Decays of New Nuclides and Isomers Beyond the Proton Drip Line - The Influence of Neutron Configurations

The energy of the vh(9/2) orbital in nuclei above N = 82 drops rapidly in energy relativeto the vf(7/2) orbital as the occupancy of the pi h(11/12) orbital increases. These two neutron orbitals become nearly degenerate as the proton drip line is approached. In this work, we have discovered the new nuclides Os-161 and W-157, and studied the decays of the proton emitter Re-160 in detail. The Os-161 and Re-160 nuclei were produced in reactions of 290, 300 and 310 MeV Ni-58 ions with an isotopically enriched Cd-106 target, separated in-flight using the RITU separator and implanted into the GREAT spectrometer. The Os-161 alpha decays populated the new nuclide W-157, which decayed by beta-particle emission. The beta decay fed the known alpha-decaying 1/2(+) and 11/2 states in Ta-157. which is consistent with a vf(7/2) round state in W-157. The measured alpha-decay energy and half-life for Os-161 correspond to a reduced a-decay width that is compatible with,wave alpha-particle emission, implying that its ground state is also a vf(7/2) state. Over 7000 Re-160 nuclei were produced and the gamma decays of a new isomeric state feeding the pi d(3/2) level in Re-160 were discovered, but no evidence for the proton or a decay of the expected pi h(11/2) state could be found. The isomer decays offer a natural explanation for this non-observation and provides a staking example of the influence of the near degeneracy of the vh(9/2) and vf(7/2) orbitals on the properties of nuclei in this region.