Multichannel features of the continuous spectrum of 6He

Abstract

It is shown that the continuous spectrum of the 6He nucleus immediately above the threshold for its breakup into an alpha particle and two neutrons has some features that are not peculiar to the continuous spectrum of binary systems. This spectrum has an infinite degeneracy multiplicity; hence, there exist an infinite number of decay channels even if we fix the angular momentum and the parity of a channel. In states characterized by a fixed value of the grand orbital, the potential energy of the 6He nucleus decreases with increasing hyperradius in inverse proportion to its cube. This circumstance is reflected in the behavior of the S-matrix elements for 3 → 3 scattering at low above-threshold energies. Because of the effect of the Pauli exclusion principle, the grand orbital generally ceases to be an appropriate quantum number for classifying decay channels, and the resulting conventional situation requires invoking superpositions of states corresponding to different values of the grand orbital. Within the method of three-cluster hyperharmonics and the approximation of an asymptotic potential, we calculate the eigenphases of the scattering matrix and reveal regularities in the behavior of these phases as functions of energy.