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

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 ¹⁶⁰Gd , specifically the K^π=4⁺ bands, as well as study the β decay of ¹⁶⁰Eu into ¹⁶⁰Gd .

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

Results: Two new excited states and ten new transitions were observed in ¹⁶⁰Gd . The β-decaying half-lives of the low- and high-spin isomers in ¹⁶⁰Eu were determined, and the low-spin state's half-life was measured to be t_1/2 = 26.0 ( 8 ) s, ≈ 16% shorter than previous measurements. Lifetimes of the two K^π = 4⁺ bandheads in ¹⁶⁰Gd 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 ¹⁶⁰Gd 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 ¹⁶⁰Gd 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 ¹⁶⁰Eu .