Stable and vibrational octupole modes in neutron rich nuclei

Evidence for stable octupole deformation in neutron-rich nuclei bounded by Z=54–58 and N=85–92 is described. More vibrational type octupole modes are observed to either side of this region in 140Ba and 152,154Nd. The largest stable octupole deformation (β3∼0.1) is reported in 144Bagg. The theoretically predicted quenching (β3→0) of stable octupole deformation with increased rotation is observed in 146Ba. There is good agreement between theory and experiment for the strongly varying electric dipole moments as a function of mass for 142–148Ba. In odd-A 143Ba and odd-Z, 145La, two pairs of positive and negative parity bands with opposite spins, parity doublets are observed. In 145La the ground band with symmetric shape coexists with the asymmetric octupole shape which stabilizes above about spin 19/2+. The isotope 109Mo was identified and a new region of stable octupole deformation is identified in 107,109Mo centered around N=64–66 as earlier predicted. This is the first case of stable octupole deformation i...