arXiv subjects

Kosuke Nomura

Publications and source records attributed to Kosuke Nomura.

At least 19 recordsLinked to original sources

arXiv2026

Octupole correlation effects on two-neutron transfer intensity in rare-earth nuclei

Impacts of octupole correlations on the low-lying $0^+$ states and two-neutron transfer intensities in rare-earth nuclei are investigated in terms of the interacting boson model that is based on the nuclear density functional theory. The octupole degrees of freedom are not only essential building blocks to describe properties of negative-parity states in the model, but also influence low-spin positive-parity states including excited $0^+$ states. The calculation produces a large number of low-energy $0^+$ states that contain significant amounts of octupole components, indicating important roles played by the octupole degrees freedom in this mass region. Octupole correlations are shown to make sizable contributions to the $(p,t)$ and $(t,p)$ transfer intensities and, in particular, to reproduce the discontinuous changes of these quantities near those nuclei with $N\approx88$ or 90, which are observed experimentally as a signature of the shape phase transition.

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arXiv2026

Effects of shape coexistence and configuration mixing on low-lying states in tellurium isotopes

Low-energy quadrupole collective states in even-even tellurium (Te) isotopes are studied using the interacting boson model with configuration mixing. The corresponding Hamiltonian is determined by means of the microscopic nuclear structure calculations within the self-consistent mean-field method employing a given energy density functional and pairing interaction. Calculated low-energy levels for nonyrast states show a parabolic behavior characteristic of the shape-coexisting structure. The intruder prolate-shape configuration is shown to mix strongly with the normal oblate-shape configuration, and play an important role in determining the low-lying structure in the Te isotopes near the middle of the neutron major shell closures.

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arXiv2025

Neutrinoless $ββ$ decay in the interacting boson model based on the nuclear energy density functionals

The neutrinoless $ββ$ ($0νββ$) decay nuclear matrix elements (NMEs) are calculated in the interacting boson model (IBM), which is based on the nuclear energy density functional (EDF) theory. The Hamiltonian of the IBM that gives rise to the energies and wave functions of the ground and excited states of $0νββ$ decay emitting isotopes and corresponding final nuclei is determined by mapping the self-consistent mean-field deformation-energy surface obtained with a given EDF onto the corresponding bosonic energy surface. The transition operators are formulated using the generalized seniority scheme, and the pair structure constants are determined by the inputs provided by the self-consistent calculations. The predicted values of the $0νββ$-decay NMEs with the nonrelativistic and relativistic EDFs are compared with those resulting from different many-body methods. Sensitivities of the predicted NMEs to the model parameters and assumptions are discussed.

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arXiv2025

Configuration mixing effects on neutrinoless $ββ$-decay nuclear matrix elements

Mixing and coexistence of intrinsic nuclear shapes play an important role to determine the low-energy structure of heavy nuclei, and are expected to affect nuclear matrix elements (NMEs) of neutrinoless double beta ($0νββ$) decay. This problem is addressed in the interacting boson model with configuration mixing that is formulated by using the nuclear energy density functional theory. It is shown that significant amounts of mixing of normal and deformed intruder configurations are present in the ground and excited $0^+$ states in the even-even nuclei that are parent or daughter nuclei of the $0νββ$ decay. An illustrative application to the $0νββ$ decays of $^{76}$Ge, $^{96}$Zr, $^{100}$Mo, $^{116}$Cd, and $^{150}$Nd shows that the inclusion of the configuration mixing reduces the NMEs for most of the $0^+_1$ $\to$ $0^+_1$ $0νββ$ decays.

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arXiv2025

Mapped interacting boson model for nuclear structure studies

The present status of the mapped interacting boson model studies on nuclear structure is reviewed. With the assumption that the nuclear surface deformation induced by the multi-nucleon dynamics is simulated by bosonic degrees of freedom, the interacting-boson Hamiltonian that provides energy spectra and wave functions is determined by mapping the potential energy surface that is obtained from self-consistent mean-field calculations based on the energy density functional onto the corresponding energy surface of the boson system. This procedure has been shown to be valid in general cases of the quadrupole collective states, and has allowed for systematic studies on spectroscopic properties of medium-heavy and heavy nuclei, including those that are far from the line of $β$ stability. The method has been extended to study nuclear structure phenomena that include shape phase transitions and coexistence, octupole deformation and collectivity, and the coupling of the single-particle to collective degrees of freedom, which is crucial to describe structures of odd nuclei, and $β$ and $ββ$ decays.

