Astromers in the radioactive decay of r-process nuclei

Abstract

We study the impact of astrophysically relevant nuclear isomers (astromers) in the context of the rapid neutron capture process (r-process) nucleosynthesis. We compute thermally mediated transition rates between long-lived isomers and the corresponding ground states in neutron-rich nuclei. We calculate the temperature-dependent β-decay feeding factors which represent the fraction of material going to each of the isomer and ground state daughter species from the β-decay parent species. We simulate nucleosynthesis by including as separate species nuclear excited states with measured terrestrial half-lives greater than 100 µs. We find a variety of isomers throughout the chart of nuclides are populated, and we identify those most likely to be influential. We comment on the capacity of isomer production to alter radioactive heating in an r-process environment.

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

“…In this paper, we treat nuclear isomers as in Refs. [31,43] and extend the results of those works. In particular, we examine the impact of the unknown properties of the intermediate states that facilitate GS ↔ isomer transitions in neutron-rich r-process nuclei.…”

“…We included in our study those nuclei with isomers in the neutron-rich r-process region between mass numbers A = 69 and A = 209 with half-lives T 1/2 > 100 µs. We highlight the specific astromers of likely import in the r process listed in Table I of Misch et al [31]. In identifying those astromers, that work held the Weisskopf rates fixed and used a single temperature-density trajectory [46,47] in the Jade network of Sprouse et al [43].…”

“…In what follows, we present detailed results for the nuclei appearing in Table I of Misch et al [31]. For each detailed isotope, we show a figure indicating the uncertainty bands for the effective transition rates in each of these nuclei.…”

“…The ground state of 26 Al has a β-decay half-life of ∼700 kyr, while its isomer decays with a half-life of ∼6 s. This nuclide-the first radioisotope to be observed in the heavens-is an important tracer of star formation [26][27][28]. Other well-known nuclei with astromers include 34 Cl (possibly visible in nova bursts, isomer at 146 keV [20,29]) and 85 Kr (in the slow neutron capture (s) process a branch point, in the r process a β-decay accelerant and possible electromagnetic source; isomer at 305 keV [30,31]). Isomers, including the 130 keV isomer in 38 K, may play a role in the rapid proton capture process (rp process) [32][33][34][35].…”

“…In their study, several hand-picked isomers in the second r-process peak were shown to impact the radioactive heating of a kilonova. Misch et al [31] recently studied the dynamic population, de-population, and decay of nuclear isomers in the r process by including in a radioactive-decay network all isomers in the ENSDF database with a half-life greater than 100 µs. As the temperature of an r-process event drops below each isomer's thermalization temperature (temperature below which the nuclide cannot reach thermal equilibrium), the isomer will freeze out as an astromer and affect the subsequent heating and evolution of isotopic abundances.…”