0 Decay of 0 + 2 States in the Rare-Earth Region : The Case of 156 Dy and 160

The branching between the E0 0+2 → 0+1 and the E2 0+2 → 2+1 transitions in Dy and Er were measured following the ε decay of Ho and Tm, respectively. The experimental results are compared to theoretical calculations, using a generalized potential in the 4 order of the deformation parameter β, which describes the violation of spherical symmetry in nuclei. This potential was chosen to cover the whole transitional path from from a spherical harmonic vibrator to an axially deformed rotor, since a dynamic symmetry transition, denoted X(5), is suggested to occur for the N = 90 rare-earth nuclei. The lifetimes of these states provide independent information for the E2 strength and are used for the extraction the E0 strength. Introduction In the geometrical description of collective nuclear motion there are three limiting cases, corresponding to the harmonic vibrator, the symmetrically deformed rotor and the triaxial rotor. Each of them is associated with a particular nuclear shape, spherical, axial-ellipsoidal, and triaxial. The transition from a spherical harmonic vibrator to an axially deformed rotor was described analytically by Iachello, introducing a dynamic symmetry, denoted X(5) [1]. It is interesting in this context to investigate the E0 transition strength, in the decay of the first excited 0 state, because this quantity is known to have particularly large values in the transitional region between spherical and deformed nuclei [2,3]. This fact can be explained by strong mixing of states with different deformation [4], or within the framework of the Interacting Boson Model (IBM) [5], by mixing of states with different number of bosons [3]. Electric monopole, E0, transitions are forbidden by the γ-decay selection rules and can occur only via the emission