The electric dipole response of $^{76}$Se above 4 MeV

TL;DR

This study investigates the electric dipole response of $^{76}$Se between 4 and 9 MeV using polarized photon scattering, revealing a pygmy dipole resonance and unobserved excitation strength, supported by theoretical calculations.

Contribution

It provides the first polarized photon scattering analysis of $^{76}$Se in this energy range, clarifies parity assignments, and combines experimental and theoretical insights.

Findings

Dipole response dominated by $E1$ excitations.

Evidence of unobserved excitation strength due to branching transitions.

Theoretical calculations support experimental observations.

Abstract

The dipole response of $^{76}_{34}$Se in the energy range 4 to 9 MeV has been analyzed using a $(\vec\gamma,{\gamma}')$ polarized photon scattering technique, performed at the High Intensity $\gamma$-Ray Source facility, to complement previous work performed using unpolarized photons. The results of this work offer both an enhanced sensitivity scan of the dipole response and an unambiguous determination of the parities of the observed J=1 states. The dipole response is found to be dominated by $E1$ excitations, and can reasonably be attributed to a pygmy dipole resonance. Evidence is presented to suggest that a significant amount of directly unobserved excitation strength is present in the region, due to unobserved branching transitions in the decays of resonantly excited states. The dipole response of the region is underestimated when considering only ground state decay branches. We investigate the electric dipole response theoretically, performing calculations in a 3D cartesian-basis time-dependent Skyrme-Hartree-Fock framework.