Kiyomi Ikeda

Unified studies of chemical bonding structures and resonant scattering in light neutron-excess systems, 10,12Be.

The generalized two-center cluster model (GTCM), which can treat covalent, ionic and atomic configurations in general systems with two inert cores plus valence nucleons, is formulated in the basis of the microscopic cluster model. In this model, the covalent configurations constructed by the molecular orbital (MO) method and the atomic (or ionic) configuration obtained by the valence bonding (VB) method can be handled in a consistent manner. The GTCM is applied to the light neutron-rich system (10,12)Be = α + α + Xn (X = 2, 4). The continuous and smooth changes of the neutron orbits from the covalent MO states to the ionic VB states are clearly observed in the adiabatic energy surfaces (AESs), which are the energy curves obtained with a variation of the α-α distance. The energy levels obtained from the AESs nicely reproduce the recent observations over a wide energy region. The individual spectra are characterized in terms of chemical-bonding-like structures, such as the covalent MO or ionic VB structures, according to analysis of their intrinsic wave functions. From the analysis of AESs, the formation of the mysterious 0(2)(+) states in (10,12)Be, which have anomalously small excitation energies in comparison to a naive shell-model prediction, is investigated. A large enhancement in a monopole transition from a ground MO state to an ionic α + (6,8)He VB state is found, which seems to be consistent with a recent observation. In the unbound region, the structure problem, which handles the total system of α + α + Xn (X = 2, 4) as a bound or quasi-bound state, and the reaction problem, induced by the collision of an asymptotic VB state of α + (6,8)He, are combined by the GTCM. The properties of unbound resonant states are discussed in close connection to the reaction mechanism, and some enhancement factors originating from the properties of the intrinsic states are predicted in the reaction observables.

Tensor-Optimized Shell Model with Bare Nucleon-Nucleon Interaction for 4He

The pion exchange between nucleons generates a strong tensor interaction, which provides a large attractive contribution for the binding energy of nucleus. This noncentral tensor interaction is difficult to handle in the shell model framework, which hinders full understanding of nuclear structure. We develop the tensor-optimized shell model (TOSM) for the strong tensor interaction and now we are able to use bare nucleon-nucleon interaction with the help of the unitary correlation operator method (UCOM) for the short-range hard core. We adopt the nucleon-nucleon interaction, AV8 � , and calculate explicitly the ground state of 4 He and make a detailed comparison with rigorous few-body model calculations. We show a large amount of success of the tensor-optimized shell model with bare nucleon-nucleon interaction for 4 He. Subject Index: 205, 206, 210, 211, 213 It is important to develop a theoretical framework to calculate nuclear structure with many nucleons using the realistic nucleon-nucleon interaction, which is obtained from two nucleon scattering. Recently, it has become possible to calculate nuclei up to a mass of approximately A ∼ 12 1)–3) using the realistic nucleon-nucleon interaction. The method used for the calculation is the Green’s function MonteCarlo method (GFMC) with the use of relative nucleon coordinates. This method introduces various correlation functions with many variational parameters in the nuclear wave function. In GFMC, the nuclear structures and binding energies were successfully reproduced by including three-body interaction. One big surprise is the extremely large contribution of the one pion exchange interaction, which is about 70 ∼ 80% of the entire nucleon-nucleon interaction. In principle, they can extend this method to calculate heavier nuclei. It is, however, extremely time-consuming even with the present computer power. Hence, it is strongly desired to develop a new method of calculating nuclei with large nucleon numbers using the nucleon-nucleon interaction. The nucleon-nucleon interaction has distinctive features, namely there exist

Effect of Expansion of the Core to the Nuclear Radius in Oxygen Isotopes

the experiments. Therefore, a consistent mechanism, which has two different behavior of the core as described above, is necessary to be considered. For this purpose, we introduce a coupled-channel picture for the structure of the 16 O-core. This coupled-channel model successfully reproduces the change radius of oxygen isotopes at 23 O [6]. In this model, the role of the Pauli forbidden states in the core is important, and the presence of valence nucleons makes the change of the nuclear size at 23 O, which is caused by the occupation in the valence-nucleon space. In this study, the relation between the structure of the core and the presence of valence nucleons is discussed by using the COSM framework.

Coupled-channel picture of the core nucleus for the expansion of the nuclear size in the drip-line region

We study the energy and radius of neutron rich nuclei toward to the drip-line region. We employ the cluster-orbital shell model approach to calculate the radius of the oxygen isotopes. The result shows that the calculated radius underestimates the observed one at 23O, though the binding energy and the drip-line of the oxygen isotopes are reproduced. Therefore, we propose a coupled-channel approach to explain the sudden increase of the radius of the oxygen isotopes at 23O. As a result, the sudden change of the radius can be descried in terms of the competition between the available valence neutron orbits and occupied orbits in the core nucleus. For the 23O case, we show that the excitation to the 1s1/2-orbit in the core nucleus is a key configuration to reproduce the radius.

