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 U-238 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 mu s following the implantation. We identified a total of 54 microsecond isomers with half-lives of similar to 0.1-10 mu s, including the discovery of 18 new isomers in very neutron-rich nuclei: Ti-59(m), As-90(m), Se-92(m), Se-93(m), Br-94(m), Br-95(m), Br-96(m), Rb-97(m), Nb-108(m), Mo-109(m), Ru-117(m), Ru-119(m), Rh-120(m), Rh-122(m), Pd-121(m), Pd-124(m), Ag-124(m), and Ag-126(m), 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 Ti-59(m), Ga-82(m), Br-92(m), Br-94(m), Br-95(m), Rb-97(m), Rb-98(m), Nb-108(m), Zr-108(m), Mo-109(m), Ru-117(m), Ru-119(m), Rh-120(m), Rh-122(m), Pd-121(m), Ag-124(m), and Ag-125(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.