Astromers in the Radioactive Decay of r-process Nuclei

摘要

Certain nuclear isomers are well known to affect nucleosynthesis with important observable consequences (e.g., Al-26 and Ta-180). We study the impact of nuclear isomers in the context of rapid neutron capture process (r-process) nucleosynthesis. We demonstrate that nuclear isomers are dynamically populated in the r process and that some are populated far from thermal equilibrium; this makes them astrophysical isomers, or "astromers." We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent beta-decay feeding factors, which represent the fraction of material going to each of the isomer and ground state daughter species from the beta-decay parent species. We simulate nucleosynthesis following the decay of a solar-like r-process composition and include as separate species nuclear excited states with measured terrestrial half-lives greater than 100 mu s. We introduce a new metric to identify those astromers most likely to be influential and summarize them in a table. Notable entries include many second peak nuclei (e.g., the Te isotopic chain) and previously overlooked isomers in stable nuclei (e.g., Sn-119, Xe-131, and Pt-195). Finally, we comment on the capacity of isomer production to alter radioactive heating in an r-process environment.