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.Received: 30 October 2002, Published online: 2 March 2004PACS: 21.10.Tg Lifetimes - 27.70. + q - 21.60.Ev Collective models - 23.20.-g Electromagnetic transitions