Lifetime measurements and the structure of some negative-parity states in ¹³⁴Ce

The lifetimes of several negative-parity states in ¹³⁴Ce excited via the reaction ¹²²Sn(¹⁶O, 4n)¹³⁴Ce at an incident beam energy of 76 MeV were measured for the first time by means of the recoil-distance Doppler-shift technique. The small B(E2) transition strengths deduced from the lifetime data at the bottom of the negative-parity yrast states show spherical shape, in qualitative accordance with predictions of the shell model. At higher excitation energy along the negative-parity yrast line, the experimental B(E2) transition strengths indicate an apparent gain of collectivity with increasing spin. The quadrupole deformation derived from the B(E2) values at the bottom of the negative-parity yrast band confirms the predictions of the cranked Nilsson-Strutinsky Bogoliubov calculations. The systematic evolution in structure of the negative yrast states in Ce isotopes and N=76 isotones is discussed.