arXiv subjects

N. Itagaki

Publications and source records attributed to N. Itagaki.

At least 37 records · Page 2Linked to original sources

arXiv2017

Effects of cluster-shell competition and BCS-like pairing in $^{12}$C

The antisymmetrized quasi-cluster model (AQCM) was proposed to describe α-cluster and $jj$-coupling shell models on the same footing. In this model, the cluster-shell transition is characterized by two parameters; $R$ representing the distance between α clusters and α describing the breaking of α clusters, and the contribution of the spin-orbit interaction, very important in the $jj$-coupling shell model, can be taken into account starting with the α cluster model wave function. Not only the closure configurations of the major shells, but also the subclosure configurations of the $jj$-coupling shell model can be described starting with the α-cluster model wave functions; however, the particle hole excitations of single particles have not been fully established yet. In this study we show that the framework of AQCM can be extended even to the states with the character of single particle excitations. For $^{12}$C, two particle two hole (2p2h) excitations from the subclosure configuration of $0p_{3/2}$ corresponding to BCS-like pairing are described, and these shell model states are coupled with the three α cluster model wave functions. The correlation energy from the optimal configuration can be estimated not only in the cluster part but also in the shell model part. We try to pave the way to establish a generalized description of the nuclear structure.

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arXiv2017

Improved version of simplified method for including tensor effect in cluster models

The tensor contribution can be directly incorporated in the cluster model in a simplified way. In conventional $α$ cluster models, the contribution of the non-central interactions exactly cancels because of the antisymmetrization effect and spatial symmetry of $α$ clusters. The mixing of breaking components of $α$ clusters to take into account the spin-orbit and tensor effects is needed. Previously we proposed a simplified method to include the spin-orbit effect, and also for the tensor part, a simplified model to directly take into account the contribution of the tensor interaction (called SMT) was introduced; however the contribution of the tensor interaction was quite limited. Here we improve SMT, which is called $i$SMT. Using newly proposed $i$SMT, the contribution of the tensor interaction in $^4$He is more than $-40$ MeV, four times larger than the previous version. The method is applied to four-$α$ cluster structure of $^{16}$O. In $^{16}$O, the tensor contribution is also large, and this is coming from the finite size effect for the distances among $α$ clusters with a tetrahedral configuration.

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arXiv2016

Consistent description of 12C and 16O using finite range three-body interaction

Consistent description of 12C and 16O has been a long standing problem of microscopic alpha cluster models, where the wave function is fully antisymmetrized and the effective interaction is applied not between alpha clusters but between nucleons. When the effective interaction is designed to reproduce the binding energy of 16O (four alpha), the binding energy of 12C (three alpha) becomes underbound by about 10 MeV. In the present study, by taking into account the coupling with the jj-coupling shell model components and utilizing Tohsaki interaction, which is phenomenological but has finite-range three-body interaction terms, we show that consistent understanding of these nuclei can be achieved. The original Tohsaki interaction gives small overbound of about 3 MeV for 16O, and this is improved by slightly modifying three-body Majorana exchange parameter. Also, the coupling with the jj-coupling shell model wave function strongly contributes to the increase of the binding energy of 12C. So far the application of Tohsaki interaction has been limited to 4N nuclei, and here, we add Bartlet and Heisenberg exchange terms in the two-body interaction for the purpose of applying it to neutron-rich systems, and it is applied to 6He.

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arXiv2016

Cluster-shell competition and its effect on the $E0$ transition probability in $^{20}$Ne

$^{20}$Ne has been known as a typical example of a nucleus with $α$ cluster structure ($^{16}$O+$α$ structure). However according to the spherical shell model, the spin-orbit interaction acts attractively for four nucleons outside of the $^{16}$O core, and this spin-orbit effect cannot be taken into account in the simple $α$ cluster models. We investigate how the $α$ cluster structure competes with independent particle motions of these four nucleons. The antisymmetrized quasi-cluster model (AQCM) is a method to describe a transition from the $α$ cluster wave function to the $jj$-coupling shell model wave function. In this model, the cluster-shell transition is characterized by only two parameters; $R$ representing the distance between clusters and $Λ$ describing the breaking of $α$ clusters, and the contribution of the spin-orbit interaction, very important in the $jj$-coupling shell model, can be taken into account by changing $α$ clusters to quasi clusters. In this article, based on AQCM, we apply $^{16}$O plus one quasi cluster model for $^{20}$Ne. Here we focus on the $E0$ transition matrix element, which has been known as the quantity characterizing the cluster structure. The $E0$ transition matrix elements are sensitive to the change of the wave functions from $α$ cluster to $jj$-coupling shell model.

