Decays of new nuclides and isomers beyond the proton drip line - The influence of neutron configurations
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The energy of the vh 9/2 orbital in nuclei above N = 82 drops rapidly in energy relative to the vf 7/2 orbital as the occupancy of the ?h 11/2 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 161 Os and 157 W, and studied the decays of the proton emitter 160 Re in detail. The 161 Os and 160 Re nuclei were produced in reactions of 290, 300 and 310 MeV 58 Ni ions with an isotopically enriched 106 Cd target, separated in-flight using the RITU separator and implanted into the GREAT spectrometer. The 161 Os a decays populated the new nuclide 157 W, which decayed by ß-particle emission. The ß decay fed the known ?-decaying l/2 + and 11/2 - states in 157 Ta, which is consistent with a vf 7/2 ground state in 157 W. The measured a-decay energy and half-life for 161 Os correspond to a reduced ?-decay width that is compatible with s-wave ?-particle emission, implying that its ground state is also a vf 7/2 state. Over 7000 160 Re nuclei were produced and the y decays of a new isomeric state feeding the ?d 3/2 level in 160 Re were discovered, but no evidence for the proton or a decay of the expected ?h 11/2 state could be found. The isomer decays offer a natural explanation for this non-observation and provides a striking 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. © 2008 American Institute of Physics.