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The sudden onset of deformation in $A\approx100$ nuclei at $N=60$ has been described as a ground-state shape transition that has raised a lot of interest over the years from an experimental and theoretical point of view [1]. This transition is most pronounced in the Zr and Sr isotopic chains where the low-energy excited-state structure shows significant signs of deformation developing at $N=60$, as opposed to the spherical-like structure observed at $N\leq58$.
At present, the two most promising theoretical interpretations of this phenomenon are given by the Monte Carlo Shell Model (MCSM) [2] and the Interacting Boson Model with Configuration Mixing (IBM-CM) [3]. The MCSM calculations interpret the structure of $^{100}$Zr within a multiple-shape-coexistence scenario with several distinct deformed shapes predicted for the lowest $0^+$ states, with rotational bands built on top of them. In contrast, the IBM-CM calculations predict a weakly-deformed "intruder" ground-state configuration in $^{100}$Zr, with corresponding $\beta$ and $\gamma$ bands, and a low-lying spherical "normal" configuration.
In order to test these theoretical models an experiment was performed at the TRIUMF-ISAC facility to investigate the structure of $^{100}$Zr following the $\beta$ decay of $^{100}$Y by utilizing the GRIFFIN $\gamma$-ray spectrometer [4]. The 15 HPGe clover detectors of GRIFFIN were coupled with seven LaBr$_3$ detectors for fast-timing lifetime measurements using the Generalized Centroid Difference method [5].
Mainly low-spin excited states were populated in the $\beta$ decay of the $1^-$ state in $^{100}$Y. This allowed for the lifetimes of several key non-yrast excited states in $^{100}$Zr, including those of the $2^+_3$, and $0^+_3$ states, to be extracted for the first time in this study. These new results will be presented and compared to the MCSM and IBM-CM theoretical predictions.
[1] P.E. Garrett et al., Prog. Part. Nucl. Phys. 124 (2022) 103931.
[2] T. Togashi et al., Phys. Rev. Lett. 117 (2016) 172502.
[3] N. Gavrielov et al., Phys. Rev. C 99, 064324 (2019).
[4] A.B. Garnsworthy et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 918 (2019).
[5] J.-M. Régis et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 726 (2013).