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

The presence of isomeric levels with half-lives in the microsecond range has been identified in Cd-125,Cd-126,Cd-127,Cd-128. Neutron-rich Cd isotopes were produced from the fragmentation of a 120 MeV/nucleon Xe-136 beam and uniquely identified through their time-of-flight, energy loss, and total kinetic energy. gamma rays from these isomeric levels were measured with an array of Ge detectors that were gated for 15 mu s by a particle implantation trigger from a stack of Si detectors. The gamma rays observed in the decay of Cd-126,Cd-128 isomers populate low-energy levels previously identified in the beta decay of Ag-126,Ag-128. The gamma rays found in the decay of Cd-125,Cd-127 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 Sn-134, whose N/Z also exceeds 1.6.