2021/10/02 by Orce, José Nicolás, Dey, Balaram, Ngwetsheni, Cebo +4
#FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE) #Nuclear Experiment (nucl-ex) #Nuclear Theory (nucl-th)
paper · doi:10.48550/arxiv.2110.00713
The abundance of about half of the stable nuclei heavier than iron via the rapid neutron capture process or r-process is intimately related to the competition between neutron capture and β-decay rates, which ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe-Weizsäcker semi-empirical mass formula\citeweiz,bethe describes the binding energy of ground states -- i.e. nuclei with temperatures of T≈0 MeV -- with the symmetry energy parameter converging between 23-27 MeV for heavy nuclei. Here we find an unexpected enhancement of the symmetry energy at higher temperatures, T≈0.7-1.0 MeV, from the available data of giant dipole resonances built on excited states. Although these are likely the temperatures where seed elements are created -- during the cooling down of the ejecta following neutron-star mergers\citemergersnucleo or collapsars\citecollapsar -- the fact that the symmetry energy remains constant between T≈0.7-1.0 MeV, suggests a similar trend down to T≈0.5 MeV, where neutron-capture may start occurring. Calculations using this relatively larger symmetry energy yield a reduction of the binding energy per nucleon for heavy neutron-rich nuclei and inhibits radiative neutron-capture rates. This results in a substantial close in of the neutron dripline -- where nuclei become unbound -- which elucidates the long sought universality of heavy-element abundances through the r-process; as inferred from the similar abundances found in extremely metal-poor stars and the Sun.