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Register a new userMonopole and dipole transitions of the cluster states of 18O
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On the basis of an extended antisymmetrized molecular dynamics calculation, we study the cluster structure and the of the 0+ and 1- states of 18O. We discuss that several different kinds of cluster states appear in the excitation spectrum, and their monopole and dipole transitions are interesting fingerprints of unique cluster structure. We show that the monopole/dipole transitions are enhanced for the 14C+alpha cluster states, while they are hindered for the molecular-orbit state. We also point out that the ratio of the electric and isoscalar monopole transition strengths gives a good hint for the structure of the excited states.
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Nuclei in the $sd$-shell demonstrate a remarkable interplay of cluster and mean-field phenomena. The $N=Z$ nuclei, such as $^{24}$Mg and $^{28}$Si, have been the focus of the theoretical study of both these phenomena in the past. The cluster and vortical mean-field phenomena can be probed by excitation of isoscalar monopole and dipole states in scattering of isoscalar particles such as deuterons or $alpha$ particles. Inelastically scattered $alpha$ particles were momentum-analysed in the K600 magnetic spectrometer at iThemba LABS, Cape Town, South Africa. The scattered particles were detected in two multi-wire drift chambers and two plastic scintillators placed at the focal plane of the K600. In the theoretical discussion, the QRPA and AMD+GCM were used. The QRPA calculations lead us to conclude that: i) the mean-field vorticity appears mainly in dipole states with $K=1$, ii) the dipole (monopole) states should have strong deformation-induced octupole (quadrupole) admixtures, and iii) that near the $alpha$-particle threshold, there should exist a collective state (with $K=0$ for prolate nuclei and $K=1$ for oblate nuclei) with an impressive octupole strength. The results of the AMD+GCM calculations suggest that some observed states may have a mixed (mean-field + cluster) character or correspond to particular cluster configurations. A tentative correspondence between observed states and theoretical states from QRPA and AMD+GCM was established. The QRPA and AMD+GCM analysis shows that low-energy isoscalar dipole states combine cluster and mean-field properties. The QRPA calculations show that the low-energy vorticity is well localized in $^{24}$Mg, fragmented in $^{26}$Mg, and absent in $^{28}$Si.
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 $4alpha$ 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.
The properties of the alpha+28Si and 16O+16O molecular states which are embedded in the excited states of 32S and can have an impact on the stellar reactions are investigated using the antisymmetrized molecular dynamics. From the analysis of the cluster spectroscopic factors, the candidates of alpha+28Si and 16O+16O molecular states are identified close to and above the cluster threshold energies. The calculated properties of the alpha+28Si molecular states are consistent with those reported by the alpha+28Siresonant scattering experiments. On the other hand, the 16O+16O molecular state, which is predicted to be identical to the superdeformation of 32S, is inconsistent with the assignment proposed by an alpha inelastic scattering experiment. Our calculation suggests that the monopole transition from the ground state to the 16O+16O molecular state is rather weak and is not strongly excited by the alpha inelastic scattering.
To identify the 3alpha BEC state with the excess neutron, we have investigated the monopole strength of the excited states of 13C by using the theoretical framework of the real-time evolution method. The calculations have revealed several candidates of the Hoyle-analog states in a highly excited region.
The assumption of an exact isospin symmetry would imply equal strengths for mirror E1 transitions (at least, in the long-wavelength limit). Actually, large violations of this symmetry rule have been indicated by a number of experimental results, the last of which is the 67As - 67Se doublet investigated at GAMMASPHERE. Here, we examine in detail various possible origins of the observed asymmetry. The coherent effect of Coulomb-induced mixing with the high-lying Giant Isovector Monopole Resonance is proposed as the most probable process to produce a large asymmetry in the E1 transitions, with comparatively small effect on the other properties of the parent and daughter levels.
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