Clustering in Exotic Nuclei*

There is strong experimental evidence, backed up by the state-of-the art calculations, that clustering plays an important role in structure of light nuclei. Groups of levels that have large reduced alpha-width, indicating high degree of alpha clustering, have been known in some nuclei for many decades. Recent development of advanced microscopic theoretical models provided new insights into clustering problem in light nuclei. It appears that clustering is not limited to N=Z (alpha-like) nuclei and to simple alpha+core configurations. An interplay between single-nucleon and cluster degrees of freedom leads to formation of interesting structures, such as molecular-like, multi-center configurations. Manifestation of this interplay can be observed more directly in non-self-conjugate, exotic nuclei, where the thresholds for nucleon(s) and cluster(s) decays may be close and one may expect to observe nucleon(s) and cluster(s) decays from the same state. Clustering in non-self-conjugate nuclei also plays an important role in nuclear astrophysics, as near alpha-threshold cluster states often dominate the radiative alpha-capture and other reactions that involve alpha-particles and exotic nuclei, that are forming in stellar environment during certain stages of nucleosynthesis processes. A well established example of the molecular-type configuration is the alpha:2n:alpha structure in 10Be, where two valence neutrons are thought to occupy the molecular-type  pi and sigma orbitals with respect to the two alpha-particles. Recent experimental advances on identifying and characterizing these configurations in exotic nuclei will be reviewed and discussed in the context of modern theoretical understanding of clustering phenomena in atomic nuclei.