Lifetime Determination of the 2+ and 4+ Rotational Levels inGd154and the Effect of Rotation-Vibration Interaction

The lifetimes of the 2+ and 4+ rotational levels of ${\mathrm{Gd}}^{154}$ were determined by coincidence measurements between the beta spectrum of ${\mathrm{Eu}}^{154}$ and the conversion lines de-exciting the levels under investigation, utilizing a double-lens coincidence spectrometer. The measurements were carried out using the self-comparison method and a time-to-amplitude converter. The mean lifetimes of the 371- and the 123-keV levels were found to be (5.6±0.7)×${10}^{-11\mathrm{}}$ sec and (1.67±0.07)×${10}^{-9\mathrm{}}$ sec, respectively. The measured ratio for the reduced transition probabilities, $\frac{B(E2;4+\to 2+)}{B(E2;2+\to 0+)}=1.77\mathrm{}\pm 0.25$, deviates from the value 1.43 predicted by the strict rotational model. The effect of rotation-vibration interaction on the transition probability ratios within a rotational band is discussed. These ratios are related uniquely to the distortion of the energy spacings. In the case of the ground-state band of ${\mathrm{Gd}}^{154}$, theory thus predicts $\frac{B(E2;4+\to 2+)}{B(E2;2+\to 0+)}=1.62\mathrm{}$ in agreement with experiment.