A search for isomeric γ decays among fission fragments from 345 MeV/nucleon ${}^{238}$U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed γ rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 μs following the implantation. We identified a total of 54 microsecond isomers with half-lives of ∼0.1–10 μs, including the discovery of 18 new isomers in very neutron-rich nuclei: ${}^{59}$Ti${}^m$, ${}^{90}{\mathrm{As}}^m$, ${}^{92}$Se${}^m$, ${}^{93}$Se${}^m$, ${}^{94}$Br${}^m$, ${}^{95}$Br${}^m$, ${}^{96}$Br${}^m$, ${}^{97}$Rb${}^m$, ${}^{108}$Nb${}^m$, ${}^{109}$Mo${}^m$, ${}^{117}$Ru${}^m$, ${}^{119}$Ru${}^m$, ${}^{120}$Rh${}^m$, ${}^{122}$Rh${}^m$, ${}^{121}$Pd${}^m$, ${}^{124}$Pd${}^m$, ${}^{124}$Ag${}^m$, and ${}^{126}$Ag${}^m$, and obtained a wealth of spectroscopic information such as half-lives, γ-ray energies, γ-ray relative intensities, and γγ coincidences over a wide range of neutron-rich exotic nuclei. Proposed level schemes are presented for ${}^{59}$Ti${}^m$, ${}^{82}$Ga${}^m$, ${}^{92}$Br${}^m$, ${}^{94}$Br${}^m$, ${}^{95}$Br${}^m$, ${}^{97}$Rb${}^m$, ${}^{98}$Rb${}^m$, ${}^{108}$Nb${}^m$, ${}^{108}$Zr${}^m$, ${}^{109}$Mo${}^m$, ${}^{117}$Ru${}^m$, ${}^{119}$Ru${}^m$, ${}^{120}$Rh${}^m$, ${}^{122}$Rh${}^m$, ${}^{121}$Pd${}^m$, ${}^{124}$Ag${}^m$, and ${}^{125}$Ag${}^m$, based on the obtained spectroscopic information and the systematics in neighboring nuclei. The nature of the nuclear isomerism is discussed in relation to the evolution of nuclear structure.

• Received 10 August 2012

DOI:https://doi.org/10.1103/PhysRevC.86.054319