Onset of isomers inCd125,126,127,128and weakened neutron-neutron interaction strength

Abstract

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