Origin of the fine structure in the giant resonance of 16O

Abstract

The fine structure of nuclear continuum states is interpreted in the framework of the shell model as the result of coupling simple unbound configurations to more complicated many-particle-many-hole cluster-like quasi-bound states. Thus the splitting of the 22 MeV peak of the giant resonance is ascertained both theoretically and experimentally [(d, γ) experiment of M. Suffert] to be due to the existence of a two-particle-two-hole coherent quasi-bound state lying in the dip of the photoabsorption cross section. On the other hand, the 2p-2h mechanism cannot account for the 21.7 MeV structure, nor by itself for the structure above 24 MeV. Analysis of cluster transfer experiments [( 4He, γ) of M. Suffert and M. Feldman, and ( 3He, γ) of N. Puttasswamy and D. Kohler] indicate the existence of a 4p-4h quasi-bound state in the vicinity of the 21.7 MeV structure, and of 3p-3h quasi-bound states above 24 MeV.