The abundances of the chemical elements heavier than iron can be attributed in about equal parts to the r and to the s process, which are taking place in supernova explosions and during the He and C burning phases of stellar

evolution, respectively. So far, quantitative studies on the extremely short-lived neutron-rich nuclei constituting the (n, γ) network of the r process are out of reach. On the contrary, the situation for the s-process is far advanced, as the reaction path of the s process from 12C to the Pb/Bi region is located within the valley of stability. Accordingly, a comprehensive database of experimental (n, γ) cross sections has been established. While for many stable isotopes the necessary accuracy is still to be reached, reliable cross sections for the involved unstable isotopes are almost completely missing. Because of the intrinsic γ background of radioactive samples, successful time-of-flight measurements are depending on intense pulsed

neutron sources. Such data are fundamental for our understanding of branchings in the s-process reaction path, which carry important model-independent information on neutron flux and temperature in the deep stellar interior

Käppeler, F.

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DOI 10.1393/ncc/i2015-15174-2Colloquia: UCANS-VIL NUOVO CIMENTO 38 C (2015) 174Astrophysical quests for neutron capture data of unstable nucleiF. Ka¨ppelerKarlsruhe Institute of Technology, Campus North, IKP - 76021 Karlsruhe, Germanyreceived 22 February 2016Summary. — The abundances of the chemical elements heavier than iron canbe attributed in about equal parts to the r and to the s process, which are takingplace in supernova explosions and during the He and C burning phases of stellarevolution, respectively. So far, quantitative studies on the extremely short-livedneutron-rich nuclei constituting the (n, γ) network of the r process are out of reach.On the contrary, the situation for the s-process is far advanced, as the reactionpath of the s process from 12C to the Pb/Bi region is located within the valleyof stability. Accordingly, a comprehensive database of experimental (n, γ) crosssections has been established. While for many stable isotopes the necessary accuracyis still to be reached, reliable cross sections for the involved unstable isotopes arealmost completely missing. Because of the intrinsic γ background of radioactivesamples, successful time-of-flight measurements are depending on intense pulsedneutron sources. Such data are fundamental for our understanding of branchings inthe s-process reaction path, which carry important model-independent informationon neutron flux and temperature in the deep stellar interior.PACS 26.20.Kn – s-process.PACS 26.50.+x – Nuclear physics aspects of novae, supernovae, and other explosiveenvironments.PACS 29.20.-c – Accelerators.PACS 97.10.Cv – Stellar structure, interiors, evolution, nucleosynthesis, ages.1. – Abundances and production scenariosBy the pioneering work of Burbidge, Burbidge, Fowler and Hoyle (B2FH) [1] andCameron [2] the origin of the heavy elements from the Fe group to the Pb/Bi region andup to the actinides has been attributed to the slow (s) and rapid (r) neutron captureprocesses, which are termed according to their typical neutron capture times being slowor rapid compared to average β-decay half-lives. Meanwhile, astrophysical scenarios areclearly identified for the s process, which takes place during the He and C burning phasesof stellar evolution [3,4]. For the r process, supernova explosions appeared to be the mostpromising sites, but as a conclusive model is still missing, neutron star mergers are beingconsidered as alternative scenarios [5].Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 F. KA¨PPELERWhile the s and r process are holding about equal shares of the observed solar-systemabundances between Fe and U [6], a third process, the so-called p (photodissociation)process, has been invoked for explaining the origin of about 32 rare, proton-rich nuclei,which, however, contribute less than 1% to the observed abundances in general.Among these processes, only the s process has been established quantitatively, yieldingmeanwhile rather detailed information of the s component in the solar system abundancesas well as of the role the s process plays in galactic chemical evolution. In fact, twodifferent s components have been identified, the main and the weak s process, which areascribed to specific phases of stellar evolution in stars of different mass.The main s process occurs in the He-burning layers of asymptotic giant branch (AGB)stars with M ≤ 3M [3, 7] and contributes predominantly to the mass range between90Zr and 209Bi. Neutrons are alternatingly produced by (α, n) reactions on 13C and 22Neat average neutron densities of about 107 and 108 cm−3, respectively.The weak s component, which contributes only to the abundances between Fe andZr, takes place in massive stars (M ≥ 8M) during convective core-He burning andsubsequently during convective shell-C burning [4,8]. The more efficient neutron exposureduring shell-C burning is characterized by fairly high neutron densities, which start atabout 1011–1012 cm−3 and then decrease