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.

35 pages, 5 figures

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• Second RPA and correlated realistic interactions
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• Candidates for the 5$\alpha$ condensed state in ${}^{20}$Ne
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• Understanding emergent collectivity and clustering in nuclei from a symmetry-based no-core shell-model perspective
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• Monopole transitions to cluster states in ${}^{10}$Be and ${}^9$Li
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• Search for the 6$\alpha$ condensed state in ${}^{24}$Mg using the ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ scattering
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• Nuclear rainbow of the symmetric nucleus-nucleus system: Interchange of the nearside and farside scattering
• Electromagnetic Excitations and Responses in Nuclei from First Principles
• Temperature of the hot $\alpha$-source: a guidance to probe the Bose-Einstein condensation of $\alpha$ clusters in the heavy-ion collision