Onset of isomers in ^{125,126,127,128}Cd and weakened neutron-neutron interaction strength

The presence of isomeric levels with half-lives in the microsecond range has been identified in ^{125,126,127,128}Cd. Neutron-rich Cd isotopes were produced from the fragmentation of a 120 MeV/nucleon ¹³⁶Xe beam and uniquely identified through their time-of-flight, energy loss, and total kinetic energy. γ rays from these isomeric levels were measured with an array of Ge detectors that were gated for 15 μs by a particle implantation trigger from a stack of Si detectors. The γ rays observed in the decay of ^126,128Cd isomers populate low-energy levels previously identified in the β decay of ^126,128Ag. The γ rays found in the decay of ^125,127Cd isomers are consistent with expected yrast structures observed in lighter, odd-mass Cd isotopes. The appearance of these isomers at the point where N/Z exceeds 1.6 is interpreted as an indication of the onset of a weakened neutron-neutron interaction that has been proposed for ¹³⁴Sn, whose N/Z also exceeds 1.6.