Decay spectroscopy of $^{160}\mathrm{Eu}$: Quasiparticle configurations of excited states and structure of ${K}^{\ensuremath{\pi}}={4}^{+}$ bandheads in $^{160}\mathrm{Gd}$

Decay spectroscopy of ${}^{160}{Eu}$ : Quasiparticle configurations of excited states and structure of $K^{π} = 4^{+}$ bandheads in ${}^{160}{Gd}$

D. Yates et al.

Phys. Rev. C 107, 064309 – Published 15 June 2023

Abstract

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the $r$ -process path and following freeze-out region impact the resulting $r$ -process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei.

Purpose: We aim to improve the understanding of the nuclear structure of ${}^{160}{Gd}$ , specifically the $K^{π} = 4^{+}$ bands, as well as study the $β$ decay of ${}^{160}{Eu}$ into ${}^{160}{Gd}$ .

Methods: High-statistics decay spectroscopy of ${}^{160}{Gd}$ resulting from the $β$ -decay of ${}^{160}{Eu}$ was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility.

Results: Two new excited states and ten new transitions were observed in ${}^{160}{Gd}$ . The $β$ -decaying half-lives of the low- and high-spin isomers in ${}^{160}{Eu}$ were determined, and the low-spin state's half-life was measured to be $t_{1 / 2} = 26.0 (8)$ s, $\approx 16 %$ shorter than previous measurements. Lifetimes of the two $K^{π} = 4^{+}$ bandheads in ${}^{160}{Gd}$ were measured for the first time, as well as $γ − γ$ angular correlations and mixing ratios of intense transitions out of those bandheads.

Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the $K^{π} = 4^{+}$ bandheads in ${}^{160}{Gd}$ is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in ${}^{160}{Gd}$ heavily populated in $β$ decay is shown to have positive parity, which raises questions regarding the structure of the high-spin $β$ -decaying state in ${}^{160}{Eu}$ .