Type-II shell evolution in ${}^{98}$Zr

The atomic nuclei in the Zirconium isotopic chain show a ground-state shape transition uniquely pronounced in the nuclear chart with respect to its abruptness. Between the (mostly) ground-state spherical ${}^{96}$Zr and the ground-state deformed ${}^{100}$Zr, ${}^{98}$Zr lies somewhere between the beginning and the ending of this narrow transition area. This work quantifies the sudden onset of quadrupole collectivity at the transitional nucleus ${}^{98}$Zr. As measure for quadrupole collectivity, the first-time determination of the B(E2; 2${}^{+}$ $\to$ 0${}_{g.s.}^{+}$) transition strength value of ${}^{98}$Zr was performed: A Coulomb excitation experiment with a radioactive ${}^{98}$Zr ion beam at the Argonne National Laboratory (USA) yielded a decisively small upper limit for the transition strength of interest. Complementary, the respective lifetime of the 2${}_1^{+}$ level was measured with the RDDS method employing a ${}^{96}$Zr(${}^{18}$O, ${}^{16}$O)${}^{98}$Zr two-neutron transfer reaction at IFIN-HH (Romania). The analysis of the two independent experiments is described and the results combined and discussed. The findings are compared to recent theory and confirm the predicted shape coexistence in ${}^{98}$Zr as effect of type-II shell evolution. Consequently, the nucleus ${}^{98}$Zr is placed at the brink of the examined structural change and identified as closest nucleus to the critical point of the underlying quantum phase transition.