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arXiv2016

Effect of cluster-shell competition of $^{12}$C on $E0$ transitions in $^{16}$O

In $^{16}$O, we investigate the relation between the $E0$ monopole transition matrix elements and cluster-shell competition using antisymmetrized quasi cluster model (AQCM), where the dissolution of $alpha$ clusters into quasi clusters due to the effect of the spin-orbit force is introduced. We focus on the structure change when the strength of the spin-orbit force is varied. The ground state dominantly has a compact four $alpha$ structure (doubly magic structure of the $p$ shell), which is rather independent of the strength. However the first excited state ($0_2^+$) has $^{12}$C$+alpha$ cluster structure and this state is largely affected by the increase of the strength; the subclosure configuration of the $p_{3/2}$ shell for the $^{12}$C cluster part becomes more important than three $alpha$ configuration. The $E0$ transition from the ground state with compact four $alpha$ configuration to the $0^+_2$ state with $^{12}$C$+alpha$ cluster configuration is suppressed when increasing the spin-orbit strength. Although the $E0$ operator itself does not have the spin dependence, the matrix element is sensitive to the difference of intrinsic spin structures of the two states. The $E0$ transition characterizes the persistence of four $alpha$ structure in the $0^+_2$ state.

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arXiv2015

General transformation of alpha cluster model wave function to jj-coupling shell model in various 4N nuclei

The antisymmetrized quasi-cluster model (AQCM) is a method to describe a transition from the alpha-cluster wave function to the jj-coupling shell model wave function. In this model, the cluster-shell transition is characterized by only two parameters; R representing the distance between alpha clusters and Lambda describing the breaking of alpha clusters, and the contribution of the spin-orbit interaction, very important in the jj-coupling shell model, can be taken into account starting with the alpha cluster model wave function. In this article we show the generality of AQCM by extending the application to heavier region; various 4N nuclei from 4He to 52Fe. We show and compare the energy curves for the alpha+40Ca cluster configuration calculated with and without alpha breaking effect in 44Ti.

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arXiv2015

Rod-shaped Nuclei at Extreme Spin and Isospin

The anomalous rod shape in carbon isotopes has been investigated in the framework of the cranking covariant density functional theory, and two mechanisms to stabilize such a novel shape with respect to the bending motion, extreme spin, and isospin, are simultaneously discussed for the first time in a self-consistent and microscopic way. By adding valence neutrons and rotating the system, we have found the mechanism stabilizing the rod shape; i.e., the $σ$ orbitals (parallel to the symmetry axis) of the valence neutrons, important for the rod shape, are lowered by the rotation due to the Coriolis term. The spin and isospin effects enhance the stability of the rod-shaped configuration. This provides a strong hint that a rod shape could be realized in nuclei towards extreme spin and isospin.

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arXiv2014

Searching for 4$α$ linear-chain structure in excited states of $^{16}$O with a covariant density functional theory

A study of 4$α$ linear-chain structure in high-lying collective excitation states of $^{16}$O with a covariant density functional theory is presented. The low-spin states are obtained by configuration mixing of particle-number and angular-momentum projected quadrupole deformed mean-field states with generator coordinate method. The high-spin states are determined by cranking calculations. These two calculations are based on the same energy density functional PC-PK1. We have found a rotational band at low-spin with the dominated intrinsic configuration considered to be the one that 4$α$ clusters stay along a common axis. The strongly deformed rod shape also appears in the high-spin region with the angular momentum $13-18\hbar$; however whether the state is pure $4α$ linear chain or not is less obvious than that in the low-spin states.

