDipartimento di Fisica, Università degli Studi di Bari, Italy wInstituto de Física Corpuscular, CSIC — Universidad de Valencia, Spain xJoint Institute for Nuclear Research (JINR), Dubna, Russia yHoria Hulubei National Institute of Physics and Nuclear Engineering, Romania zInstitute of Physics and Power Engineering (IPPE), Obninsk, Russia aaJapan Atomic Energy Agency (JAEA), Tokai-mura, Japan bbEuropean Commission, Joint Research Centre, Geel, Retieseweg 111, B-2440 Geel, Belgium ccUniversity of York, United Kingdom ddKarlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany eeTokyo Institute of Technology, Japan ff National Technical University of Athens, Greece ggIstituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy hhIstituto Nazionale di Fisica Nucleare, Sezione di Trieste, Italy iiDipartimento di Astronomia, Università di Trieste, Italy jjDipartimento di Fisica e Astronomia, Università di Catania, Italy kkPhysikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany llUniversity of Ioannina, Greece mmUniversity of Vienna, Faculty of Physics, Vienna, Austria nnUniversity of Granada, Spain ooBhabha Atomic Research Centre (BARC), India ppCentre for Astrophysics Research, University of Hertfordshire, United Kingdom qqDepartment of Physics, University of Basel, Switzerland rrAustralian National University, Canberra, Australia Doi: 10.12693/APhysPolA.139.383 ∗e-mail: [email protected] This manuscript summarizes the results of radiative neutron capture cross-section measurements on two stable germanium isotopes, 70Ge and 73Ge. Experiments were performed at the n_TOF facility at CERN via the time-of-flight technique, over a wide neutron energy range, for all stable germanium isotopes (70,72,73,74, and 76). Results for 70Ge [Phys. Rev. C 100, 045804 (2019)] and 73Ge [Phys. Lett. B 790, 458 (2019)] are already published. In the field of nuclear structure, such measurements allow to study excited levels close to the neutron binding energy and to obtain information on nuclear properties. In stellar nucleosynthesis research, neutron induced reactions on germanium are of importance for nucleosynthesis in the weak component of the slow neutron capture processes. topics: n_TOF, neutron time-of-flight, neutron capture cross-section, MACS 1. Introduction Neutron capture reactions in stars are responsible for forming about 99% of the nuclei heavier than iron. The slow and rapid neutron capture processes contribute almost equally to the overall abundances. The sprocess takes place when neutron densities are low (108n cm−3) and radioactive decay is faster than subsequent neutron capture on radionuclides [3]; when neutron densities are high (>1020 n cm−3) nuclei can capture many neutrons, creating nuclides far from the valley of stability, and then decay by β−cascades (the so-called rprocess [4]). The weak component of the sprocess takes place in massive stars (>8solar masses M) and forms isotopes of mass 384 The 100 years anniversary of the Polish Physical Society — the APPA Originators Fig. 1. Nucleosynthesis path of the sprocess going along the stability valley. Solid boxes represent stable isotopes. Slow neutron captures and following βdecays are marked by arrow lines. Red box indicates the “s-only” 70Ge isotope, purple box indicates the “r-only” 76Ge isotope. number between 56 and 90. In this scenario, neutrons are produced by 22Ne(α,n)25Mg reaction [5] in two different burning stages of stellar evolution. In massive stars, 14N is produced via the CNO cycle, and then converted to 22Ne by the reaction sequence 14N(α, γ)18F(β+ν)18O(α, γ)22Ne. Towards the end of He core burning, at temperatures of about 0.3 GK, neutrons are released by the 22Ne(α,n)25Mg reaction. If some 22Ne survives the helium burning stage, 22Ne(α,n)25Mg source is reactivated at temperatures of about 1 GK during carbon shell burning [6–10]. Recent studies indicate that the weak sprocess is responsible for about 80% of the total germanium abundance [11]. On the other hand, 12% of germanium is produced in the main sprocess in asymptotic giant branch stars, with mass of about 1–5 M, during their thermal pulses. The remaining 8% is thought to be produced by primary explosive nucleosynthesis processes [11]. In stars, neutrons are thermalized very fast and their velocities can be described by the Maxwell– Boltzmann distribution. Therefore, the key quantity to predict abundances produced in the sprocess is the Maxwellian averaged cross-section (MACS). To study the sprocess nucleosynthesis for the weak component, MACS values should be known up to kBTvalues of about 100 keV. Figure 1 shows the sprocess path in the region of interest. The stable isotope with the lowest mass is 70Ge, which plays a special role as “s-only” nuclide (with minor contributions from proton-rich nucleosynthesis processes) since it is shielded from the rprocess production by its stable isobar 70Zn [1]. It suggests that 70Ge is mainly produced in the sprocess and its abundance can be used as a reference to compare to solar abundances. On the other hand, 76Ge due to its position on the path can be treated as an “r-only” isotope since it is shielded from the sprocess by the unstable 75Ge with short half-life of 83 min. In addition, germanium is commonly used as detector material, and hence neutron induced reactions on germanium are important for low-background experiments using HPGe detectors [12]. The data for 76Ge is currently under analysis. 