Observation of new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon 238U

A search for isomeric γ 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 γ rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 Is following the implantation. We identified a total of 54 microsecond isomers with half-lives of ∼0.110 Is, including the 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, 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 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. The nature of the nuclear isomerism is discussed in relation to the evolution of nuclear structure.