Speaker
Description
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 $\beta$-decay rates, which ultimately depends on the binding energy of neutron-rich nuclei. The well-known Bethe-Weizs\"acker semi-empirical mass formula [1,2] describes the binding energy of ground states -- i.e. nuclei with temperatures of $T\approx0$ 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, $T\approx0.7-1.0$ 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 [3] or collapsars\cite{collapsar}, the fact that the symmetry energy remains constant between $T\approx0.7-1.0$ MeV, suggests a similar trend down to $T\approx0.5$ 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, 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.
[1] C. F. von Weizsacker, Zur Theorie der Kernmassen. Z. Phys. 96 (1935) 431.
[2] H. A. Bethe and R. F. Bacher, Stationary States of Nuclei, Rev. Mod. Phys. 8 (1936) 82.
[3] F. -K. Thielemann, M. Eichler, I. V. Panov, and B. Wehmeyer, Neutron Star Mergers and Nucleosynthesis of Heavy Elements, Annu. Rev. Nucl. Part. Sci. 67 (2017) 253.
[4] D. M. Siegel, J. Barnes, and B. D. Metzger, Collapsars as a major source of r-process elements, Nature 569 (2019) 241.