2. Experimental setup and measurement The measurements of the (n,γ) cross-section of stable Ge isotopes at n_TOF were performed over a wide neutron energy range from eV to several hundreds of keV. The n_TOF facility is a part of the CERN accelerator complex, shown in Fig. 2. In the first accelerator, LINAC, protons are accelerated to energies of 1.4 GeV. Protons of energy up to 20 GeV from Proton Synchrotron are sent to the n_TOF spallation target (7×1012 protons per pulse with 0.8 Hz repetition rate). An intense neutron beam is produced by spallation reactions impinging on a cylindrical 1.3-tonne lead target (40 cm length, 60 cm diameter) [13]. The moderated neutron spectrum ranges from thermal energies (25 meV) to several GeV. Measurements were performed at Experimental Area 1 (EAR1), at a distance of 185 m from the spallation target, characterized by excellent energy resolution ranges from 3×10−4at 1 eV to 3×10−3 at 100 keV [13]. The n_TOF facility is equipped with another experimental beam line, EAR2, with an approximately ten times shorter vertical flight path resulting in a higher flux but worse energy resolution. For the detection of prompt γrays following capture reactions, we used C6D6scintillation detectors, which have been specially optimised for extremely low sensitivity to scattered neutrons. Due to this feature, C6D6detectors are most suitable for measuring isotopes with neutron capture cross-sections much lower than the elastic scattering cross-section. Samples used for the (n,γ) measurements on Ge isotopes consisted of GeO2powder pressed into pallets of cylindrical shape of 2 cm diameter. All samples were glued on to a sample holder with 6 µm backing made of Mylar. Runs with an empty sample holder were performed to study the sample independent Fig. 2. Layout of the n_TOF facility at CERN. 385 The 100 years anniversary of the Polish Physical Society — the APPA Originators background. A metallic germanium sample of natural composition was measured to identify any contributions from other germanium isotopes present in the enriched samples. 3. Data analysis As a first step, the dead-time corrected time- of-flight counting spectra were converted to neutron energy spectra with the known neutron time- of-flight and flight path length. The flight path length was determined by calibrating against well known low energy resonances in the 197Au(n,γ) reaction [14]. The limitations caused by the experimental setup itself (neutron energy resolution), allowed us to parameterize resonances up to 25 keV and 14 keV in the case of 70Ge and 73Ge isotope, respectively. The experimental capture yield can be described as Y(En) = fN(En)C(En)−B(En) cΦ(En),(1) where C(En)is the count spectrum of the measured sample, B(En)is the background, cis the detection efficiency, Φis the neutron flux, and fN(En)is a normalisation factor. Figure 3 shows the experimental capture yield compared to the yield predicted by ENDF/B- VIII [15] (taking into account all the experimental effects). As can be seen, many new resonances are observed. The background B(En)at the n_TOF facility consists of the following components: samplerelated background, beam-related background and ambient background unrelated to the beam. The environmental background is present during all measurements. It is due to natural radioactivity and cosmic rays and it is determined in runs without the beam. Beam-related background is mainly caused by interactions between the neutron beam and material in the experimental area and it is determined by measurements with an empty sample Fig. 3. Experimental capture yield of 70Ge(n,γ) reaction in comparison with ENDF/B-VIII capture yield. Fig. 4. 70Ge time-of-flight spectrum compared to empty sample holder, both measured with Al filter in the beam. The counting spectra are weighted and normalized to the total number of protons. holder. Sample-related background is specifically caused by neutrons scattered off the sample. Usually, the capture probability is a few orders of magnitude lower than the scattering probability and thus many neutrons may be scattered from the sample. Scattered neutrons can be captured by the surrounding materials and cause additional background events. In order to correct for the background due to neutron scattering, runs with neutron filters in the beam were performed. Figure 4 shows the 70Ge spectrum as well as the empty sample holder spectrum, both measured with an Al filter in the beam. The filter was made of aluminum of proper thickness to capture all neutrons from the beam at certain resonance energies. Any events in the dips of these resonances are produced by background reactions. The aluminum filter has those dips at around 35, 86, and 150 keV neutron energy. The background is scaled to these dips which are called black resonances. The 197Au sample was used to obtain a normalisation factor fNwhich corrects the capture yield since the diameter of the neutron beam is larger than the diameter of the capture sample. The sample used for this purpose had the same diameter as the Ge samples. The normalisation was determined using the 197Au resonance at 4.9 eV neutron energy, employing the saturated resonance method [16]. The detection efficiency was taken into account using the pulse height weighting technique (PHWT). It is based on applying pulse height dependent weights to the data, to achieve a capture efficiency proportional to the excitation energy of the compound nucleus. Weighting functions were obtained in detailed simulations of the experimental setup using GEANT4 [17, 18]. The neutron flux Φis measured in a separate campaign, using a variety of different detectors and well known reference reactions such as 10B(n,α) and 6Li(n,α) [19]. Neutron resonances from (n,γ) reactions on germanium isotopes were fitted with the multilevel, multi-channel R-matrix code SAMMY [20]. 