Information on nuclear shapes at high spins from lifetime measurements

Using Doppler shift techniques, lifetimes of high spin states have been measured in a number of nuclei near N-90 in order to get their dynamic electric quadrupole moments, which are a direct reflection of the collective aspects of the nuclear wave functions. The potential energy surfaces of 160Yb (N = 90) reveal that it is very shallow in the γ degree of freedom as well as in epsilon2 and thus should be susceptible to deformation driving influences. Quadrupole transition moments extracted from lifetime data show that especially in /sup 160,161/Yb this is true. Lifetime measurements were carried out on 158Er in the γ-γ coincidence mode. The 158Er data were analyzed in four ways. Shifted and unshifted peak intensities were first extracted from the total-projected coincidence spectra. Similarly, they were extracted from spectra generated by the sum of all gates below the state of interest, by the gate on the first transition above and by the gate on the second transition above the one of interest. There is evidence for centrifugal stretching in the ground band and a dropoff of collectivity in the s band. 4 figs