Speaker
Description
Accurate neutron-capture and photodisintegration reaction rates obtained within the Hauser-Feshbach statistical framework depend critically on the nuclear $\gamma$-ray strength function and nuclear level density. Uncertainties in these quantities propagate directly into Maxwellian-averaged cross sections and remain a major source of uncertainty in modeling $r$-process nucleosynthesis. In this work, we investigate the impact of low-lying pygmy dipole strength on electric dipole transitions and its consequences for $(n,\gamma)$ and $(\gamma,n)$ reaction rates in neutron-rich nuclei. The $\gamma$-ray strength functions are calculated using a fully self-consistent relativistic quasiparticle random-phase approximation based on the DD-PCX energy density functional and are subsequently employed in Hauser-Feshbach calculations of astrophysical reaction rates. We demonstrate that reaction-rate enhancements are governed greatly by the energetic alignment of the pygmy dipole strength with the neutron separation threshold, rather than only by the total amount of low-energy dipole strength. When the pygmy mode lies close to the neutron threshold, neutron capture and photodisintegration reactions are significantly affected. We find pronounced rate enhancements in nuclei such as $^{68}$Ni and $^{132}$Sn, where this alignment occurs, while thermal averaging reduces large local cross-section enhancements to more moderate but still astrophysically relevant rate increases. For photodisintegration reactions, pygmy dipole effects become important in very neutron-rich nuclei with low neutron separation energies, again leading to the notable modifications when dipole strength and threshold energies coincide. These results underline the crucial role of a realistic microscopic description of low-energy dipole strength near the neutron threshold for predictive $r$-process modeling and emphasize the need for close synergy between theory and experiments probing dipole response in neutron-rich nuclei.