Prolate-oblate shape transition in heavy neutron-rich nuclei

A large amount of experimental information was obtained on heavy neutron-rich nuclei in the Hf-Hg region during the last decade. In the majority of the cases these nuclei were populated ha relativistic energy fragmentation and studied via isomeric and beta decays. In addition, deep-inelastic studies gave information to nuclei close to stability. The experimental data were used to establish the sign of quadrupole deformation, prolate or oblate. In the present paper this information is compared with two global theoretical calculations, one which is constrained to axial symmetry and one which considers triaxiality. The prolate-oblate transitional region was reached for (Z=76) Os and Ir (Z=77), as proved by the observation of oblate isomeric states in Os-197,Os-198,Os-199 and Ir-201. For Ta (Z=73) and W (Z=74) isotopes no clear evidence of oblate deformation exists so far, with W-192 and Ta-189 being the most neutron-rich isotopes with spectroscopic information.