Decay Spectroscopy of ¹⁶⁰Eu: Quasiparticle Configurations and Structure in ¹⁶⁰Gd

Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. The structure of nuclei along the r-process path and in the region involved in the freeze-out impact the resulting r-process final abundance distribution. Knowledge of the structure of nuclei midshell in both proton and neutron number around mass A≈165 helps to understand the evolution of subshell gaps and deformation parameters in these nuclei and the r-process flow around the rare-Earth peak.

To study this region, high-statistics decay spectroscopy data of ¹⁶⁰Gd resulting from the β-decay of ¹⁶⁰Eu was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Two new excited states and ten new transitions were observed in ¹⁶⁰Gd, as well as confirmation of the majority of known β-delayed transitions in ¹⁶⁰Gd. Lifetimes of the two K^π=4⁺ band-heads in ¹⁶⁰Gd were measured for the first time, as well as γ-γ angular correlations and mixing ratios of intense transitions out of those band-heads. These new measurements provide insight into the structure of the K^π=4⁺ bands in ¹⁶⁰Gd. Additionally, the state at 2 MeV in ¹⁶⁰Gd, which is strongly populated in β-decay from the high-spin β-decaying state in ¹⁶⁰Eu, is shown to have positive parity. This result raises questions regarding the established structure of the β-decaying states in ¹⁶⁰Eu.