Abstract
Neutron reactions are responsible for the formation of the elements heavier than iron. The corresponding scenarios relate to helium burning in Red Giant stars (s process) and to supernova explosions (r and p processes). The s process, which operates in or near the valley of β -stability, has produced about half of the elemental abundances between Fe and Bi. Accurate (n, γ ) cross sections are the essential input for s process studies, because they determine the abundances produced by that process. Following a brief summary of the neutron capture processes, the focus will be set on the s process in massive stars, where the role of reliable cross section information is particularly important. Eventually, the intriguing aspects of the origin of 60 Fe will be addressed. Attempts to determine the stellar cross section of that isotope are pushing experimental possibilities to their limits and present a pertinent challenge for future facilities.
Highlights
As introduced in the pioneering work by Burbidge, Burbidge, Fowler and Hoyle (B2FH) [1], the origin of the elements heavier than iron are ascribed to the slow and rapid neutron capture processes (s and r process), which are characterized by their typical time scales compared to average β-decay half lives
time of flight (TOF) measurements are essential for defining energy-differential neutron capture cross sections over a sufficiently large energy range that Maxwellian averaged cross sections (MACS) values can be determined from these data for any stellar temperature of interest
A setup with two optimized C6D6 detectors has been adopted by the n TOF collaboration for measuring the (n, γ) cross sections of the stable Fe and Ni isotopes, corresponding to the seed nuclei of the weak s process in massive stars and its immediate progeny [27]. This experimental campaign aims at the determination of the energy-differential cross sections, σ(En), in the entire neutron energy range needed for obtaining the MACS values up to a thermal energy of kT = 90 keV
Summary
As introduced in the pioneering work by Burbidge, Burbidge, Fowler and Hoyle (B2FH) [1], the origin of the elements heavier than iron are ascribed to the slow and rapid neutron capture processes (s and r process), which are characterized by their typical time scales compared to average β-decay half lives. Steady flow conditions have been reached in low mass stars, where the σ Ns function is practically constant in the mass region between Zr and Pb/Bi (apart from the neutron magic nuclei with N = 50, 82, and 128, which act as bottle necks for the reaction flow because of their very small (n, γ) cross sections) This behavior implies that the s abundances are directly anti-correlated with the respective stellar cross sections. The fact that flow equilibrium was not attained in massive stars has important consequences for the role of the stellar (n, γ) cross section in the mass region 60 < A < 90
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