Shape changes and test of the critical-point symmetry X (5) in N = 90 nuclei

Reliable and precise lifetimes of excited states in 154Gd and 156Dy were measured using the recoil distance Doppler shift (RDDS) technique. Excited states of 154Gd were populated via Coulomb excitation with a 32S beam at 110 MeV delivered by the FN tandem accelerator at the University of Cologne. For 156Dy a coincidence plunger experiment was performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer and the Cologne coincidence plunger apparatus using the reaction 124Sn(36S,4n)156Dy at a beam energy of 155 MeV. Shape changes previously suggested to appear in the ground-state band (gsb) of 156Dy and in the s-band above the first band crossing were not supported by the transition probabilities determined in this work. The measured transition probabilities of 156Dy and 154Gd as well as the corresponding energy spectra are compared with the predictions of the recently proposed X(5) model and in the case of 156Dy also with an IBA fit.