Observation of a Core-Excited E4 Isomer in 98Cd

A core-excited I = (12+) spin-gap isomer was identified in 98Cd in an experiment at EUROBALL IV. It was found to feed the known I = (8+) seniority isomer by an E4 transition. Half-lives of T1/2 = 0.23() µs and 0.17() µs were measured for the two states at Ex = 6635 keV and 2428 keV, respectively. From the excitation energy of the core-excited isomer a 100Sn shell gap of 6.46(15) MeV is inferred. The measured E4 and E2 strengths, 100Sn core excitations and the origin of empirical polarization charges are discussed in the framework of large-scale shell model calculations. An E2 polarization charge for protons of e<0.4 e is found, which corresponds to the empirical value e = 0.45() e in the pure proton hole valence space.