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The concept of nuclear cluster forbiddenness

In nuclear cluster systems, a rigorous structural forbiddenness of virtual nuclear division into unexcited fragments is obtained. We re-analyze the concept of forbiddenness, introduced in Ref. 1 for the understanding of structural effects in nuclear cluster physics. We show that the concept is more involved than the one presented previously, where some errors were committed. Due to its importance, it is reanalyzed here. In the present contribution a simple way for the determination of forbiddenness is given, which may easily be extended to any number of clusters, though in this contribution we discuss only two-cluster systems, for illustrative reasons. A simple rule is obtained for the minimization of the forbiddenness, namely to start from a cluster system with a large SU(3) irrep (lambda,mu), but minimizing (lambda-mu), i.e. the system has to be oblate. The rule can be easily implemented in structural studies, done up to now with an oversimplified definition of forbiddenness. The new method is applied to various systems of light clusters and to some decay channels of 236U and 252Cf.