Increasing Octupole Collectivity across the Z=64 Isotopic Chain: B(E3) Values in ^{150}Gd

An enhanced electric octupole (E3) strength of 45(5) W.u. has been measured for the 3^{-}→0^{+} transition in ^{150}Gd, providing direct evidence for large octupole collectivity in this nucleus. The value was determined following two sensitive experiments, to measure the mean lifetime of the J^{π}=3^{-} level and the weak E3 decay branch direct to the ground state. The systematics of the E3 strength across the Gd isotopic chain reveals that enhanced octupole collectivity is obtained for ^{150}Gd. The increasing low-lying octupole strength in the Gd isotopes is analyzed with five state-of-the-art theoretical models. It is found to be best reproduced by quasiparticle random phase approximation calculations, providing a good understanding of octupole collectivity in the rare-earth region.