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Observation of new microsecond isomers among fission products of 345 MeV/nucleon 238U

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.