Measurement of the 154 Gd(n, γ ) cross section and its astrophysical implications
MMeasurement of the
Gd(n, γ ) cross section and its astrophysicalimplications A. Mazzone a,b , S. Cristallo c,d , O. Aberle e , G. Alaerts f , V. Alcayne g , S. Amaducci h,i , J.Andrzejewski j , L. Audouin k , V. Babiano-Suarez l , M. Bacak e,m,n , M. Barbagallo e,a , V. B´ecares g ,F. Beˇcv´aˇr o , G. Bellia h,i , E. Berthoumieux n , J. Billowes p , D. Bosnar q , A. S. Brown r , M. Busso c,s ,M. Caama˜no t , L. Caballero l , M. Calviani e , F. Calvi˜no u , D. Cano-Ott g , A. Casanovas u , D.M.Castelluccio v,w , F. Cerutti e , Y. H. Chen k , E. Chiaveri p,x,e , G. Clai v,w , N. Colonna a , G. P. Cort´es u ,M. A. Cort´es-Giraldo x , L. Cosentino h , L. A. Damone a,y , M. Diakaki z , M. Dietz aa , C.Domingo-Pardo l , R. Dressler ab , E. Dupont n , I. Dur´an t , Z. Eleme ac , B. Fern´andez-Dom´ıngez t , A.Ferrari e , I. Ferro-Gonc¸alves ad , P. Finocchiaro h , V. Furman ae , R. Garg aa , A. Gawlik j , S.Gilardoni e , T. Glodariu af , K. G¨obel ag , E. Gonz´alez-Romero g , C. Guerrero x , F. Gunsing n , S.Heinitz ab , J. Heyse f , D. G. Jenkins r , E. Jericha m , Y. Kadi e , F. K¨appeler ah , A. Kimura ai , N.Kivel ab , M. Kokkoris z , Y. Kopatch ae , S. Kopecky f , M. Krtiˇcka o , D. Kurtulgil ag , I. Ladarescu l , C.Lederer-Woods aa , J. Lerendegui-Marco x , S. Lo Meo v,w , S.-J. Lonsdale aa , D. Macina e , A.Manna v,aj , T. Mart´ınez g , A. Masi e , C. Massimi v,aj, ∗ , P. F. Mastinu ak , M. Mastromarco e,p , F.Matteucci al,am , E. Maugeri ab , E. Mendoza g , A. Mengoni v,w , V. Michalopoulou z , P. M. Milazzo al ,F. Mingrone e , R. Mucciola v,aj , A. Musumarra h,i , A. Negret af , R. Nolte an , F. Og´allar ao , A.Oprea af , N. Patronis ac , A. Pavlik ap , J. Perkowski j , L. Piersanti c,d , I. Porras ao , J. Praena ao , J. M.Quesada x , D. Radeck an , D. Ramos Doval k , R. Reifarth ag , D. Rochman ab , C. Rubbia e , M.Sabat´e-Gilarte x,e , A. Saxena aq , P. Schillebeeckx f , D. Schumann ab , A. G. Smith p , N. Sosnin p , A.Stamatopoulos z , G. Tagliente a , J. L. Tain l , Z. Talip ab , A. E. Tarife˜no-Saldivia u , L.Tassan-Got e,z,k , P. Torres-S´anchez ao , A. Tsinganis e , J. Ulrich ab , S. Urlass e,ar , S. Valenta o , G.Vannini v,aj , V. Variale a , P. Vaz ad , A. Ventura v , D. Vescovi c,as,d , V. Vlachoudis e , R. Vlastou z , A.Wallner at , P. J. Woods aa , R. Wynants f , T. J. Wright p , P. ˇZugec q a Istituto Nazionale di Fisica Nucleare, Bari, Italy b Consiglio Nazionale delle Ricerche, Bari, Italy c Istituto Nazionale di Fisica Nazionale, Perugia, Italy d Istituto Nazionale di Astrofisica - Osservatorio Astronomico d’Abruzzo, Italy e European Organisation for Nuclear Research (CERN), Switzerland f European Commission, Joint Research Centre, Geel, Retieseweg 111, B-2440 Geel,Belgium g Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT),Spain h INFN Laboratori Nazionali del Sud, Catania, Italy i Dipartimento di Fisica e Astronomia, Universit`a di Catania, Italy j University of Lodz, Poland k IPN, CNRS-IN2P3, Univ. Paris-Sud, Universit´e Paris-Saclay, F-91406 Orsay Cedex,France l Instituto de F´ısica Corpuscular, CSIC - Universidad de Valencia, Spain m Technische Universit¨at Wien, Austria n CEA Saclay, Irfu, Universit´e Paris-Saclay, Gif-sur-Yvette, France o Charles University, Prague, Czech Republic p University of Manchester, United Kingdom q Department of Physics, Faculty of Science, University of Zagreb, Croatia r University of York, United Kingdom s Dipartimento di Fisica e Geologia, Universit`a di Perugia, Italy t University of Santiago de Compostela, Spain u Universitat Polit`ecnica de Catalunya, Spain v Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Italy w Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy x Universidad de Sevilla, Spain y Dipartimento di Fisica, Universit`a degli Studi di Bari, Italy a r X i v : . [ nu c l - e x ] F e b National Technical University of Athens, Greece aa School of Physics and Astronomy, University of Edinburgh, United Kingdom ab Paul Scherrer Institut (PSI), Villigen, Switzerland ac University of Ioannina, Greece ad