The s-process – overview and selected developments

Abstract

Abstract. Almost all of the heavy elements are produced via neutron capture reactions in a multitude of stellar production sites. The predictive power of the underlying stellar models is currently limited because they contain poorly constrained physics components such as convection, rotation or magnetic fields.Neutron captures measurements on heavy radioactive isotopes provide a unique opportunity to largely improve these physics components, and thereby address important questions of nuclear astrophysics. Such species are branch-points in the otherwise uniquely defined path of subsequent neutron captures along the s-process path in the valley of stability. These branch points reveal themselves through unmistakable signatures recovered from pre-solar meteoritic grains that originate in individual element producing stars.Measurements on radioactive isotopes for neutron energies in the keV region represent a stringent challenge for further improvements of experimental techniques. This holds true for the neutron sources, the detection systems and the technology to handle radioactive material. IntroductionAbout 50% of the element abundances beyond iron are produced via slow neutron capture nucleosynthesis (s process). Starting at iron-peak seed, the s-process mass flow follows the neutron rich side of the valley of stability via a sequence of neutron captures and β − decays (see Figure 1) synthesizing the elements between iron and bismuth.If different reaction rates are comparable, the s-process path branches and the branching ratio reflects the physical conditions in the interior of the star. Such nuclei are most interesting, because they provide the tools to effectively constrain modern models of the stars where the nucleosynthesis occurs. As soon as the β − decay is significantly faster than the typically competing neutron capture, no branching will take place. Therefore experimental neutron capture data for the s process are only needed, if the respective neutron capture time under stellar conditions is similar or smaller than the β − decay time, which includes all stable isotopes. Depending on the actual neutron density during the s process, the "line of interest" is closer to or farther away from the valley of stability.The modern picture of the main s-process component refers to the He shell burning phase in AGB stars [1]. Nuclei with masses between 90 and 209 are mainly produced during the main component. The highest neutron densities in this model occur during the 22 Ne(α,n) phase and are up to 10 12 cm −3 with temperatures around kT = 30 keV. The other extreme can be found during the 13 C(α,n) phase where neutron densities as low as 10 7 cm −3 and temperatures around kT = 5 keV are possible. Similarly to the main component, also the weak component referring to different evolutionary stages in massive stars has to phases [2,3]. Mainly nuclei with masses

A statistical method for the identification of stars enriched in neutron-capture elements from medium-resolution spectra

“…[1]), τ n , that is τ β << τ n , creating elements close to the floor of the valley of stability, which is formed by long-lived heavy nuclides (Busso et al 1999;Sneden et al 2008;Heil et al 2009;Käppeler et al 2011). Instead, in r-process reactions the time related to β-decay is large compared to the neutron capture time, τ β >> τ n , allowing the capture of several neutrons before the nuclei have time to undergo radioactive decay (Reifarth 2010;Thielemann et al 2011;Arnould et al 2007). As a result, the r-process typically synthesizes the heaviest isotopes of every heavy element.…”

Alpha-induced reactions on selenium between 11 and 15 MeV

“…The difficulty is that 85 Kr is a gaseous radioactive isotope with a half life of t 10.8 yr 1 2 = [12], which sets strict limits on the number of atoms possible inside a γ-calorimeter [13]. We plan to measure the 85 Kr(n, γ) cross section in the astrophysically interesting energy region between 1 and 100 keV at the FRANZ facility [14][15][16][17]. The production of the necessary material could be achieved by irradiating a sample of 82 Se with α-particles [18] and the material could be contained in titanium spheres [19,20].…”