The low-lying states in ${}^{106}\mathrm{Zr}$ and ${}^{108}\mathrm{Zr}$ have been investigated by means of $\beta \mathrm{\text{−}}\gamma$ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of $620\pm 150\mathrm{ns}$ has been identified in ${}^{108}\mathrm{Zr}$. For the sequence of even-even Zr isotopes, the excitation energies of the first $2^{+}$ states reach a minimum at $N=64$ and gradually increase as the neutron number increases up to $N=68$, suggesting a deformed subshell closure at $N=64$. The deformed ground state of ${}^{108}\mathrm{Zr}$ indicates that a spherical subshell gap predicted at $N=70$ is not large enough to change the ground state of ${}^{108}\mathrm{Zr}$ to the spherical shape. The possibility of a tetrahedral shape isomer in ${}^{108}\mathrm{Zr}$ is also discussed.

• Received 20 December 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.202501