Abstract
The abundance of heavy elements above iron through the rapid neutron capture process or r-process is intimately related to the competition between neutron capture and $\beta$ decay rates, which ultimately depends on the binding energy of atomic nuclei. The well-known Bethe-Weizsacker semi-empirical mass formula describes the binding energy of ground states in nuclei with temperatures of T~0 MeV, where the nuclear symmetry energy saturates between 23-26 MeV. Here we find a larger saturation energy of ~30 MeV for nuclei at T~0.7-1.3 MeV, which corresponds to the typical temperatures where seed elements are created during the cooling down of the ejecta following neutron-star mergers and collapsars. This large symmetry energy yields a reduction of the binding energy per nucleon for neutron-rich nuclei; hence, the close in of the neutron dripline, where nuclei become unbound. This finding constrains exotic paths in the nucleosynthesis of heavy elements -- as supported by microscopic calculations of radiative neutron-capture rates -- and further supports the universal origin of heavy elements, as inferred from the abundances in extremely metal-poor stars and meteorites.
New Thermonuclear Reaction Rate Equations for Radiative Neutron Capture
