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A sudden onset of deformation is observed in the ground states of Zr and Sr appearing sharply at $N=60$ [1]. This unique feature will be discussed in the context of state-of-the-art Monte Carlo Shell Model (MCSM) calculations which were the first to successfully reproduce the rapid increase of collectivity and dramatic changes in the low-energy spectra of the Zr isotopes [2]. According to the MCSM calculations, an inversion of nearly-spherical and well-deformed configurations in $^{98}$Zr and $^{100}$Zr appears with small to no mixing, causing an abrupt ground-state shape transition. Furthermore, the presumably prolate ground state of $^{100}$Zr is predicted to coexist with multiple excited configurations possessing different intrinsic shapes.
The MCSM theoretical predictions were put to the test in a recent $\beta$-decay study of $^{100}$Zr performed at the TRIUMF ISAC I facility. A mixture of $^{100}$Rb and $^{100}$Sr ions was delivered onto a mylar tape in the center of the powerful GRIFFIN spectrometer [3], comprising $15$ large-volume HPGe clover detectors coupled to seven LaBr$_3$ detectors for fast-timing lifetime measurements. The collected $\gamma-\gamma$ coincidence data was used to substantially extend the level scheme of $^{100}$Zr and firmly assign the spins of key states via $\gamma-\gamma$ angular correlations. Those include several newly found $0^+$ states, some of which were recently reported in Ref. [4], and a potential candidate spin-2 member of a band built on the $0_4^+$ state [5].
Selected results will be presented, including the lifetime of the $2_2^+$ state, extracted for the first time in this work. The shape-coexistence scenario in $^{100}$Zr will be put on firm ground and striking structural similarities between $^{100}$Zr and $^{98}$Sr [6] will be revealed.
[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] A.B. Garnsworthy et al., NIM Phys. Res. A, 918 (2019).
[4] J. Wu et al., Phys. Rev. C 109, (2024) 024314.
[5] D. Kalaydjieva et al., Acta Phys. Pol. Proc. Supp. 16 (2023) 4-A15.
[6] E. Clément et al., Phys. Rev. Lett. 116 (2016) 022701.
