Multiple shape coexistence in ¹⁰⁰Zr

The ground states of nuclei with A ≈ 100 undergo a sudden shape transition, appearing sharply at N = 60. Equally dramatic changes are observed in the energy spectra, suggesting a possible shape-coexistence scenario. These unique features of Zr isotopes were successfully reproduced by large-scale Monte-Carlo shell model (MCSM) calculations. The coexistence of states with multiple different intrinsic shapes was predicted, accompanied by an inversion of configurations in ⁹⁸Zr and ¹⁰⁰Zr, appearing with small to no mixing due to type-II shell evolution.

A β-decay study of ¹⁰⁰Y was performed at the TRIUMF-ISAC facility in November 2021. A radioactive ion beam mixture of ¹⁰⁰Rb and ¹⁰⁰Sr was used to populate the nucleus of interest in a series of β decays. The experimental setup consisted of 15 large-volume GRIFFIN HPGe detectors, coupled to seven LaBr₃ detectors for fast-timing lifetime measurements.

Using the rich γ-ray coincidence data, the level scheme of ¹⁰⁰Zr was largely extended. The spins of several key states were unambiguously assigned via γ-γ angular correlations, including several newly observed low-lying 0⁺ states. The lifetime of the 2₂⁺ state was measured for the first time, allowing to put on firm ground the shape-coexistence scenario.

Selected results will be presented and discussed in the context of the MCSM calculations. Structural similarities between ¹⁰⁰Zr and ⁹⁸Sr, for which shape coexistence has already been established, will be highlighted.