Speaker
Description
The evolution of ground-state shapes usually proceeds smoothly, however for Rb, Sr, Y, and Zr nuclei there are abrupt shape transitions that occur at $N=60$ (see Refs. [1,2] for reviews). Some recent calculations, using the Monte Carlo Shell Model (MCSM) [3] 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 further predict multiple shape coexistence occurring in the Zr isotopes. Moving towards higher $Z$, the abruptness of the transition becomes attenuated with a smoother evolution observed in Mo and Ru, 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 [5-7] and fission [8-11]. Coulomb excitation studies have provided important matrix elements and quadrupole moments [12-17] and have been complemented by lifetime measurements [18-21] revealing the collectivity of the band structures. A conversion electron study has provided a measurement of the change in radii of states in $^{98}$Zr and $^{100}$Zr [22].
We bring additional data to provide a more comprehensive view to the region, focused especially on how the structures in Zr evolve with increasing $N$ from $^{90}$Zr. Using our $\gamma$-ray spectrometers at the TRIUMF-ISAC facility, we have performed studies of $^{92m}$Nb and $^{98,100}$Y decay to study $^{92,98,100}$Zr, respectively. We have established a collective band in $^{92}$Zr [23], and have firmly identified the deformed band built on the $0^+_3$ state in $^{98}$Zr [24]. Our lifetime measurements of levels in $^{100}$Zr reveal that the structure built on the $0^+_2$ state is weakly collective, and bears remarkable similarities with the $0^+_2$ band in $^{98}$Sr [25]. Examining all data available for the Zr isotopes leads us to the suggestion of triple shape coexistence occurring. We place the deformations extracted from these data in the context of the systematics of the wider region.
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[25] D. Kalaydjieva et al., in preparation (2026).