Instituto Superior T´ecnico, Lisbon, Portugal ae Joint Institute for Nuclear Research (JINR), Dubna, Russia af Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH),Bucharest, Magurele, Romania ag Goethe University Frankfurt, Germany ah Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany ai Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan aj Dipartimento di Fisica e Astronomia, Universit`a di Bologna, Italy ak Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy al Istituto Nazionale di Fisica Nazionale, Trieste, Italy am Dipartimento di Fisica, Universit`a di Trieste, Italy an Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany ao University of Granada, Spain ap University of Vienna, Faculty of Physics, Vienna, Austria aq Bhabha Atomic Research Centre (BARC), India ar Helmholtz-Zentrum Dresden-Rossendorf, Germany as Gran Sasso Science Institute, L’Aquila, Italy at Australian National University, Canberra, Australia
Abstract
The neutron capture cross section of
Gd was measured from 1 eV to 300 keV in the experimen-tal area located 185 m from the CERN n TOF neutron spallation source, using a metallic sampleof gadolinium, enriched to 67% in
Gd. The capture measurement, performed with four C D scintillation detectors, has been complemented by a transmission measurement performed at theGELINA time-of-flight facility (JRC-Geel), thus minimising the uncertainty related to samplecomposition. An accurate Maxwellian averaged capture cross section (MACS) was deducedover the temperature range of interest for s process nucleosynthesis modeling. We report a valueof 880(50) mb for the MACS at kT =
30 keV, significantly lower compared to values available inliterature. The new adopted
Gd(n, γ ) cross section reduces the discrepancy between observedand calculated solar s-only isotopic abundances predicted by s-process nucleosynthesis models. Keywords: s process,
Gd, Neutron time of flight, n TOF ∗ Corresponding Author
URL:
[email protected] (C. Massimi )
Preprint submitted to Physic Letters B February 7, 2020 . Introduction
All the elements heavier than those in the iron group are produced by a sequence of neutroncapture reactions and β decays taking place in a hot stellar environment, during di ff erent phasesof stellar evolution. The two main processes involved are the slow (s) and the rapid (r) neutroncapture processes. The s process [1, 2] owes its name to the neutron-capture time scale, whichallows β decay to occur between consecutive capture events. Consequently, a series of thesereactions produce stable isotopes by moving along the β -stability valley. On the other hand,when the neutron densities are high enough [3], the neutron capture sequence is much faster thanthe β decays and the path, the r process path, can proceed toward many short-lived isotopes,approaching the neutron drip line.Most nuclei receive a contribution from both the s and the r processes (see e.g. [4]). However,a few isotopes cannot receive any contribution from the r process because they are shieldedagainst β decays by stable isobars and for this reason are called s-only isotopes. This is thecase of the two gadolinium isotopes Gd and
Gd which are shielded against the β -decaychains from the r-process region by their stable samarium isobars, as shown in Figure 1. To beprecise, Gd may receive an additional contribution from the p process, which proceeds viaphoto-disintegration. The amount of the p-process contribution to the
Gd abundance is stillfar from being precisely determined (see [5] and references therein), while a minor p-processcontribution to the
Gd abundance cannot be excluded as well.The almost pure s-process origin of
