Evolution of the X(5) critical-point symmetry in rotating ¹⁷⁶Os

Lifetimes of excited states above 10⁺ in the yrast band in ¹⁷⁶Os have been measured using the Doppler shift attenuationmethod via the fusion evaporation reaction ¹⁵²Sm(²⁸Si, 4n) ¹⁷⁶Os. Lifetimes of states over 12⁺ were measured for the first time. The deduced transitional quadruple moments (Qt), together with the previous data using a recoil distance method, are compared with theoretical calculations based on the X(5) model and the interaction boson model. For the states below spin 10⁺, the data of previous work support an X(5)-like structure, but for the states above 10⁺, the value of Q_t remains almost constant, being the characteristic of a symmetric rotor. The present result suggests that the shape of the nucleus ¹⁷⁶Os changes from X(5) critical-point symmetry to an axially deformed rotor with increasing spin.