Coincidence Recoil-Distance Doppler-Shift Lifetime Measurements in 128Ba

Mean lifetimes of 24 excited states in Ba-128 populated via the Cd-116(O-16,4n) reaction at E = 76 MeV were measured by means of the recoil-distance Doppler-shift method in coincidence mode. A new version of the differential decay-curve method which takes into account the velocity distribution of the recoils and the effect of the Doppler-shift attenuation in the stopper was applied for the lifetime determination. The B(E2) values in the ground-state band indicate a reduction of the transition strengths which may be partly explained by the O(6) limiting case of the interacting boson model or alternatively, by rotationally induced changes in the single-particle level structure at spins higher than I-pi = 4(+). The general collective (or Frankfurt) model is found to describe well the intraband transition strengths in the quasi-gamma band which also exhibit the features characteristic for a K = 2 (h) over bar rotational band. However, none of the collective models applied can describe in a consistent way all observed properties of the low-lying collective states. The newly measured B(E2,I-->I-2) and B(M1,I-->I-1) transition strengths reconfirm the configuration pi h(11/2)d(5/2) of the negative-parity semide-coupled bands. The lowest levels of the higher-lying "dipole'' band in Ba-128, which is built on the high-K four-quasiparticle prolate configuration (pi h(11/2)d(5/2)) x (nu h(11/2)g(7/2)), are studied.