Gd (as for other s-only isotopes), makes its capturecross section crucial for testing various stellar models aiming at understanding s process nu-cleosynthesis in Asymptotic Giant Branch (AGB) stars, the most important stellar site for thes process. In particular, relevant hints on the shape and extension of the main s process neu-tron source, the so-called C pocket [6], can be derived. Recently, several studies [7, 8, 4]have been conducted analyzing the solar s process abundances in the framework of a GalacticChemical Evolution (GCE) model to investigate the e ff ect of di ff erent internal structures of the C pocket, which may a ff ect the e ffi ciency of the C( α ,n) O reaction. In addition, Trippellaand Collaborators [9] carried out a similar analysis based on single stellar models. Cristallo etal. [8] and Prantzos et al. [4] have obtained an under-production (-30%) of the Gd s-only nu- In the following, an individual isotope or / and its abundance is imply indicated by the symbol (e.g. Gd), to simplifythe notation. igure 1: (Color online) The yellow line represents the main s-process path in the Sm-Eu-Gd region; the s-only isotopesare highlighted with a yellow dashed box. Stable elements are in black, β + , β − and β + radioisotopes are in orange, blueand red, respectively. cleus compared to the s-only isotope Sm, which is an un-branched isotope, usually assumedas a reference for the s process flow. This result is at odds with observations. The authors ofref. [8], have suggested that part of such a deviation could be connected to uncertainties in theadopted nuclear physics inputs, taken from the Karlsruhe Astrophysical Database of Nucleosyn-thesis in Stars (KADoNiS) version 0.3 [10], including the neutron capture cross section of
Gd.In the work by Trippella et al. [9], problems for the s process production of Gd were found aswell, although in this case, the discrepancy between the abundances of
Sm and
Gd is lesspronounced. The need for a new
Gd(n, γ ) measurement was underlined by the unreasonableprediction for the over-productions of Gd and
Gd with respect to their solar abundances. Inparticular, the ratio Gd / Gd), turned out to be lower than unity, while it is thought that
Gdshould exhibit a higher p-process contribution as compared to
Gd [9]. In addition,
Gd wasfound to be produced insu ffi ciently compared to the lighter s-only Sm and
Nd, producedmostly by the s process and possibly partly by the p process.The close correlation between stellar abundances and neutron capture cross sections callsfor an accurate determination of the
Gd(n, γ ) cross section. In addition, the reduction of theuncertainty related to nuclear physics inputs could rule out one of the possible causes of presentdiscrepancies between observation and model predictions of the abundances. In fact, refinedstellar models require a full set of Maxwellian Averaged Capture Cross Section (MACS) for4hermal energies in the range kT = −
30 keV. In the case of
Gd, 80% of the MACS at kT = . −
300 keV- hereafter we refer to this energy region as Unresolved Resonance Region (URR). At kT = Gd(n, γ ) cross section measurements are reported in literature, by Shorin et al. [11], Beer and Macklin [12] and Wisshak et al. [13]. They all cover the URR and therespective MACS exhibit large di ff erences and at kT =
30 keV, they range from 878(27) mb to1278(102) mb. Therefore, the data present in the literature, so far, are not conclusive enough toconstrain stellar model calculations.The large spread in the available experimental data could be related to the corrections forisotopic impurities which are necessarily applied in the data analysis. In particular, the poorknowledge of their cross sections, given the low natural abundance of
Gd (2.18%). Di ff erentdiscrepancies may be related to: i) the detectors used in the past experiments, which in somecases could have su ff ered from high neutron sensitivity; ii) the experimental determination of theneutron flux, which might have been biased in some previous measurements; iii) the quality ofoxide samples and the need of canning for the container to avoid loss of material.The present measurement reduced the impact of these limiting factors, by using the well-established, low neutron-sensitivity C D detectors [14], combined to a self-sustaining metallicsample enriched in Gd, and exploiting the results of the recent
Gd(n, γ ) measurement per-formed at n TOF [15]. Moreover, the gadolinium sample was characterised by a transmissionmeasurement at the neutron time-of-flight facility GELINA at EC-JRC-Geel (Belgium).
