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A sudden ground-state shape transition is known to occur sharply at $N=60$ for several nuclei in the $A\approx\!100$ region [1]. Dramatic changes are observed in the energy spectra of Sr and Zr, including an appearance of low-energy $0^+$ states that are associated to competing configurations characterized by different nuclear shapes. In contrast, in Mo isotopes the ground state shape evolution appears to be more gradual, in accordance with the moderate change in $E_x(2_1^+)$ across $N=60$ [1], proposed to be the result of emerging triaxiality [2].
Recent state-of-the-art Monte Carlo Shell Model (MCSM) calculations [3] reproduced the ground-state band properties throughout the Zr isotopic chain and suggested the appearance of multiple shape coexistence in $^{100}$Zr. In addition, it was proposed that the abrupt shape transition at $N=60$ is caused by an inversion of a spherical and prolate-deformed configurations, corresponding to the ground states of $^{98}$Zr and $^{100}$Zr, respectively, appearing with small to no mixing between them due to type-II shell evolution [3].
To test the MCSM predictions and investigate in detail this fascinating region, a $\beta$-decay study of $A=100$ isotopes was carried out at the TRIUMF-ISAC facility. A radioactive ion beam mixture of $^{100}$Rb and $^{100}$Sr was used and population of excited states in isotopes ranging from $^{100}$Sr to $^{100}$Mo was observed. The powerful GRIFFIN array [4] coupled to a tape station allowed to explore the level structure of several nuclei with a main focus on $^{100}$Zr ($N=60$). In addition, $^{100}$Mo ($N=58$) was studied with the aim of observing low-intensity $\gamma$-ray transitions and establishing spins of excited states that have remained undetermined to date. While a low-energy Coulomb-excitation study [5] revealed that the triaxial ground state of $^{100}$Mo coexists with a prolate-deformed $0_2^+$ level, little is known for the higher-lying $0^+$ states.
Selected results will be highlighted, including the identification of new $0^+$ states in $^{100}$Zr via $\gamma$-$\gamma$ angular correlations and candidates for $2^+$ states built on them. The mixing of coexisting configurations will be addressed relying on high-precision branching and mixing ratios obtained in this work. In addition, a comparison between Zr isotopes with $N=58$,$60$ will be presented, reporting on recent key findings from our group in $^{98}$Zr [6]. Finally, for the first time results concerning newly discovered structures in $^{100}$Mo will be presented and a possible multiple-shape coexistence scenario will be discussed.
[1] P.E. Garrett et al., Prog. Part. Nucl. Phys. 124 (2022) 103931.
[2] R. Rodriguez-Guzman et al., Phys. Lett. B 691, 202 (2010).
[3] T. Togashi et al., Phys. Rev. Lett. 117 (2016) 172502.
[4] A.B. Garnsworthy et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 918 (2019).
[5] K. Wrzosek-Lipska et al., Phys. Rev. C 86, 064305 (2012).
[6] K. Mashtakov et al., in preparation.