Electric dipole response of sd-shell nuclei within the Configuration-Interaction Shell Model approach

TL;DR

This paper uses the Configuration Interaction shell model to systematically analyze the electric dipole response of sd-shell nuclei, achieving good agreement with experimental data and providing insights into low-energy dipole strength and pygmy resonances.

Contribution

It demonstrates the effectiveness of the shell-model approach in describing nuclear dipole excitations and offers detailed insights into the structure of low-energy dipole strength in light nuclei.

Findings

Good agreement with experimental photoabsorption data after operator renormalization.

Confirmation of the isovector nature of the Giant Dipole Resonance.

Identification of fragmented wave functions and isovector transition densities at low energies.

Abstract

Reliable theoretical predictions of nuclear dipole excitations are crucial for various nuclear applications, particularly in nuclear astrophysics. Calculations of radiative capture cross sections often rely on theoretical gamma strength functions, with the electric dipole response being the dominant component. We aim at a systematic description of the E1 strength of nuclei with mass numbers between 17 and 40 of interest for various applications and to better understand the nature of the low-energy dipole strength in neutron-rich nuclei, known as pygmy dipole resonances. We use the Configuration Interaction shell-model framework in the p-sd-pf valence space with a previously established empirical Hamiltonian. Systematic results of photoabsorption strength show good agreement with experimental data, provided a renormalization of the dipole operator is applied. Transition densities are computed in 26Ne and confirm the pure isovector character of the Giant Dipole Resonance. The strength at 7-10MeV is shown to have a distinct structure with largely fragmented wave functions and transition densities of isovector character at the edge of the nucleus. The Configuration Interaction shell model is proved to be a valuable tool in the description of the photoresponse of light nuclei, providing more accurate results than the usually employed approaches.