Electromagnetic transition strengths in 156Dy

Reliable and precise lifetimes of excited states in 156Dy were measured by means of the recoil distance Doppler-shift technique in the coincidence mode. The experiment was performed at the Laboratori Nazionali di Legnaro with the GASP array and the Cologne coincidence plunger apparatus using the reaction 124Sn(36S,4n)156Dy at a beam energy of 155 MeV. New values of the branching ratios of transitions depopulating the levels of the first excited band have been derived. The measured transition probabilities of 156Dy in the ground-state band and the first excited band as well as the energy spectra are compared to the predictions of the recently proposed X(5) model and to an interacting boson approximation fit. The comparison reveals a different behavior of the intraband transition strengths and indicates a possible coexistence of a normal deformed ground-state band and an X(5)-like first excited band. It also reveals that in 156Dy, the γ degree of freedom plays a more important role than it does in the well-established X(5) nuclei with N=90. A fit of the data using the general collective model suggests that a deeper collective potential V(β,γ) may also be a reason for the differences in the spectroscopic properties of 156Dy and those nuclei.