Properties of drip-line nuclei with an m-scheme cluster-orbital shell model approach

In the drip-line region of oxygen isotopes, an abrupt increase of the r.m.s.radius of 23O is observed from the analysis of the reaction cross section. We develop an m-scheme approach of COSM and perform calculations for oxygen isotopes. We examine the interaction dependence to the calculated energies and r.m.s.radii. Further, the relation between the density and nucleon-nucleon interaction is discussed.

Role of Tensor Force in Light Nuclei based on The Tensor Optimized Shell Model

We propose a new method to describe nuclear structure using bare nuclear interaction, in which the tensor and short‐range correlations are described by using the tensor optimized shell model (TOSM) and the unitary correlation operator method (UCOM), respectively.

Study of the tensor correlation in a neutron-rich sd-shell region with mean-field and beyond-mean-field methods

We study the effect of the tensor force on nuclear structure with mean‐field and beyond‐mean‐field methods. An important correlation induced by the tensor force is a two‐particle‐two‐hole (2p2h) correlation, which cannot be treated with a standard mean‐field method. To treat the 2p2h tensor correlation, we develop a new framework [charge‐ and parity‐projected Hartree‐Fock (CPPHF) method], which is a beyond‐mean‐field method. In the CPPHF method, we introduce single‐particle states with parity and charge mixing. The parity and charge projections are performed on a total wave function before variation. We apply the CPPHF method to oxygen isotopes including neutron‐rich ones. The potential energy from the tensor force has the same order of magnitude as that from the LS force and becomes smaller with neutron number, which indicates that excess neutrons do not contribute to the 2p2h tensor correlation significantly. We also study the effect of the tensor force on spin‐orbit‐splitting (ls‐splitting) in a neutro...

Study of the tensor correlation in a neutron‐rich sd‐shell region with the charge‐ and parity‐projected Hartree‐Fock method

We study the effect of the tensor force on nuclear structure with mean‐field and beyond‐mean‐field methods. An important correlation induced by the tensor force is two‐particle–two‐hole (2p2h) correlation, which cannot be treated with a usual mean‐filed method. To treat the 2p2h tensor correlation, we develop a new framework (charge‐ and parity‐projected Hartree‐Fock (CPPHF) method), which is a beyond‐mean‐field method. In the CPPHF method, we introduce single‐particle states with parity and charge mixing. The parity and charge projections are performed on a total wave function before variation. We apply the CPPHF method to oxygen isotopes including neutron‐rich ones. The potential energy from the tensor force has the same order of magnitude with that from the LS force and becomes smaller with neutron number, which indicates that excess neutrons do not contribute to the 2p2h tensor correlation significantly. We also study the effect of the tensor force on spin‐orbit‐splitting (ls‐splitting) in a neutron‐rich fluorine isotope 23F. The tensor force reduces the ls‐splitting for the proton d‐orbits by about 3 MeV. This effect is important to reproduce the experimental value. We also find that the 2p2h tensor correlation does not affect the ls‐splitting in 23F.We study the effect of the tensor force on nuclear structure with mean‐field and beyond‐mean‐field methods. An important correlation induced by the tensor force is two‐particle–two‐hole (2p2h) correlation, which cannot be treated with a usual mean‐filed method. To treat the 2p2h tensor correlation, we develop a new framework (charge‐ and parity‐projected Hartree‐Fock (CPPHF) method), which is a beyond‐mean‐field method. In the CPPHF method, we introduce single‐particle states with parity and charge mixing. The parity and charge projections are performed on a total wave function before variation. We apply the CPPHF method to oxygen isotopes including neutron‐rich ones. The potential energy from the tensor force has the same order of magnitude with that from the LS force and becomes smaller with neutron number, which indicates that excess neutrons do not contribute to the 2p2h tensor correlation significantly. We also study the effect of the tensor force on spin‐orbit‐splitting (ls‐splitting) in a neutron‐r...

Role of the Tensor Correlation on the Spin-Orbit Splitting in Neutron Halo Nuclei

We investigate the tensor correlation in neutron halo nuclei 6He and 11Li. We use the extended (4He/9Li)+n+n models for two nuclei and a special attention is paid for core nuclei (4He and 9Li) to incorporate the tensor correlation in the shell model type wave function. We discuss the Pauli‐blocking between core nuclei and last two neutrons and its effect on the spin‐orbit splitting of halo nuclei. For 6He, 02+ state is affected by this blocking from the 0s1/2 → 0p1/2 excitation in 4He. For 11Li, the blocking makes the (1s)2 and (0p)2 configurations close to each other in energy and hence mix equal amount of two configurations to develop the halo structure.

Properties of the tensor correlation in He isotopes

We investigate the roles of the tensor correlation on the structures of 4,5He. For 4He, we take the high angular momentum states as much as possible with the 2p2h excitations of the shell model type method to describe the tensor correlation. Three specific configurations are found to be favored for the tensor correlation. This correlation is also important to describe the scattering phenomena of the 4He+nsystem including the higher partial waves consistently.