2. Measurements
The neutron capture cross section measurement was performed at the neutron time-of-flightfacility n TOF at CERN. In this facility, neutrons are produced by spallation reactions inducedon a lead target by 20 GeV / c protons from the CERN Proton Synchrotron (PS), which provides atotal of 2 × neutrons / pulse, generated by a 7 × protons / pulse primary beam. The initiallyfast neutrons are moderated and then collimated through two flight paths of di ff erent lengths. Thepresent measurement was performed at the experimental area located 185 m from the spallationtarget. This long flight-path, combined with the 7 ns width of the proton bunches from the PS,results in a high energy resolution ranging from 3 × − at 1 eV to 3 × − a 100 keV [16].5he neutron capture events were observed via the detection of the prompt γ -ray cascade from Gd excited states. Four C D detectors, modified for minimizing their neutron sensitivity [14],were arranged at 125 ◦ relative to the neutron beam direction and about 10 cm upstream from thegadolinium sample position. This configuration minimised the e ff ect of anisotropic emission of γ cascades while reducing the background from in-beam photons scattered by the sample. TheTotal Energy detectors (see [17] and references therein) were used in combination with PulseHeight Weighting Technique (PHWT) [18, 17].The sample consisted of 0.263 g of metallic gadolinium enriched at 66.78% in Gd. Themain contaminant, i.e.
Gd, was declared by the sample provider at 17.52%. A detailed reso-nance analysis of the capture data, based on the recent results obtained by time of flight on
Gd(n, γ ) at n TOF, allowed us to estimate a content of Gd equal to 20.2%. This higher value wasconfirmed by a transmission measurement on the same sample carried out at GELINA.A Au sample of the same diameter was used to normalise the measured yield, by applying thesaturated resonance technique [19]. Also, two other samples with the same diameter of 3 cm wereused. A lead sample enabled the estimate of the background, while a natural gadolinium sampleallowed to assign observed resonances to the correct gadolinium isotope, besides confirming theisotopic content of
Gd in the enriched sample.As mentioned above, the gadolinium sample was further studied through the transmissionmeasurement at a 10-m station of the GELINA facility. The transmission, T , which was experi-mentally obtained from the ratio of Li-glass spectra resulting from a sample-in and a sample-outmeasurement, is related to the total cross section σ tot by the equation: T ( E n ) = e − n σ tot ( E n ) , (1)where n = (1 . ± . × − atoms / b denotes the areal density of the gadolinium sample.GELINA is particularly suitable for high-resolution transmission measurement, because of itstime characteristic and the small dimensions of the neutron producing target. For this experi-ment, the neutron beam was collimated to a diameter of 10 mm at the sample position and filterswere placed near the sample to absorb slow neutrons from the previous neutron-burst and to con-tinuously monitor the background. The neutron beam passing through the sample was detectedby a 6.4 mm thick and 76 mm wide Li-glass scintillator enriched to 95% in Li. The detectorwas placed at 10.86 m from the neutron production target.6 . Data analysis and results
The experimental capture yield Y c , i.e. the probability for an incident neutron to be capturedin the sample, can be deduced from the measured count rate, corrected for the detection e ffi ciencyof capture events. By applying the PHWT, the count rate, C w , is weighted in order to make thedetection e ffi ciency independent of the cascade path and γ multiplicity. The weighting functionswere calculated simulating the response of the full apparatus by a GEANT4 [20] Monte-Carlosimulation. The capture yield can be written as: Y ( E n ) = N C w ( E n ) − B w ( E n ) Φ ( E n ) (2)where N is a normalisation factor, B w is the weighted count rate representing the background and Φ is the neutron flux impinging on the sample. The neutron energy E n was determined from themeasured time of flight using an e ff ective flight path determined from well-known low energyresonances in Au [21].The normalisation factor groups together geometrical factors, such as the area of the sampleand its beam-interception factor, the solid angle subtended by the capture and flux monitors, andthe detection e ffi ciency. It was obtained with the saturated resonance technique applied to the4.9-eV resonance in Au. This normalisation, based on the Au capture data, was within 1.3%consistent with the normalisation derived from a fit to the capture data using the parameters fromthe transmission data.The background, B w , includes di ff erent contributions: i) ambient background, which wasdetermined from a measurement in absence of the neutron beam; ii) the sample-independentbackground (also referred to as empty background), due to the neutron beam, which was esti-mated from a measurement with neutron beam impinging on an empty sample; iii) the sample-dependent background, either due to sample-scattered neutrons (subsequently captured in theenvironmental material and generating γ -rays in the experimental area) or due to γ -rays pro-duced at the spallation target and reaching the experimental