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

On the basis of the supermultiplet potential model for the interaction of extremely 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 1s shell of light nuclei, a systematics of near-threshold states in the t + d + d, t + t + d, and t + t + t systems 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 α + d + d, α + t + d, and α + t + t systems (where they appear owing to the binding effect of the alpha particle) is discussed. The three-cluster states being considered may be important not only for nuclear physics proper but also for nuclear astrophysics