Interpretation of metastable states in the $N>70$ Zr region

International audienceIsomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon $^{238}$U beam. Isomers reported in nuclei spanning a predicted prolate-oblate shape change boundary, $^{111}$Zr ($E=283.1~$keV; $\tau =0.326(63)$ $\upmu $s), $^{112}$Nb ($E=44.2$ keV; $\tau =0.094(26)$ $\upmu $s), $^{113}$Nb ($E=135.4$ keV; $\tau =0.846(80)$ $\upmu $s), and $^{115}$Mo ($E=198.6$ keV; $\tau =63(4)$ $\upmu $s), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from $\gamma $-ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for $N>70$ in this region. In addition, isomers in $^{108}$Nb, $^{109}$Nb, $^{113}$Tc, $^{117}$Ru, $^{119}$Ru, $^{120}$Rh, and $^{122}$Rh, not spanning the prolate-oblate transition discussed, are presented