Alpha Cluster Structures and Isoscalar Monopole Excitations in Light Nuclei

Abstract

Isoscalar monopole excitations to cluster states in light nuclei are in general strong as to be comparable with the single particle strength. Their experimental strengths share about 20 % of the sum rule value in the case of 4He, 11B, 12C, and 16O etc. We discuss the isoscalar monopole strength function in 16O, which is typical in light nuclei, is discussed up to Ex ~ 40 MeV. It is found that 1) two different types of monopole excitations exist in 16O; one is the monopole excitation to cluster states which is dominant in the lower energy part, and the other is the monopole excitation of the mean-field type such as one-particle one-hole (1p1h) which is attributed mainly to the higher energy part, and 2) this character of the monopole excitations originates from the fact that the ground state of 16O with the dominant doubly closed shell structure has a duality of the mean-field-type as well as alpha-clustering character.