Mean lifetimes of 24 excited states in ${}^{128}\mathrm{Ba}$ populated via the ${}^{116}\mathrm{Cd}{(}^{16}\mathrm{O},4n)$ reaction at $E=76\mathrm{}\mathrm{MeV}$ 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 $B\left(E2\mathrm{}\right)\mathrm{}$ 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 $I^{\pi }{=4\mathrm{}}^{+}.$ The general collective (or Frankfurt) model is found to describe well the intraband transition strengths in the quasi-γ band which also exhibit the features characteristic for a $K=2\mathrm{}ħ$ 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 $B(E2,\overrightarrow{I}I-2)$ and $B(M1,\overrightarrow{I}I-1)$ transition strengths reconfirm the configuration $\pi h_{11/2}{d}_{5/2}$ of the negative-parity semidecoupled bands. The lowest levels of the higher-lying “dipole” band in ${}^{128}\mathrm{Ba},$ which is built on the high-K four-quasiparticle prolate configuration $\left(\pi h_{11/2}{d}_{5/2}\right)\otimes \left(\nu h_{11/2}{g}_{7/2}\right),$ are studied.

• Received 31 January 2000

DOI:https://doi.org/10.1103/PhysRevC.62.014314