Hot pygmy dipole strength in nickel isotopes

TL;DR

This study investigates how finite temperature influences low-energy dipole strength in nickel isotopes, revealing significant temperature-dependent enhancements in pygmy dipole strength, especially in neutron-rich nuclei, with implications for astrophysics.

Contribution

It provides the first comprehensive theoretical analysis of hot pygmy dipole strength in Ni isotopes across a range of temperatures using finite-temperature RQRPA.

Findings

Neutron-rich Ni isotopes show up to 2.5 times more dipole strength at higher temperatures.

Near N=Z isotopes develop pronounced hot pygmy dipole strength as temperature increases.

Predicted E1 strength and B(E1) values serve as benchmarks for experimental validation.

Abstract

At finite temperatures, nuclear excitations are significantly modified, most notably through the emergence of additional low-energy dipole strength, which can critically impact astrophysical reaction rates. Ongoing fusion-evaporation experiments on Ni isotopes provide a unique opportunity to investigate the hot pygmy dipole strength (HPDS), underscoring the need for reliable theoretical predictions and a comprehensive understanding of this emerging phenomenon. In this work, the HPDS is investigated in Ni isotopes from $N = Z$ to neutron-rich systems ($^{56\text{--}70}$Ni) over a temperature range of $T=$ 0$-$2~MeV using the finite-temperature relativistic quasiparticle random phase approximation. In neutron-rich Ni isotopes, the pygmy dipole strength at higher temperatures exceeds up to 2.5 times its value observed at zero temperature. In contrast, near $N \approx Z$ isotopes show negligible low-energy dipole strength at $T = 0$ MeV but develop a pronounced HPDS as the temperature increases. Predicted E1 energy-weighted strength ($S_{\text{EWS}}$) and cumulative $B$(E1) values for HPDS are presented across the Ni isotopic chain for various low-energy intervals and temperatures, providing essential benchmarks to support and guide experimental studies.