Dipole collectivity and reflection asymmetric nuclear shapes

Collective dipole bands observed in light and heavy nuclei, are characterized experimentally by states of alternating parities and enhanced E1 decays. Such states arise from an intrinsic state which is reflection asymmetric and, therefore, is not invariant under the parity operation. This state may correspond to any configuration that is reflection asymmetric (e.g cluster state, or stable octupole deformed state). Dipole collective states have been observed in /sup 18/O and the Ra-Th region. The experimental signatures for dipole collectivity in these nuclei are discussed, and similarity and differences between the cluster model and stable octupole shape model are discussed. Search for such states in other regions of the chart of nuclei are proposed