exponentially.In contrast to the s process, where neutron capture times of the order of days to yearsconfine the reaction path to or close to the valley of β stability, extremely high neutrondensities of ≥ 1022 cm−3 are reached in the r process, giving rise to capture times of theorder of milliseconds, indicating an explosive scenario for the r process either related tosupernovae or neutron star mergers. In either case, the extreme neutron densities implythat (n, γ) reactions are becoming much more rapid than β decays, thus driving thereaction path close to the neutron drip line at the limits of stability. After the explosion,the initial, short-lived reaction products decay to the valley of stability.The reaction paths of the s and r process are sketched in fig. 1. The neutron-richisotopes outside the s path can be ascribed to the r process. Apart from these r-onlyisotopes the r process contributes also to most of the other isotopes, except for those,Fig. 1. – The main neutron-capture processes responsible for the observed abundances betweeniron and the actinides. The reaction path of the s process follows the stability valley, while ther-process path is shifted to very neutron-rich nuclei, which later decay (dashed arrows) and aremixing with the s component. Note also the occurrence of local branchings in the s-processpath (circles) at unstable isotopes with similar neutron capture and β decay rates.ASTROPHYSICAL QUESTS FOR NEUTRON CAPTURE DATA OF UNSTABLE NUCLEI 3which are shielded by stable isobars. The corresponding ensemble of s-only isotopesis important for the separation of the respective abundance distributions and, moreimportantly, to set constraints for the overall s-process efficiency and for the parametersgoverning the abundance pattern in s-process branchings. The subset of r-only nucleican be used as calibration points for the overall r-process distribution. About 32 stableisotopes on the proton-rich side, which cannot be produced by stellar neutron reactions,are attributed to the p process, which is likely to occur also in supernova explosions [9].It is to be noted for all s-process scenarios that the reaction path is confined to thevalley of β stability, because neutron capture times are on average much longer than βhalf-lives. This feature implies that the evolving s abundances are strongly correlatedwith the respective neutron capture cross sections, which, therefore, constitute the mostimportant nuclear physics input for any s-process study.2. – Neutron reactions and available dataThe neutron spectrum typical of the various s-process sites is described by a Maxwell-Boltzmann distribution, because neutrons are quickly thermalized in the dense stellarplasma. The effective stellar reaction cross sections are therefore obtained by the averag-ing the experimental data over that spectrum. The resulting Maxwellian averaged crosssections (MACS)〈σ〉kT = 2√π∫∞0σ(En) En e−En/kT dEn∫∞0En e−En/kT dEnare commonly compared for a thermal energy of kT = 30 keV, but for realistic s-processscenarios a range of thermal energies has to be considered, from about 8 keV typical ofthe 13C(α, n) source in low mass AGB stars to about 90 keV during shell-C burning inmassive stars. To cover this full range, energy-differential cross sections σ(En) are neededin the energy region 0.1 ≤ En ≤ 500 keV.Although an extensive set of experimental cross sections is meanwhile available, thesituation is far from being satisfactory. The data for many of the stable isotopes do notmeet the required accuracy of ≤ 5%, are discrepant, or cover only part of the energy rangeof interest. For the set of unstable nuclei, which play an essential role in understandingthe s-process branchings, reliable data are yet largely missing, because they are verydifficult to access experimentally.The current status has been summarized in the KADoNiS compilation [10,11], wheredata sets for 356 isotopes, including 77 radioactive nuclei on or close to the s-processpath are listed. While measurements exist for the (n, γ) cross sections of almost all 277stable isotopes, experimental data are available for only 14 radioactive nuclei, includingthe branch points 63Ni, 129I, 135Cs, 147Pm, 151Sm, 154Eu, and 163Ho. However, mostmeasurements on unstable isotopes have been carried out via activation in a quasi-stellarspectrum for kT = 25 keV [12], thus requiring extrapolation to lower and higher tempera-tures by means of theoretical cross sections obtained with the Hauser-Feshbach statisticalmodel that are typically affected by uncertainties of 25 to 30%.The MACS values of unstable isotopes are crucial for the interpretation of s-processbranchings as illustrated in fig. 2. The local abundance pattern produced by the branch-ings is reflecting the stellar neutron flux that determines the neutron capture flow atthe branching point compared to the strength of the β-decay channel. In some cases,however, this mechanism is more complicated, because the stellar β-decay rate can be4 F. KA¨PPELERFig. 