Low-spin collective excitations in deformed nuclei: When is a β a β vibration?

The 02+ level in deformed nuclei has historically been given the label β, even though its properties vary drastically from one nucleus to another. By reviewing models from where the original term β vibration arose, and appealing to microscopic calculations, guidelines are proposed for the properties of a true β vibration. An examination of available data, including B(E2) values, ρ2(E0) values, and single- and two-nucleon-transfer populations, shows that very few 02+ levels satisfy the criteria for a β vibration. It is suggested that pairing excitations may be playing a dominant role