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arXiv2014

High-spin torus isomers and their precession motions

We systematically investigate the existence of exotic torus isomers and their precession motions for a series of $N=Z$ even-even nuclei from $^{28}$Si to $^{56}$Ni. We analyze the microscopic shell structure of the torus isomer and discuss why the torus shape is generated beyond the limit of large oblate deformation. We use the cranked three-dimensional Hartree-Fock (HF) method with various Skyrme interactions in a systematic search for high-spin torus isomers. We use the three-dimensional time-dependent Hartree-Fock (TDHF) method for describing the precession motion of the torus isomer. We obtain high-spin torus isomers in $^{36}$Ar, $^{40}$Ca, $^{44}$Ti, $^{48}$Cr, and $^{52}$Fe. The emergence of the torus isomers is associated with the alignments of single-particle angular momenta, which is the same mechanism as found in $^{40}$Ca. It is found that all the obtained torus isomers execute the precession motion at least two rotational periods. The moment of inertia about a perpendicular axis, which characterizes the precession motion, is found to be close to the classical rigid-body value. The high-spin torus isomer of $^{40}$Ca is not an exceptional case. Similar torus isomers exist widely in nuclei from $^{36}$Ar to $^{52}$Fe and they execute the precession motion. The torus shape is generated beyond the limit of large oblate deformation by eliminating the $0s$ components from all the deformed single-particle wave functions to maximize their mutual overlaps.

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arXiv2013

Pure Collective Precession Motion of High-Spin Torus Isomer

We investigate the precession motion of the exotic torus configuration in high-spin excited states of $^{40}$Ca. For this aim, we use the three-dimensional time-dependent Hartree-Fock (TDHF) method. Although the high-spin torus isomer is a unique quantum object characterized by the alignment of angular momenta of independent single-particle motions, we find that the obtained moment of inertia for rotations about an axis perpendicular to the symmetry axis is close to the rigid-body value. We also analyze the microscopic structure of the precession motion using the random-phase approximation (RPA) method for high-spin states. In the RPA calculation, the precession motion of the torus isomer is generated by coherent superposition of many one-particle-one-hole excitations across the sloping Fermi surface that strongly violates the time-reversal symmetry. By comparing results of the TDHF and the RPA calculations, we find that the precession motion obtained by the TDHF calculation is a pure collective motion well decoupled from other collective modes.

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arXiv2013

Investigation of 3/2$_2^-$ state of $^{9}$Li nucleus with microscopic structure and reaction models

The low-lying states of the $^{9}$Li nucleus are investigated with a unified framework of microscopic structure and reaction models. In the structure model, the wave function is fully antisymmetrized and the $^{9}$Li nucleus is described as an $α$ + $t$ + $n$ + $n$ four-body system, and low-lying 1/2$^{-}$, 3/2$^{-}$, 5/2$^{-}$, and 7/2$^{-}$ states are obtained by the stochastic multi-configuration mixing method. Using these wave functions, the quasi-elastic cross section at $E/A$ = 60 MeV and the elastic and inelastic cross sections at $E/A$ = 50 MeV on the $^{12}$C target are calculated in the framework of the microscopic coupled channel (MCC) method. The characteristic inelastic angular distribution is seen in the 3/2$_{2}^{-}$ state, whose $α+t$ cluster structure and valence neutron configurations are discussed in detail. We find the possibility of triaxial deformation and mixing of di-neutron components in the $^{9}$Li nucleus.

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arXiv2012

Existence of exotic torus configuration in high-spin excited states of $^{40}$Ca

We investigate the possibility of the existence of the exotic torus configuration in the high-spin excited states of $^{40}$Ca. We here consider the spin alignments about the symmetry axis. To this end, we use a three-dimensional cranked Skyrme Hartree-Fock method and search for stable single-particle configurations. We find one stable state with the torus configuration at the total angular momentum $J=$ 60 $\hbar$ and an excitation energy of about 170 MeV in all calculations using various Skyrme interactions. The total angular momentum J=60 $\hbar$ consists of aligned 12 nucleons with the orbital angular momenta $Λ=+4$, +5, and +6 for spin up-down neutrons and protons. The obtained results strongly suggest that a macroscopic amount of circulating current breaking the time-reversal symmetry emerges in the high-spin excited state of $^{40}$Ca.

