Isoscalar monopole strength function in $^{ 16}$O up to $E_{ x}\simeq40$ MeV is discussed. We found that the fine structures at the low energy region up to $E_{ x} \simeq 16$ MeV in the experimental monopole strength function obtained by the $^{ 16}$O$(\alpha,\alpha^{ \prime})$ reaction can be rather satisfactorily reproduced within the framework of the $4\alpha$ cluster model, while the gross three bump structures observed at the higher energy region ($16 \lesssim E_{ x} \lesssim 40$ MeV) look likely to be approximately reconciled by the mean-field calculations such as RPA and QRPA. In this paper, it is emphasized that two different types of monopole excitations exist in $^{ 16}$O; one is the monopole excitation to cluster states which is dominant in the lower energy part ($E_{ x} \lesssim 16$ 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 ($16 \lesssim E_{ x} \lesssim 40$ MeV). It is found that this character of the monopole excitations originates from the fact that the ground state of $^{ 16}$O with the dominant doubly closed shell structure has a duality of the mean-field-type { as well as} $\alpha$-clustering { character}. This dual nature of the ground state seems to be a common feature in light nuclei.
Categories: nucl-th nucl-ex