Title:Cluster structure and isoscalar monopole excitation in light nuclei
Isoscalar monopole excitation to cluster states in light nuclei is in
general strong as to be comparable with the single particle strength and
shares an appreciable portion of the sum rule value. We discuss the
isoscalar monopole strength function S(E) in 16O up to E_x <40 MeV. We found
that the fine structures at the low energy region up to E_x \simeq 16 MeV in
the experimental S(E) obtained by the 16O(alpha,alpha') reaction can be
rather satisfactorily reproduced within the framework of the 4alpha cluster
model, while the gross three bump structures observed at the higher energy
region (16 < E_x < 40 MeV) look likely to be approximately reconciled by the
mean-field calculations such as RPA and QRPA. In this talk, it is emphasize
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 (E_x < 16 MeV), 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 (16 < E_x < 40 MeV), 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. We also stress that
the isoscalar monopole excitation is useful to search for cluster states in
light nuclei.
