Lifetime measurements in the transitional nucleus ^{138}Gd

Lifetime measurements have been made in the ground-state band of the transitional nucleus (138)Gd from coincidence recoil-distance Doppler-shift data. (138)Gdnuclei were produced using the (106)Cd ((36)Ar, 2p2n) reaction with a beam energy of 190 MeV. Reduced transition probabilities have been extracted from the lifetime data collected with the Koln plunger placed at the target position of the JUROGAM-II array. The B(E2) values have been compared with predictions from X(5) critical-point calculations, which describe the phase transition between vibrational and axially symmetric nuclear shapes, as well as with IBM-1 calculations at the critical point. While the excitation energies in (138)Gdare consistent with X(5) predictions, the large uncertainties associated with the measured B(E2) values cannot preclude vibrational and rotational contributions to the low-lying structure of (138)Gd. Although experimental knowledge for the low-lying gamma and beta-vibrational bands in (138)Gd is limited, potential-energy surface calculations suggest an increase in gamma softness in the ground-state band. In order to more fully account for the effects of gamma softness, the X(5) and IBM-1 calculations need to be extended to include the gamma degree of freedom for (138)Gd.