Isomeric 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; τ= 0.326 (63) μ s), 112 Nb (E= 44.2 keV; τ= 0.094 (26) μ s), 113 Nb (E= 135.4 keV; τ= 0.846 (80) μ s), and 115 Mo (E= 198.6 keV; τ= 63 (4) μ 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 γ -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.
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This work was supported by JSPS KAKENHI Grant nos. 26800117 and 25247045. UK authors were supported by STFC Grant nos. ST/J000132/1, ST/J000051/1 and ST/K502431/1. P.H.R. acknowledges support from the UK National Measurement Office. P.-A.S. was financed by JSPS Grant no. 23 01752 and the RIKEN Foreign Postdoctoral Researcher Program. V.W. was supported by DOE Grant no. DE-FG02-91ER-40609 and the German BMBF Grant No. 05P12RDFN8. J.T. was financed by Spanish Ministerio de Ciencia e Innovación under Contract Nos. FPA2009-13377-C02 and FPA2011-29854-C04. J.G.L and F.R.X acknowledge the National Natural Science Foundation of China under Grants no. 12205340, 11835001, 11921006 and 12035001. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors.
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