Speaker
Description
The evolution of ground-state shapes usually proceeds smoothly, however for Rb, Sr, Y, and Zr nuclei nuclei there is an abrupt shape transition that occurs at $N=60$ (see Refs. [1,2] for reviews). Some recent calculations, using the state-of-the-art Monte Carlo Shell Model (MCSM) [3,4] and the Interacting Boson Model employing the Intertwined Quantum Phase Transition (IQPT) [5], have been able to reproduce this abrupt change for the Zr isotopes and predict that shape coexistence occurs both above and below the critical $N=60$ point. The MCSM calculations also predict multiple shape coexistence in Zr. Moving away from $Z=40$, the abruptness of the transition becomes attenuated, with a smoother evolution observed in Mo, for example [2].
Over the past decade, there has been a large number of experimental investigations, using a variety of probes, that are bringing new insights into nuclei in the $N=60$ region. Deformed band structures have been revealed through detailed $\gamma$-ray spectroscopy following $\beta$-decay [6-9] and fission [10-13]. Coulomb excitation studies have provided important matrix elements and quadrupole moments [14-16] and have been complemented by lifetime measurements [17-20]. A conversion electron study has provided a measurement of the change in radii of the $^{98}$Zr and $^{100}$Zr nuclei [21].
Using $\beta$-decay and Coulomb excitation measurements, we have recently added high-precision data on excited states of $^{94}$Zr, $^{96}$Zr, and $^{98}$Zr. This has enabled us to characterize the excited states belonging to deformed structures. For $^{94}$Zr, our Coulomb excitation results provide firm evidence for an oblate-triaxial deformed structure for the $0^+_2$ state [22]. In $^{96}$Zr, we have refined the in-band $B(E2;2^+_2\rightarrow 0^+_2)=39(6)$ W.u. [23] from the previous value of 37(11) W.u. [24]. The increased level of precision of the $B(E2)$ value, combined with other properties of the $0^+_2$ band, has led us to suggest that its configuration is different from the corresponding state in $^{94}$Zr [23]. Our data also firmly identify the deformed band built on the $0^+_3$ state in $^{98}$Zr [25]. Considering these data, and through comparison with other nuclei, especially $^{96,98}$Sr, leads us to the suggestion of triple shape coexistence in the Zr isotopes.
References
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[17] P. Singh et al., Phys. Rev. Lett. 121 (2018) 192501.
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[21] G. Tocabens et al., Phys. Rev. C 111 (2025) 034306.
[22] N. Marchini et al., Phys. Lett. B submitted (2026).
[23] M. Zielinska et al., Phys. Lett. B submitted (2026).
[24] C. Kremer et al., Phys. Rev. Lett. 117 (2016) 172503.
[25] K. Mashtakov et al., to be published (2026).
| Contribution category | Experiment |
|---|---|
| Presenter status | Faculty/Staff |