  A search for isomeric gamma-decays among fission fragments from 345
MeV/nucleon 238U 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 gamma-rays were detected using three clover-type high-purity
germanium detectors located at the focal plane within a time window of 20
microseconds following the implantation. We identified a total of 54
microsecond isomers with half-lives of ~0.1 - 10 microseconds, including
discovery of 18 new isomers in very neutron-rich nuclei: 59Tim, 90Asm, 92Sem,
93Sem, 94Brm, 95Brm, 96Brm, 97Rbm, 108Nbm, 109Mom, 117Rum, 119Rum, 120Rhm,
122Rhm, 121Pdm, 124Pdm, 124Agm and 126Agm, and obtained a wealth of
spectroscopic information such as half-lives, gamma-ray energies, gamma-ray
relative intensities and gamma-gamma coincidences over a wide range of
neutron-rich exotic nuclei. Proposed level schemes are presented for 59Tim,
82Gam, 92Brm, 94Brm, 95Brm, 97Rbm, 98Rbm, 108Nbm, 108Zrm, 109Mom, 117Rum,
119Rum, 120Rhm, 122Rhm, 121Pdm, 124Agm and 125Agm, based on the obtained
spectroscopic information and the systematics in neighboring nuclei. Nature of
the nuclear isomerism is discussed in relation to evolution of nuclear
structure.
