On the two-quasineutron rotational band in Ba

The lifetimes of five levels of the two-quasineutron band in 128 Ba populated via the 116 Cd( 16 O ,4 n) reaction were measured for the first time by means of the recoil-distance Doppler-shift method. For the data-analysis, a new version of the differential decay-curve method in singles-like mode was developed which takes into account the velocity distribution of the recoils and the effect of the Doppler-shift attenuation in the stopper. The high values of 〈K 2 〉 and K eff characterising the band are confirmed by the determined B (E2 ,I→I−2 ) and B (M1 ,I→I−1 ) transition strengths. At the same time, the rigid-rotor model is not able to describe the behaviour of the experimental B (E2 ,I→I−1 ) and B (M1 ,I→I−1 ) values. A new theoretical effort seems to be needed for a consistent explanation of the intraband transition strengths within two-quasiparticle bands in γ -soft nuclei.