Threshold-Aligned Pygmy Dipole Strength in Astrophysical $(n,\gamma)$ and $(\gamma,n)$ Reactions

TL;DR

This paper demonstrates that the alignment of pygmy dipole strength with neutron separation energy significantly influences reaction rates in r-process nucleosynthesis, emphasizing the need for precise microscopic models and experimental data.

Contribution

It reveals that the reaction-rate enhancements depend on the energy alignment of pygmy dipole strength with neutron separation, not just the total low-energy strength.

Findings

Reaction-rate enhancements are governed by PDS energy alignment with $S_n$.

Nuclei like $^{68}$Ni and $^{132}$Sn show strong effects due to this alignment.

Accurate microscopic descriptions of low-energy dipole strength are crucial for reliable r-process modeling.

Abstract

Reaction-rate calculations relevant to r-process nucleosynthesis depend sensitively on the nuclear $\gamma$-strength function ($\gamma$SF). Here we investigate the impact of low-lying pygmy dipole strength (PDS) in $(n,\gamma)$ and $(\gamma,n)$ reactions using $\gamma$SF based on relativistic nuclear energy density functional theory and propagate these strengths into Hauser--Feshbach statistical model calculations of the reaction rates. We show that considerable reaction-rate enhancements at temperatures relevant for r-process nucleosynthesis are governed by the alignment of the pygmy dipole strength energy with the neutron separation threshold $S_n$ rather than by the total low-energy strength. Consequently, nuclei such as $^{68}$Ni and $^{132}$Sn, where the PDS energy-$S_n$ alignment occurs, exhibit the strongest effects on reaction-rate enhancements. These results demonstrate that modeling reliable reaction rates in r-process nucleosynthesis necessitates accurate microscopic descriptions of low-energy dipole strength, in close synergy with experimental investigations in the vicinity of neutron threshold.