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arXiv2012

Search for three alpha states around an $^{16}$O core in $^{28}$Si

We investigate the existence of weakly coupled gas-like states comprised of three $α$ particles around an $^{16}$O core in $^{28}$Si. We calculate the excited states in $^{28}$Si using the multi-configuration mixing method based on the $^{16}$O + 3$α$ cluster model. We also include the $^{16}$O + $^{12}$C and $^{24}$Mg + $α$ basis wave functions prepared by the generator coordinate method. To identify the gas-like states, we calculate the isoscalar monopole transition strengths and the overlap of the obtained states with the geometrical cluster wave function and the Tohsaki-Horiuchi-Schuck-Röpke (THSR) wave function. The results show that the obtained fourth and twelfth states significantly overlap with the THSR wave function. These two states clearly coexist with the $^{16}$O + $^{12}$C cluster states, emerging at similar energies. The calculated isoscalar monopole strengths between those two states are significantly large, indicating that the states are members of the excitation mode. Furthermore, the calculated root-mean-squared (RMS) radii for these states also suggest that a layer of gas-like three $α$ particles could exist around the surface of the $^{16}$O core, which can be described as a "two-dimensional gas" in the intermediate state before the Hoyle-like three $α$ states emerge.

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arXiv2011

Linear chain structure of four-$α$ clusters in $^{16}$O

We investigate the linear-chain configurations of four-$α$ clusters in $^{16}$O using a Skyrme cranked Hartree-Fock method and discuss the relationship between the stability of such states and angular momentum. We show the existence of a region of angular momentum (13-18 $\hbar$) where the linear chain configuration is stabilized. For the first time we demonstrate that stable exotic states with a large moment of inertia ($\hbar^2/2Θ$ $\sim$ 0.06-0.08 MeV) can exist.

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arXiv2011

Static and Dynamic Chain Structures in the Mean-Field Theory

We give a brief overview of recent work examining the presence of $α$-clusters in light nuclei within the Skyrme-force Hartree-Fock model. Of special significance are investigations into $α$-chain structures in carbon isotopes and $^{16}$O. Their stability and possible role in fusion reactions are examined in static and time-dependent Hartree-Fock calculations. We find a new type of shape transition in collisions and a centrifugal stabilization of the $4α$ chain state in a limited range of angular momenta. No stabilization is found for the $3α$ chain.

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arXiv2011

Three triton states in 9Li

We focus on a characteristic non-alpha cluster structure in light neutron-rich nuclei; three triton structure in 9Li. This is an analogy to the case of three alpha state in 12C (Hoyle state). The alpha clusters behave as bosons. however tritons have Fermionic nature, and how the three cluster structure is different from 12C is an intriguing problem. For this purpose, we introduce three triton wave functions. In addition, alpha+t+n+n wave functions are prepared to describe other low-lying states of 9Li, and the coupling effect between them is taken into account. The states with dominantly the three triton components appear below the three triton threshold energy, where three triton correlation is important, however the root mean square radius is not enhanced contrary to the alpha gas states in 12C and 16O.

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arXiv2011

Gas-like state of $α$ clusters around $^{16}$O core in $^{24}$Mg

We have studied gas-like states of $α$ clusters around an $^{16}$O core in $^{24}$Mg based on a microscopic $α$-cluster model. This study was performed by introducing a Monte Carlo technique for the description of the THSR (Tohsaki Horiuchi Schuck Röpke) wave function, and the coupling effect to other low-lying cluster states was taken into account. A large isoscalar monopole ($E0$) transition strength from the ground to the gas-like state is discussed. The gas-like state of two $α$ clusters in $^{24}$Mg around the $^{16}$O core appears slightly below the 2$α$-threshold e

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arXiv2011

Symplectic structure and monopole strength in 12C

The relation between the monopole transition strength and existence of cluster structure in the excited states is discussed based on an algebraic cluster model. The structure of $^{12}$C is studied with a 3$α$ model, and the wave function for the relative motions between $α$ clusters are described by the symplectic algebra $Sp(2,R)_z$, which corresponds to the linear combinations of $SU(3)$ states with different multiplicities. Introducing $Sp(2,R)_z$ algebra works well for reducing the number of the basis states, and it is also shown that states connected by the strong monopole transition are classified by a quantum number $Λ$ of the $Sp(2,R)_z$ algebra.

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