Test of the critical point symmetry X(5) in N=90 nuclei and A approximate to 180 OS isotopes

Reliable and precise lifetimes of excited states in Gd-154 and Dy-156 were measured using the recoil distance Doppler shift (RDDS) technique. Excited states of Gd-154 were populated via Coulomb excitation with a S-32 beam at 110 MeV delivered by the FN tandem accelerator at the University of Cologne. For Dy-156 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 Sn-124(S-36,4n)Dy-156 at a beam energy of 155 MeV. The measured transition probabilities in Dy-156 and Gd-154 as well as the corresponding energy spectra are compared with the predictions of the recently proposed X(5) model and in the case of Dy-156 also with an IBA fit. In addition, criteria for finding new X(5) regions are given and the Os nuclei around A=180 were found to be very good X(5) candidates.