Isomer-delayed γ -ray spectroscopy of A=159-164 midshell nuclei and the variation of K -forbidden E1 transition hindrance factors

Excited states have been studied in \iso{159}{Sm}, \iso{161}{Sm}, \iso{162}{Sm} (Z~=~62), \iso{163}{Eu} (Z~=~63), and \iso{164}{Gd} (Z~=~64), populated by isomeric decay following 238U projectile fission at RIBF, RIKEN. The isomer half-lives range from 50 ns to 2.6 \mus. In comparison with other published data, revised interpretations are proposed for \iso{159}{Sm} and \iso{163}{Eu}. The first data for excited states in \iso{161}{Sm} are presented, where a 2.6 \mus isomer is assigned a three-quasiparticle, K^\pi = 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 163Sm, 164Eu and 165Eu.