area, where they can be scattered bythe sample into the detectors. This third component was estimated by a measurement with a leadsample in the neutron beam. Previous measurements and Monte-Carlo simulations showed thatthe contribution of the in-beam γ -ray background is relevant only in a limited energy window of1-100 keV [16]. Therefore, it was possible to disentangle the two sample-dependent backgroundcomponents in the time-of-flight spectrum measured with the lead and with the gadolinium sam-7les, properly scaled. The neutron background was scaled for the elastic cross section and theareal density of the gadolinium and lead sample, while the γ background was scaled for the ef-fective atomic number of gadolinium and lead samples. The same procedure for the estimationof the total background was adopted in the study of the Au(n, γ ) cross section. In particular,in the energy region from 5 keV to 500 keV, where this cross section is very well-known (andabove 200 keV, considered as a standard) the capture cross section from this study resulted to bein good agreement with evaluations.In figure 2 the weighted number of counts, registered with the Gd sample, are shown to-gether with the sample-independent (empty) background and the sample-dependent backgroundcomponents. The empty-sample background component dominates the total background overthe energy region of interest, whereas the sample-dependent background contributes by less than4% to the total background. The absolute magnitude of the background was additionally verifiedwith dedicated runs using black resonance filters [17] and a fair agreement was found. In theregion between 5 and 300 keV, the signal-to-total background ratio is 2.5. Although the totalbackground level is significant, its uncertainty is small since the background is dominated by theempty-sample component, which is known within 1%.
Figure 2: (Color online) Weighted spectrum measured with the
Gd sample compared to the background measuredwith an empty-sample holder and the one estimated from a measurement with a lead sample. The last spectrum wasobtained by scaling the in-beam γ -ray background and the component due to the neutrons scattered by the sample, seetext for details. The neutron flux was evaluated with a dedicated measurement campaign using di ff erent de-8ection systems and with neutron cross section standards [22]. The estimated systematic un-certainty on the flux determination was within 1% below 3 keV [22] and of 3.5 % up to 300keV.In the energy region up to 2.7 keV - hereafter referred to as Resolved Resonance Region(RRR) -, the experimental capture yield was analysed with the Bayesian R-matrix analysis codeSAMMY [23]. The code can manage experimental e ff ects such as Doppler and resolution broad-ening, self-shielding and multiple interactions of neutrons in the sample. Sizable discrepancieswere found for some neutron resonances compared to the yields obtained using the ENDF / B-VIII.0 evaluation data set [24]. These di ff erences were further confirmed by the transmissiondata. An example is shown in figure 3, where the present results of resonance shape analysis arecompared to the expected values obtained using resonance parameters from the evaluated dataset. Up to 2.75 keV, we analysed 156 resonances and 3 new resonances were found at 183.17(2), Figure 3: (Color online) Measured capture yield and transmission of for n + Gd. Experimental data are shown byred symbols, the n TOF R-matrix fit in blue and the yield calculated from ENDF / B-VIII.0 parameters in green, both incontinuous lines. S = . × − , an average level spacing D = . Γ γ = ff ects was lower than 2%.The contribution of the contaminants present in the measured samples was taken into ac-count in the experimental data analysis. In particular, for the main contaminant Gd, the crosssection was assumed from the previous n TOF measurement on
Gd. In figure 4, the capturecross section extracted from this study is compared to the data of Shorin et al. [11], Beer andMacklin [12], Wisshak et al. [13] and the ENDF / B-VIII evaluation. In the URR, the present datafairly agree with the data by Beer and Macklin [12] and they are substantially lower than thedata reported by Wisshak et al. [13] and Shorin et al. [11]. This comparison seems to indicatethat the results obtained with similar experimental setups are in fair agreement, while they areinconsistent if the adopted measurement technique is di ff erent. A discussion of potential reasonsfor the disagreement is beyond the scope of this article. Figure 4: (Color online)
Gd(n, γ ) cross section from the present study compared to previous measurements (colouredsymbols) and evaluation (continuous line). The HF calculation has been normalized by a factor 0.737 (see text). MACS as a function of the thermal energy kT were calculated from the present capture datain the RRR and in the URR. The cross section in the energy region above this range (i.e. above300 keV) was taken into account by calculations performed using the Hauser Feshbach (HF)statistical model theory, as implemented in the TALYS code [26]. The average resonance pa-rameters, obtained in the present analysis of the RRR, were constrained to be reproduced by the10alculations by adjusting the level density and the γ -ray strength function. An overall normalisa-tion by a factor 0.737 of the resulting capture cross section was still necessary to reproduce thepresent experimental MACS at kT =
