Shigeo Ohkubo

Local Potential Approach to the Alpha-Nucleus Interaction and Alpha-Cluster Structure in Nuclei

We review the merits of the local potential approach in the description of elastic alpha-particle scattering and of alpha-cluster structure in medium-weight nuclei. We recall how optical model analyses of elastic scattering angular distributions, displaying an anomalous large angle enhancement, yielded unique optical potentials, whose real part is determined with good accuracy up to small internucleus distances, and we investigate their properties; we then analyze the general compatibility of these potentials with the microscopic requirements resulting from the constraints due to antisymmetrization, in the case of a system which was thoroughly investigated from a microscopic point of view, and whose spectroscopy is well agreed upon, that is, the 20Ne = α + 16O system. The local potential approach is then applied to the 44Ti = α + 40Ca system - which is the analogue of 20Ne in the fp-shell -whose α-cluster spectroscopy remained unclear for a long time and for which microscopic calculations led to contradictory interpretations; not only does this simple model give a nice account of the energy location and intraband electromagnetic transition probabilities for the members of the 44Ti ground state band, but it predicts the existence of excited alpha-cluster bands, which were subsequently identified experimentally in alpha-transfer experiments. The same approach is applied to the 40Ca = α + 36Ar system, where similar predictions have recently been confirmed experimentally, and to other systems close to the sd-shell closure region. Finally the persistence of alpha-cluster structure in medium-weight and heavy nuclei is discussed within the local potential approach.

Microscopic Study of Coexistence of Alpha-Cluster and Shell-Model Structure in the 40Ca-44Ti Region

The structure of the nuclei in the mass range 40 ≤ A ≤ 44 is investigated by the microscopic α + core orthogonality condition model. The energy spectra, electromagnetic transitions and α-spectroscopic factors are calculated. The calculated properties are in good overall agreement with the available experimental data. It is shown that the α-cluster states yield similar rotational bands which are characterized by the weak coupling feature and parity-doublet. The observed E2 intra-band transitions in the cluster bands, which show strong enhancement, and other transitions are systematically reproduced by the model. The model also gives a good account for enhancement and spreading of the α-spectroscopic factors. It is discussed that the coexistence and interference of α-cluster states and shellmodel states are important for understanding these features of the nuclei.

Alpha-cluster bands in 40Ca observed via the (6Li, d) reaction

Abstract The Kπ = 0− band of the parity doublet bands in 40Ca, which exhibits an alpha-cluster structure, has been observed via the (6Li, d) reaction. As a possible candidate of the members of the band, the states from the 8.15 MeV, 1− up to the 12.65 MeV, 7− were observed. Furthermore, the 8+ state of the Kπ = 0+ band was observed at 10.34 MeV.

Cluster structure and collective behavior of the nucleus 48Cr

Abstract The structure of the nucleus 48 Cr was investigated by the 40 Ca +α+α orthogonality condition model (OCM). The energy spectra, electromagnetic transitions, and spectroscopic factors for the α+ 44 Ti and 8 Be + 40 Ca channels were calculated and are discussed herein. The observed energies and E2 transitions of the yrast band, which show collective behavior, were found to be well reproduced by the cluster model. Some exotic 2α -cluster states with “Ca– α – α ”- and “ α –Ca– α ”-like configurations are predicted near the 2α threshold. The structures of the states were analyzed through the wave functions. The mechanism of backbending of the yrast band can be explained naturally in terms of the cluster-coupling scheme.

Airy structure in rainbow scattering and nucleus-nucleus interaction

Abstract Mechanism of Airy structure in nuclear rainbow scattering and cluster structure for the 16 O+ 16 O and 16 O+ 12 C systems are investigated by using global deep potentials.

Cluster structure of 48Cr

The structure of the nucleus 48Cr is investigated by the 40Ca+α+α orthogonality condition model. The energy spectra and electromagnetic transitions are calculated. The observed energies and E2 transitions of the yrast band, which show interesting collective behavior, are well reproduced by the cluster model.

Alpha scattering from nonclosed shell nucleus 12C and alpha-nucleus interaction

In order to understand the α-nucleus interactions for the α+12C∼α+16O systems systematically, α scattering from 12C is studied in a microscopic coupled-channel model using a folding potential and realistic microscopic wave functions of 12C. The experimental angular distributions of elastic and inelastic scattering to the 2+(4.43 MeV), 3− (9.64 MeV), and 02+ (7.66 MeV) states in the range of Eα = 41–172.5 MeV are analyzed.

Rainbow, Airy structure, and molecular structure in the 16O + 16O System

Rainbow, Airy structure, and molecular structure in the 16O + 16O system are investigated from the viewpoint of a unified description of the composite system. The potential for the 16O + 16O system determined from rainbow scattering is applied to low-energy scattering near the Coulomb barrier. Quasi-bound and bound molecular states are calculated in the complex scaling method. Evidence for the existence of the lowest N=24 and N=28 molecular bands with the 16O + 16O structure in 32S is shown.

16 O+ 16 O Molecular Structure and Superdeformation in 32 S

The molecular structure with the 16O+16O configuration in 32S and the superdeformed structure in 32S are discussed from the viewpoint of the cluster model.

Interpretation of Airy minima in 16O + 16O and 16O + 12C elastic scattering in terms of a barrier-wave/internal-wave decomposition

The observation of refractive effects in 16O+16O and 16O+12C elastic scattering data has definitively established the fact that the optical potential for some light heavy-ion systems is relatively transparent and that its real part is deep. Most of the interpretations of the rainbow features of these data rely on the so-called nearside-farside decomposition of the scattering amplitude. Starting from recent optical model analyses of 16O+16O and 16O+12C elastic scattering around 100 MeV incident energy as an example, we present an alternative interpretation based on the barrier-wave/internal-wave decomposition first proposed by Brink and Takigawa. This method, which complements the nearside-farside approach, demonstrates clearly the exceptional transparency of the 16O+16O, and to a lesser extent 16O+12C, interactions at the investigated energies and makes possible the extraction of the two contributions whose interference explains the Airy oscillations seen in the farside amplitude.