Fine Structure and Quasibound States

With the exception of single-particle resonances, or of the gross structure of the giant resonance, the nature of unbound nuclear states is still largely unexplored. However, this field of research is becoming rapidly a most promising one. This is due on one hand to the extension of shell model concepts and techniques to the microscopic description of nuclear modes above particle emission threshold, and on the other hand to the recent development of the technology of cluster transfer reactions (i.e. reactions where a number of nucleons are added to a target nucleus). The fine structure of nuclear continuum states is interpreted in the framework of the shell model as the result of coupling unbound configurations to many particle - many hole quasibound states. The density of the resonances is linked to the density of shell model quasibound states which can be formed while the widths of the peaks are related to the overlap between the wave-functions of the quasibound states and of the unbound configurational states. This description is amenable to experimental verification. For example an interference shape in the cross-section curve for a process induced by a one body operator may be attributed to a n particle - n hole quasibound state when a resonance in the excitation cross-section via the transfer of a n-p cluster to a n-h target occurs at the same energy. As an illustration the present experimental situation for the 16O continuum is reviewed. (author)