Abstract
Neutron reactions are responsible for the formation of the elements heavier than iron. The corresponding scenarios relate to the He- and C- burning phases of stellar evolution (s process) and to supernova explosions (r and p processes). The s process, which is characterized by low neutron densities, operates in or near the valley of β stability and has produced about half of the elemental abundances between Fe and Bi in the solar system and in the Universe. Because the s abundances are essentially determined by the (n, γ ) cross sections along the reaction path, accurate neutron data constitute the key input for s process studies. Important constraints for the physical conditions at the stellar sites can be inferred by comparison of the abundance patterns from current s -process models with solar system material or presolar grains. The experimental methods for the determination of stellar (n, γ ) rates are outlined at the example of recent cross section measurements and remaining quests will be discussed with respect to existing laboratory neutron sources and new developments.
Highlights
The concept of neutron capture nucleosynthesis as the origin of the heavy elements beyond the Fe group has been formulated half a century ago in the pioneering work of Burbidge, Burbidge, Fowler and Hoyle (B2FH) [1] and of Cameron [2]
Since the formulation of these concepts, considerable progress has been achieved in the quantitative description of the s process, which provides rather detailed information of the s component in the solar system abundances as well as of the role the s process plays in galactic chemical evolution
The activation method represents a well established and accurate approach for Maxwellian averaged cross sections (MACS) measurements at a thermal energy of kT = 25 keV by irradiating samples in a quasi-stellar neutron spectrum that can be produced via the 7Li(, )7Be reaction [32, 33]
Summary
The concept of neutron capture nucleosynthesis as the origin of the heavy elements beyond the Fe group has been formulated half a century ago in the pioneering work of Burbidge, Burbidge, Fowler and Hoyle (B2FH) [1] and of Cameron [2]. The elements heavier than iron are predominantly produced either by the slow (s) or the rapid (r) neutron capture process, which are characterized by their typical neutron capture times with respect to the average -decay half lives Both processes are contributing about half of the observed solar abundances between Fe and U. In contrast to the s process, where neutron capture times of the order of days to years confine the reaction path to or close to the valley of stability, extremely high neutron densities of 1022 cm 3 are reached in the r process, giving rise to capture times of the order of milliseconds These parameters are clearly indicating an explosive scenario for the r process either related to supernovae or neutron star mergers. These p abundances are much smaller than the s and r components
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