Isomer Decay Spectroscopy of <mml:mmultiscripts>Sm 164 </mml:mmultiscripts> and <mml:mmultiscripts>Gd 166 </mml:mmultiscripts> : Midshell Collectivity Around N =100

Excited states in the N =102 isotones <mml:mmultiscripts>Gd 166 </mml:mmultiscripts> and <mml:mmultiscripts>Sm 164 </mml:mmultiscripts> have been observed following isomeric decay for the first time at RIBF, RIKEN. The half-lives of the isomeric states have been measured to be 950(60) and 600(140) ns for <mml:mmultiscripts>Gd 166 </mml:mmultiscripts> and <mml:mmultiscripts>Sm 164 </mml:mmultiscripts> , respectively. Based on the decay patterns and potential energy surface calculations, including β₆ deformation, a spin and parity of 6^- has been assigned to the isomeric states in both nuclei. Collective observables are discussed in light of the systematics of the region, giving insight into nuclear shape evolution. The decrease in the ground-band energies of <mml:mmultiscripts>Gd 166 </mml:mmultiscripts> and <mml:mmultiscripts>Sm 164 </mml:mmultiscripts> (N =102 ) compared to <mml:mmultiscripts>Gd 164 </mml:mmultiscripts> and <mml:mmultiscripts>Sm 162 </mml:mmultiscripts> (N =100 ), respectively, presents evidence for the predicted deformed shell closure at N =100 .