Supermultiplet symmetry and near-threshold levels in systems featuring twoor three extremely light clusters

On the basis of the supermultiplet potential model for the interaction of e xtremely light clusters (formalism of Young diagrams), it is explained why the d + t system does not feature a (1/2)(+) (L = 0) level allied to (3/2)( +) (L = 0) thermonuclear resonance. By using known data on the quasielastic knockout of nucleons from the inner Is shell of light nuclei, a systematic s of near-threshold states in the t + d + d, t + t + d, and t + t + t syste ms is constructed in terms of excited Young diagrams like {f} = {322}. The emergence of similar states in going over from the d + d and t + t systems (which feature no such states) to the alpha + d + d, alpha + t + d, and alp ha + t + t systems (where they appear owing to the binding effect of the al pha particle) is discussed. The three-cluster states being considered may b e important not only for nuclear physics proper but also for nuclear astrop hysics. (C) 2000 MAIK "Nauka/lnterperiodica".