30 keV.The uncertainty on the MACS takes into account the uncorrelated uncertainty attributableto counting statistics and systematic uncertainties. The uncertainty components originate fromthe normalisation of the capture data and the PHWT (1.3%), the shape of the neutron flux (1%below 3 keV and 3.5% above) and the subtraction of the background (on average 2%, dependingon the energy region.). Another minor uncertainty is associated with the alignment of the sampleand its geometrical shape. As a result, over the thermal energy range of kT = −
100 keV, theuncertainty on the MACS ranges between 5 and 7%. In table 1, the present MACS are reported
Table 1: Maxwellian Averaged Capture Cross (in mb) calculated for the n TOF data in the energy range kT = − Energy n TOF KADoNiS KADoNiS(keV) 0.3 1.05 2160(90) 2801 2947(190)8 1820(80) – 2195(87)10 1590(80) 1863 1924(61)15 1270(80) 1477 1537(33)20 1080(60) 1258 1326(24)25 970(50) 1124 1188(19)30 880(50) 1028(12) 1088(16)40 770(40) 898 950(14)50 690(40) 810 856(12)60 640(40) 745 786(12)80 560(30) 653 690(15)100 510(30) 591 626(19)for the thermal energy grid proposed by KADoNiS [10, 27]. In the whole energy range, the11ACS values from our measurement are significantly lower than KADoNiS. It is interesting tonote that the disagreement worsened with the new version of the evaluation, the deviation beingbetween 10% and 20%.
4. Astrophysical implications
As discussed above, a new determination of the
Gd neutron capture cross section wasmotivated by a discrepancy between stellar models and observations, as highlighted by [8] andconfirmed by [4], where a lower theoretical Gd / Sm ratio with respect to that measured inthe Sun (and derived for the early-solar system) was found. In fact, theoretical values, whichinclude yields from the Asymptotic Giant Branch (AGB) phase of low and intermediate massstars (taken from the FRUITY database of AGB star nucleosynthesis [28, 29]), show an under-production of
Gd with respect to
Sm: Gd / Sm = Gd itself.The use of the present cross section leads to an increase of the theoretical solar
Gd abun-dance by 10% on average . The di ff erence in the Gd surface abundances is lower than thechange of the neutron capture cross sections (on average 15% compared to KaDoNiS 0.3). Thisis because the
Gd production / destruction strongly depends on the branching at Eu, whichis an unstable isotope (its decay lifetime in the terrestrial condition is 8.6 y). This branching isby-passed when the major neutron source in AGB stars (the C( α ,n) O reaction) is activated,due to its short lifetime for the timescale characterizing neutron captures in this regime. The situ-ation may be di ff erent during thermal pulses when the higher temperature can e ffi ciently activatethe Ne( α ,n) Mg neutron source (which produces a definitely higher neutron flux). In sucha case, the neutron capture channel is competitive compared to the β decay channel and, as aconsequence, the main s-process path may by-pass Gd. The delicate balance between neutroncaptures on
Gd and β decays from Eu determines the final abundance of
Gd.In summary, the present experimental value leads to a better agreement between modelcalculations and observations, although it is not able to completely remove the mismatch. In Note that only a limited number of AGB models have been investigated with the present cross section. The evaluationof the e ff ect in a full GCE model will be published separately. abundance to be ±
15% (and ±
5% for samarium). The adoption of the present
Gdneutron capture cross section, eventually leads to a new Gd / Sm ratio of 0.77 in FRUITYmodels. When taking into consideration the lower limit of the present neutron capture cross sec-tion, we obtain Gd / Sm (cid:39) ± Nd,
Sm and
Gd is completely erased(due to the larger production of
Gd), at the same time, the ratio Gd / Sm attains a value(1.15) consistent with observations, within uncertainties. In general, it appears that the approachin ref. [31] produces a flatter distribution of s process isotopes, although this was obtained inpost-process computations and not in full stellar models. Therefore, a clear suggestion emergingfrom the present
Gd(n, γ ) cross section measurement is that some of the remaining modelambiguities might be solved by a merging of the mixing approaches presented in FRUITY andref. [9], something that is in an advanced stage of implementation [32].Another important outcome from this combined experimental and theoretical study is relatedto the Gd abundance, which largely depends on the branching at
Eu. For this isotope, noexperimental data are available for both the neutron capture cross section and the temperature-dependent β decay rate. Therefore, s process calculations are based on purely theoretical esti-mations (see ref. [33] and ref. [34], respectively). A lower Eu(n, γ ) cross section or a faster Eu( β − ) Gd decay would lead to a larger
Gd surface abundance with respect to
Sm (andvice-versa). Therefore, the present result suggests that additional e ff orts should be spent in thisdirection from the experimental side, to provide experimental values as detailed as possible tostellar modelers.