2. – The s-process network in the Sm-Eu-Gd region with branchings of the reaction path at151Sm, 152,154,155Eu, and 153Gd. Due to the competition between neutron capture and β-decay,the abundance patterns of branchings carry important information on neutron flux and temper-ature at the s-process site.enhanced at stellar temperatures. The branching at 151Sm is of particular interest be-cause the 93 yr half-life of 151Sm decreases by about a factor of 30 in an s-processenvironment [13]. Accordingly, branchings can be considered as diagnostic tools for thestellar interior, provided that temperature and neutron flux can be constrained by recon-structing the abundance pattern on the basis of reliable cross section data for the involvedisotopes.3. – Laboratory work: activation vs. time of flightStellar (n, γ) cross sections are commonly measured either by activation in a quasi-stellar neutron field [12] or using the time-of-flight (TOF) technique. The activationmethod has the advantage of superior sensitivity, thus allowing one to limit the samplematerial to sub-μg amounts and to avoid excessive backgrounds from the activity of thesample. For most branch points, however, activation is not applicable, because they aresituated between stable isotopes (see 151Sm in fig. 2).In view of the relevant keV energies, TOF measurements of stellar cross sectionsdepend on accelerator-based neutron facilities, using either moderated neutrons producedby high-energy pulsed electron or proton beams or neutrons directly originating from(p, n) reactions with low-energy protons. While moderated sources, e.g. GELINA atGeel/Belgium [14,15], n TOF at CERN [16], or LANSCE at Los Alamos [17], cover verywide neutron energy ranges from thermal to nearly the beam energy, the neutron spectraproduced with low-energy machines can be tailored to the astrophysically relevant keVregion, e.g. at the FRANZ facility in Frankfurt/Germany [18,19].Compared to activation measurements the sensitivity of the TOF method suffers fromthe lower (pulsed) beam currents and from the (much) larger distance between neutronsource and sample. These drawbacks must be compensated by increasing the samplemass, by increasing the peak currents of the pulsed neutron sources, or by reducing theflight path. Because the sample mass for unstable isotopes is limited by the specificγ-activity, neutron flux and flight path have to optimized instead. The key parameterfor TOF experiments is the achievable flux at the sample position, which is compared infig. 3 for the main existing and future facilities. The expected performance of FRANZ isbased on the experience with the Karlsruhe Van de Graaff accelerator, scaled with thebeam current on target.ASTROPHYSICAL QUESTS FOR NEUTRON CAPTURE DATA OF UNSTABLE NUCLEI 5Fig. 3. – Integrated neutron flux in the astrophysically relevant energy interval from 10 to100 keV at existing TOF facilities and at the future FRANZ accelerator.While the 20m stations at LANSCE and n TOF represent the best choices amongthe moderated sources, the shorter flight path at FRANZ of only 1 m will make thisdesign a very attractive option for measurements on unstable samples. Note, however,that additional aspects for such measurements, e.g. the resolution in neutron energy, theeffect of different repetition rates, γ-flash intensities, and facility-specific backgroundsare not considered in this comparison.4. – OutlookIn summary, intense pulsed sources of keV neutrons are instrumental for a wealthof applications in astrophysics, especially for cross section measurements on unstableisotopes. At present, TOF measurements on most s-process branch point isotopes ap-pear to be feasible and are partially under investigation at n TOF/CERN [20]. Apartfrom these s-process aspects, there is also a multitude of requests for MACS data con-cerning other scenarios with even higher neutron densities. Under certain conditions,stars may experience convective-reactive nucleosynthesis episodes if H-rich material isconvectively mixed into a He-burning zone [21, 22], resulting in neutron densities up toabout 1015 cm−3, a situation intermediate between the s and the r-process. Accordingly,the neutron-capture path is shifted by a few mass units from the valley of stability, asneutron capture times are only minutes or hours. Explosive nucleosynthesis in the rand p process are another field, where neutron reactions on unstable isotopes are of keyimportance, essentially during the freeze-out phase when free neutrons are captured bythe primary reaction products.The data requests for these scenarios are yet outside the reach of existing facili-ties. Once FRANZ starts operation, it might be possible to reduce the flight path toabout 10 cm, thus extending the possibilities for measurements on unstable samples sig-nificantly. Recently, neutron-induced reactions in inverse kinematics have been pro-posed [23] by combining a beam of low-energy radioactive ions cycling in a storagering similar to the CRYRING at GSI/FAIR [24] with a reactor core as the neutrontarget.6 F. KA¨PPELERREFERENCES[1] Burbidge E., Burbidge G., Fowler W. and Hoyle F., Rev. Mod. 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Astrophysical quests for neutron capture data of unstable nuclei

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Astrophysical quests for neutron capture data of unstable nuclei

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