Isoscalar monopole excitations in ${}^{16}$O: $\alpha$-cluster states at low energy and mean-field-type states at higher energy

Abstract

Isoscalar monopole strength function in ${}^{16}$O up to $E_x\simeq 40$ 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.

Comments: 35 pages, 5 figures