Speaker
Description
Accurate neutron-capture and photodisintegration reaction rates within the Hauser-Feshbach statistical framework are strongly governed by the nuclear $\gamma$-ray strength function and the nuclear level density. Uncertainties in these key nuclear inputs propagate directly into Maxwellian-averaged cross sections and constitute one of the dominant sources of uncertainty in modeling the astrophysical $r$-process. In this work, we examine the role of low-lying pygmy dipole strength in electric dipole transitions and assess its impact on $(n,\gamma)$ and $(\gamma,n)$ reaction rates in neutron-rich nuclei. The $\gamma$-ray strength functions are computed using a fully self-consistent relativistic quasiparticle random-phase approximation based on the DD-PCX energy density functional. These microscopic strength functions are then employed as input to Hauser-Feshbach calculations of astrophysical reaction rates. Our results demonstrate that enhancements in reaction rates are primarily dictated by the energetic alignment of the pygmy dipole strength with the neutron separation threshold, rather than solely by the total amount of low-energy dipole strength. When the pygmy mode is located close to the neutron threshold, both neutron-capture and photodisintegration reaction rates can be significantly enhanced. Pronounced rate enhancements are observed in nuclei such as $^{68}$Ni and $^{132}$Sn, where this alignment occurs. While thermal averaging moderates large local cross-section enhancements, the resulting rate increases remain astrophysically significant. For photodisintegration reactions, pygmy dipole effects become particularly important in very neutron-rich nuclei with low neutron separation energies, again leading to notable modifications when dipole strength and threshold energies coincide. These findings highlight the essential role of a realistic microscopic description of low-energy dipole strength near the neutron threshold for reliable $r$-process modeling and underscore the importance of close synergy between theoretical developments and experimental investigations of dipole response in neutron-rich nuclei.