5. Acknowledgements
The isotope used in this research was supplied by the United States Department of EnergyO ffi ce of Science by the Isotope Program in the O ffi ce of Nuclear Physics.This research was supported by the EUFRAT open access programme of the Joint ResearchCentre at Geel. No info on isotopic uncertainties are currently available.
1] E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Rev. Mod. Phys. 29 (1957) 547–650.[2] A. G. W. Cameron, Publications of the Astronomical Society of the Pacific 69 (1957) 201.[3] F.-K. Thielemann, et al., Progress in Particle and Nuclear Physics 66 (2) (2011) 346.[4] N. Prantzos, et al., Monthly Notices of the Royal Astronomical Society 491 (2) (2019) 1832.[5] C. Travaglio, et al., ApJ 854 (1) (2018) 18.[6] R. Gallino, C. Arlandini, M. Busso, M. Lugaro, C. Travaglio, O. Straniero, A. Chie ffi , M. Limongi, ApJ 497 (1)(1998) 388–403.[7] S. Bisterzo, et al., ApJ 787 (1) (2014) 10.[8] S. Cristallo, C. Abia, O. Straniero, L. Piersanti, ApJ 801 (1) (2015) 53.[9] O. Trippella, et al., ApJ 787 (1) (2014) 41.[10] I. Dillmann, M. Heil, F. K¨appeler, R. Plag, T. Rauscher, F.-K. Thielemann, in: AIP Conf. Proc. 819, 123; online athttp: // / /2013).[15] M. Mastromarco, et al., Eur. Phys. J. A 55 (1) (2019) 9.[16] C. Guerrero, et al., Eur. Phys. J. A 49 (2) (2013) 27.[17] P. Schillebeeckx, et al., Nucl. Data Sheets 113 (12) (2012) 3054.[18] A. Borella, et al., Nucl. Instrum. & Methods A 577 (3) (2007) 626.[19] R. Macklin, J. Gibbons, Physical Review 159 (4) (1967) 1007.[20] S. Agostinelli, et al., Nucl. Instrum. & Methods A 506 (3) (2003) 250.[21] C. Massimi, et al., Journal of Korean Physical Society 59 (2011) 1689.[22] M. Barbagallo, et al., Eur. Phys. J. A 49 (12) (2013) 156.[23] N. M. Larson, Tech. rep., Oak Ridge National Lab., TN (US) (1998).[24] D. A. Brown, et al., Nucl. Data Sheets 148 (2018) 1.[25] F. Mingrone, , et al., Phys. Rev. C 95 (2017) 034604.[26] A. J. Koning, D. Rochman, Nucl. Data Sheets 113 (12) (2012) 2841.[27] I. Dillmann, R. Plag, F. K¨appeler, T. Rauscher, in: Proc. EFNUDAT 2009 Workshop, 2010, p. 55.[28] S. Cristallo, et al., ApJ Supplement Series 197 (2) (2011) 17.[29] S. Cristallo, O. Straniero, L. Piersanti, D. Gobrecht, ApJ Supplement Series 219 (2) (2015) 40.[30] K. Lodders, H. Palme, H. Gail, JE Tr¨umper 4 (2009) 44.[31] O. Trippella, et al., ApJ 818 (2) (2016) 125.[32] D. Vescovi, et al., Tech. rep., In preparation.[33] T. Rauscher, F.-K. Thielemann, Atomic Data and Nuclear Data Tables 75 (1-2) (2000) 1.[34] K. Takahashi, K. Yokoi, Atomic Data and Nuclear Data Tables 36 (3) (1987) 375.