Isomer-delayed Γ -Ray Spectroscopy of A=159–164 Midshell Nuclei and the Variation of K -Forbidden

Excited states have been studied in 159 Sm, 161 Sm, 162 Sm (Z = 62), 163 Eu (Z = 63), and 164 Gd (Z = 64), populated by isomeric decay following 238 U projectile ssion at RIBF, RIKEN.The isomer half-lives range from 50 ns to 2.6 µs.In comparison with other published data, revised interpretations are proposed for 159 Sm and 163 Eu.The rst data for excited states in 161 Sm are presented, where a 2.6 µs isomer is assigned a three-quasiparticle, K π = 17/2 -structure.The interpretation is supported by multi-quasiparticle Nilsson-BCS calculations, including the blocking of pairing correlations.A consistent set of reduced E1 hindrance factors is obtained.Limited evidence is also reported for isomeric decay in 163 Sm, 164 Eu and 165 Eu.