Coincidence recoil-distance Doppler-shift lifetime measurements in Ba-128 - art. no. 014314

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 D oppler-shift method in coincidence mode. A new version of the differential decay-curve method which takes into account the velocity distribution of th e recoils and the effect of the Doppler-shift attenuation in the stopper wa s applied for the lifetime determination. The B(E2) values in the ground-st ate band indicate a reduction of the transition strengths which may be part ly explained by the O(6) limiting case of the interacting boson model or al ternatively, by rotationally induced changes in the single-particle level s tructure at spins higher than I-pi = 4(+). The general collective (or Frank furt) 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.