Nuclear Physics of thesProcess

Abstract

Abstract:Starting from a sketch of the s-process concept formulated 50 years ago, the nuclear physics data for s-process calculations are briefly reviewed with emphasis on the status of neutron capture cross sections and beta decay rates. Accurate and comprehensive experimental data are mandatory as direct input for s-process calculations as well as for improving the complementary information from nuclear theory. The current challenges of the field are discussed in the light of new or optimized methods and state-of-theart facilities, indicating the potential for accurate measurements and the possibility to study cross sections of radioactive isotopes. These opportunities will be considerably enriched by the enormous improvements provided by new facilities. The s Process in RetrospectLooking back on the founding papers of Nuclear Astrophysics published 50 years ago by Burbidge et al. (1957, hereafter referred to as B 2 FH) and by Cameron (1957) it is inspiring to see that already the entire s-process panorama was sketched in its essential parts. The visionary view of these papers is the more admirable as the basics were still pretty incomplete. This is certainly also true for the s process. In spite of the fact that neither the neutron source reactions nor the neutron capture cross sections in the astrophysically relevant energy range were known apart from some rudimentary information, all essential features had been inferred from these meager information. The product of the stellar (n, γ) cross sections and of the resulting s abundances, σ N s , which represents the reaction flow, was found to be a smooth function of mass number A. From the composite slope of this function two different s processes were postulated. The steep decline between A ≈ 63 and 100 was interpreted as the result of an s-process site with not enough neutrons available per 56 Fe seed to build the nuclei to their saturation abundances. In the mass region beyond A ≈ 100, the much smaller slope was suggesting that steady flow was achieved and that all of these nuclei reached their saturation abundances. It was concluded that 'the two different processes might have occurred in two different red-giant stars ' (Burbidge et al. 1957), completely consistent with what is now called the weak s process during core He burning 1,2 and shell C burning (Raiteri et al. 1991(Raiteri et al. , 1993Limongi et al. 2000) in massive stars 1 http://www.ucolick.org/∼alex 2 http://www.mporzio.astro.it/∼limongi on the one hand and the main s component contributed by thermally pulsing low mass asymptotic giant branch (AGB) stars (Straniero et al. 1995;Gallino et al. 1998;Arlandini et al. 1999) on the other hand. Eventually, the identification of the s-process sites has been shown to be an important ingredient for quantitative studies of galactic chemical evolution (Travaglio et al. 1999(Travaglio et al. , 2001(Travaglio et al. , 2004.The main difficulty that prevented further conclusions was due to the lack of reliable neutron capture cross sections. In total, the cros...

Activation Experiments forp-Process Nucleosynthesis

“…During stellar burning phases, the slow neutron capture process (s process) takes place on a time scale of several 10000 years at moderate temperatures of about 10 8 K and neutron densities ranging from 10 6 cm −3 to 10 12 cm −3 . Given these conditions, a reaction path is determined by a series of subsequent neutron-capture reactions and β decays that is close to the valley of β stability [1,2,3]. In contrast, the rapid neutron-capture process (r process) happens in explosive scenarios lasting only for a few seconds and producing extreme conditions: Temperatures of more than 10 9 K are combined with neutron-densities higher than 10 20 cm −3 .…”

International workshop on next generation gamma-ray source

“…It is thought to occur in explosive scenarios such as supernovae [78,79] and was recently verified in neutron star merger via gravitational wave observations [80]. In contrast, the average neutron densities during s-process nucleosynthesis (s: slow neutron capture) are rather small (around 10 8 cm −3 ), so that the neutron capture rate λ n is normally well below the β-decay rate λ β , and the reaction path is therefore close to the valley of β stability [81][82][83]. However, during the peak neutron densities, branching occurs at unstable isotopes with half-lives as low as several days.…”

Expanding Nuclear Physics Horizons with the Gamma Factory

“…[ 72,192 ] In contrast, average neutron densities during s‐process nucleosynthesis (s: slow neutron capture) are rather small (≈10 8 cm −3 ), that is, the neutron capture rate $$\lambda _\text{n}$$ is normally well below the β‐decay rate $$\lambda _{\beta }$$ and the reaction path is close to the valley of β stability. [ 193–195 ] However, when the s‐process reactions occur at peak neutron densities, reaction path branchings occur at unstable isotopes with half‐lives as low as several days. The half‐lives at these branching points are normally known with high accuracy, at least under laboratory conditions.…”