Isomer-delayed γ -ray spectroscopy of A=159–164 midshell nuclei and the variation of K -forbidden

Abstract

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 fission 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 first 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.

Insight into nuclear midshell structures by studying K isomers in rare-earth neutron-rich nuclei

“…Therefore all the moments of inertia display a gradual and smooth variation with frequency ω. [4,9] and Yokoyama et al [5] independently. The configuration is assigned as a two-neutron ν 1 2 − [521]⊗ν 7 2 + [633] configuration by Nilsson-BCS [4,9], deformed Hartree-Fock [5], projected shell model [55] and present PNC-CSM calculations.…”

“…The experimental data are taken from Refs. [4,8,11,12,52,53]. states are mainly gained from the enlarged Z = 62 energy gap of the single proton Nilsson levels with non-zero ε 6 (see Fig.…”

“…The nuclear structure inputs of the rare-earth nuclei can lead to an improved understanding of the r−process nucleosynthesis [3]. However, due to their neutron excess, detailed structure informations are very difficult to be revealed by the experiment in these rare-earth neutron-rich nuclei.The recent experimental progresses in the neutron-rich A = 150 − 170 region [4][5][6][7][8][9][10][11][12][13] are attributed to a great extent to the existence of the nuclear isomeric state. A nucleus can be "traped" in an aligned spin orientation relative to its symmetric axis to form the K isomeric state (or K-isomer), where K is a quantum number representing the projection of the total nuclear spin along the symmetry axis of the nucleus.…”

“…Investigations of K-isomers have been reviewed recently in Refs. [14][15][16][17][18].An impressive progress has been achieved in experimental studies of samarium and gadolinium isotopes very recently [4,5,8,9,11,12]. The most exiting discoveries include: 160 Sm became the lightest nucleus with a known four-quasiparticle K-isomer [8], K-isomers were firstly observed in very neutron-rich nuclei 164 Sm and 166 Gd [11] and in odd-A samarium isotopes [4,5].…”

“…[14][15][16][17][18].An impressive progress has been achieved in experimental studies of samarium and gadolinium isotopes very recently [4,5,8,9,11,12]. The most exiting discoveries include: 160 Sm became the lightest nucleus with a known four-quasiparticle K-isomer [8], K-isomers were firstly observed in very neutron-rich nuclei 164 Sm and 166 Gd [11] and in odd-A samarium isotopes [4,5]. Besides the important structure informations mentioned above, these experimental data will provide a good test ground of the present nuclear theories in the most unknown territory in the nuclear landscape.…”