Coincidence recoil-distance Doppler-shift lifetime measurements in128Ba

Abstract

Mean lifetimes of 24 excited states in ${}^{128}\mathrm{Ba}$ populated via the ${}^{116}\mathrm{Cd}{(}^{16}\mathrm{O},4n)$ reaction at $E=76\mathrm{MeV}$ 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 ${I}^{\ensuremath{\pi}}{=4}^{+}.$ The general collective (or Frankfurt) model is found to describe well the intraband transition strengths in the quasi-\ensuremath{\gamma} band which also exhibit the features characteristic for a $K=2\ensuremath{\Elzxh}$ 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 $B(E2,\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}2)$ and $B(M1,\stackrel{\ensuremath{\rightarrow}}{I}I\ensuremath{-}1)$ transition strengths reconfirm the configuration $\ensuremath{\pi}{h}_{11/2}{d}_{5/2}$ 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 $(\ensuremath{\pi}{h}_{11/2}{d}_{5/2})\ensuremath{\bigotimes}(\ensuremath{\nu}{h}_{11/2}{g}_{7/2}),$ are studied.