386 The 100 years anniversary of the Polish Physical Society — the APPA Originators Fig. 5. Example for some SAMMY fits of the 70Ge experimental capture yield. Solid line shows the result of SAMMY fit, dots are the experimental capture yield measured at n_TOF. SAMMY takes into account all experimental effects, such as broadening of resonance shape and self-shielding, and multiple scattering corrections. In addition, sample impurities due to other Ge isotopes and oxygen were taken into account. From a single capture measurement only the capture kernel kcan be extracted. It is defined as: k=gΓnΓγ Γn+Γγ ,(2) where gis a statistical factor for different spin combinations. It is given by g=(2J+ 1) (2s+ 1)(2I+ 1) (3) where Jis the resonance spin, sis the neutron spin and Iis the ground state spin of the target nucleus. The partial neutron and radiative widths, Γnand Γγ, respectively, and the energy were obtained with SAMMY. For known resonances, initial parameters were taken from the Atlas of Neutron Resonances [21]. Resonance structures can be observed until the experimental broadening is larger than the distance between subsequent resonances. This energy region is called the resolved resonance region (RRR). A few examples of 70Ge resonance fits are shown in Fig. 5. Average radiative width Γγand the average resonance spacing D0were determined using the resonances in the RRR. Only strong resonances, for which the SAMMY fit yielded Γn>10Γγ, were chosen. Detailed results and averaged parameters for 70Ge and 73Ge can be found in [1] and [2], respectively. With increasing neutron energy, due to the worsening of the experimental resolution combined with lower statistics and increasing resonance widths, analysis of individual resonance parameters is no longer possible. This energy region is called the unresolved resonance region (URR). The level of the background also increases as a result of which a fraction of smaller resonances may be missed, resulting in an underestimation of the cross-section. The unresolved capture cross-section was obtained after subtraction of backgrounds due to neutron scattering and contributions due to impurities, and applying corrections for multiple scattering and selfshielding effect. 4. Stellar capture cross-section We calculated Maxwellian averaged crosssections (MACS), defined as: MACS =2 √π 1 (kBT)2 ∞ Z 0 dE E σ(E)e−E/(kBT), (4) using resonance information from SAMMY fits combined with the unresolved cross-section which we determined up to neutron energies of 300 keV. Final uncertainties in the MACS values are ≈5%. Figure 6 shows the ratio between experimental results obtained from n_TOF data and values from the Karlsruhe Astrophysical Database of Nucleosynthesis in Stars [22], which is based on previous data and theoretical predictions. In the case of 70Ge, there are only small changes in the resultant 70Ge abundances produced in massive stars using the new 70Ge MACS, about 3%. The reason for that is a very good agreement with previous results published by Walter and Beer in 1985 [23]. In the case of 73Ge, MACSs are a factor about 1.5 larger than MACSs recommended to be used in stellar models, which were based on KADoNiS-1.0. It is worth mentioning that it was the first capture cross-section measurement on 73Ge isotope. This results in a reduction of the 73Ge abundances by over 30%. The new data indicate that the germanium isotopic abundance pattern produced in the sprocess in a low metallicity massive star is compatible with the isotopic abundance pattern of the solar system [2]. 5. Conclusions Neutron capture cross-sections on Ge have impact on abundances produced in the weak component of the slow neutron capture process. Recently, cross-sections on all stable Ge isotopes have been measured at the n_TOF facility at CERN. Results for 70Ge and 73Ge have already been published in [1, 2], while data analysis for the remaining isotopes is still ongoing. Fig. 6. Ratio between MACSs values obtained at n_TOF [1, 2] and values from KADoNiS-1.0 database [22]. 387 The 100 years anniversary of the Polish Physical Society — the APPA Originators Acknowledgments This work was supported by the Austrian Science Fund FWF (J3503), the Adolf Messer Foundation (Germany), the UK Science and Facilities Council (ST/M006085/1), and the European Research Council ERC-2015-StG No. 677497. We also acknowledge the support of the National Science Centre, Poland, under the grant UMO- 2016/22/M/ST2/00183, the MSMT of the Czech Republic and the Croatian Science Foundation under the project IP-2018-01-8570. References [1] A. Gawlik et al. (The n_TOF Collaboration), Phys. Rev. C 100, 045804 (2019). [2] C. Lederer-Woods et al. (The n_TOF Collaboration), Phys. Lett. B 790, 458 (2019). [3] R. Reifarth, C. Lederer, F. Käppeler, J. Phys. G 41, 053101 (2014). [4] F.-K. Thielemann, A. Arcones, R. Käppeli et al., Prog. Part. Nucl. Phys. 66, 346 (2011). [5] G. Cescutti, R. 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