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

Z. Patel et al. -- 10 pags., 18 figs., 11 tabs.Excited states have been studied in Sm159, Sm161, Sm162 (Z=62), Eu163 (Z=63), and Gd164 (Z=64), populated by isomeric decay following U238 projectile fission at RIBF, RIKEN. The isomer half-lives range from 50 ns to 2.6μs. In comparison with other published data, revised interpretations are proposed for Sm159 and Eu163. The first data for excited states in Sm161 are presented, where a 2.6-μs isomer is assigned a three-quasiparticle, Kπ=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 Sm163, Eu164, and Eu165.This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN, and CNS, University of Tokyo. All UK authors are supported by STFC. P.H.R. is partially supported by the UK National Measurement Office (NMO). P.-A.S. was financed by JSPS Grant No. 2301752 and the RIKEN Foreign Postdoctoral Researcher Program. J.T. was financed by Spanish Ministerio de Ciencia e Innovación under ContractsNo. FPA2009-13377-C02 and No. FPA2011-29854- C04. Zs.V. and I.K. were supported by OTKA Contract No. K100835. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. This work was also supported by JSPS KAKENHI Grant No. 25247045Peer Reviewe