Two-quasiparticle excitations and collectivity in the weakly-deformed transitional isotopes 104, 106, 108Cd

Abstract

Using the generalized centroid-shift method on the Rutgers tandem, the following half-lives of 106Cd excited states were measured in the reaction 93Nb( 16O, p2n): T 1 2 (3679.0 keV) = 0.7 +0.1 −0.3 ns, T 1 2 (3507.8 keV) = 1.2 ± 0.4 ns, T 1 2 (3044.2 keV) = 0.4 ± 0.1 ns, and T 1 2 (2330.7 keV) = 0.6 ± 0.2 ns . With the same method applied on the Rossendorf cyclotron, the following half-lives were measured in the reactions 102, 106 Pd(α, 2 n): T 1 2 (2902.0 keV) = 0.8 +0.2 −0.1 ns ( 104 Cd) as well as T 1 2 (3737.3 keV) = 0.2 ± 0.1 ns , T 1 2 (3223.7 keV) = 0.2 ± 0.1 ns , T 1 2 (3057.4 keV) = 0.10 ± 0.05 ns , T 1 2 (2975.3 keV) = 0.15 ± 0.10 ns , T 1 2 (3110.5 keV) = 0.3 ± 0.1 ns , and T 1 2 (2565.2 keV) = 0.2 ± 0.1 ns ( 108 Cd) . The results reveal the non-collective (two-quasiparticle) character of several states above 2.9 MeV in 104, 106, 108Cd, in qualitative accordance with predictions of the slightly-deformed-rotor model. They concern completely aligned [h 11 2 g 7 2 ] (9 −−11 −-13 −, etc.) as well as semi-decoupled [h 11 2 d 5 2 ] (6 −-8 −-10 −, etc.) two-quasineutron band structures. Further, the possible character of 8 + (two-quasiproton) excitations, 5 + (two-quasineutron) states and of other intrinsic excitations is discussed. The experimental findings present a challenge to current theories of transitional nuclei for a quantitative treatment of absolute γ-ray transition strengths.