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arXiv2024

Effects of pairing strength on the nuclear structure and double-$β$ decay predictions within the mapped interacting boson model

The low-energy nuclear structure and two-neutrino double-$β$ ($2νββ$) decay are studied within the interacting boson model (IBM) that is based on the nuclear energy density functional (EDF). The IBM Hamiltonian describing the initial and final even-even nuclei, and the interacting boson fermion-fermion Hamiltonian producing the intermediate states of the neighboring odd-odd nuclei are determined by the microscopic inputs provided by the self-consistent mean-field (SCMF) calculations employing a relativistic EDF and a separable pairing force. Sensitivities of the low-lying structure and $2νββ$-decay properties to the pairing strength are specifically analyzed. It is shown that the SCMF calculations with decreased and increased pairing strengths lead to quadrupole-quadrupole interaction strengths in the IBM that are, respectively, significantly enhanced and reduced in magnitude. When the increased pairing is adopted, in particular, the energy levels of the excited $0^+$ states are lowered, and the predicted $2νββ$-decay nuclear matrix elements (NMEs) increase in magnitude systematically. The mapped IBM employing the increased pairing force generates effective NMEs and half-lives that are in a reasonable agreement with the experimental data for the $^{76}$Ge$\to^{76}$Se, $^{82}$Se$\to^{82}$Kr, and $^{100}$Mo$\to^{100}$Ru decays in particular, whereas the calculation with the standard pairing strength is adequate to provide an overall good description of the effective NMEs in agreement with data.

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arXiv2024

Weakening of $N=28$ shell gap and the nature of $0_2^+$ states

The work reports a novel application of the interacting boson model in light-mass region around $^{48}$Ca, that takes into account intruder states and configuration mixing. The model is shown to provide a reasonable description of the observed low-lying yrast and yrare states of the $N=28$ even-even isotones from Si to Fe, and even-even Ca isotopes. The nature of $0_2^+$ states is addressed in terms of the competition between spherical and deformed intruder configurations, and the rigidity of the $N=28$ shell gap is tested. The $0_2^+$ isomer in $^{44}$S is shown to arise from a weak mixing between the two configurations. The unresolved low-lying spectra in $^{46}$Ar is also approached using the core-excitations across $N=28$ shell. The SU(3) structure in $^{42}$Si is supported by the present calculation. The nearly spherical nature of $^{50}$Ti, $^{52}$Cr, $^{54}$Fe and $^{42,44,46}$Ca isotopes is found, while the core excitations are found to be essential for the description of yrare states. Shell model calculation is also performed for comparison.

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arXiv2024

Low-energy structure and $β$ decay properties of neutron-rich nuclei in the region of a shape phase transition

The low-energy structure and $β$ decay properties of the neutron-rich even-mass nuclei near the neutron number $N=60$ that are experimentally of much interest are investigated within the framework of the nuclear density functional theory and the interacting boson-fermion-fermion model. By using the results of the constrained self-consistent mean-field calculations based on the relativistic energy density functional, the interacting-boson Hamiltonian describing the even-even core nuclei, the boson-fermion, and the fermion-fermion interactions are determined. The Gamow-Teller transition strengths, with the corresponding operator being constructed without introducing further phenomenological adjustment, are computed by using the wave functions of the initial and final nuclei of the $β$ decay. The triaxial quadrupole potential energy surfaces computed for the $N=60$ even-even isotones suggest a pronounced $γ$ softness. The calculated energy spectra for the even-even and odd-odd nuclei in the Rb to Cd isotopic chains exhibit an abrupt change in nuclear structure around $N=60$, as suggested experimentally. The predicted $β$-decay $\log{}_{10}ft$ values underestimate the measured values for the nuclei with low $Z$ and with $N \leqslant 60$, exhibit a rapid increase for $N>60$, reflecting the nuclear structure evolution, and agree rather well with the measured values for those nuclei with $Z$ being not far from the proton major shell closure $Z=50$. Sensitivity of the predicted $β$-decay properties to the model assumptions and parameters employed in the nuclear structure calculations is discussed, specifically, by comparing results obtained based on the different choices of the underlying energy density functional.

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arXiv2024

Systematic shell-model study for structure and isomeric states in $^{200-210}$Po isotopes

We report systematic large-scale shell-model calculation for Po isotopes with $A=$ 200 to 210. We have performed calculations using KHH7B interaction in the model space $Z$ = 58-114 and $N$ = 100-164 around doubly-magic $^{208}$Pb. We allow valence neutrons to occupy in the $1f_{5/2}$, $2p_{3/2}$, $2p_{1/2}$, and $0i_{13/2}$ orbitals, while two valence protons beyond $Z=82$ are occupied in $0h_{9/2}$, $1f_{7/2}$ and $0i_{13/2}$ orbitals. The calculated energies and electromagnetic properties are compared with the available experimental data and predicted where experimental data are not available. We have also reported shell-model results for different isomeric states of these nuclei.

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arXiv2022

Overview of Seniority Isomers

Nuclear isomers are the metastable excited states of nuclei. The isomers can be categorized into a few classes including spin, seniority, \emph{K}, shape and fission isomers depending upon the hindrance mechanisms. In this paper, we aim to present an overview of seniority isomers, which is a category related to the seniority quantum number. The discussion is mainly based on the concepts of seniority and generalized seniority. Various aspects of seniority isomers and their whereabouts have been covered along with the situations where seniority mixing prevents the isomerism.

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arXiv2022

Microscopic formulation of the interacting boson model for reflection asymmetric nuclei

Reflection asymmetric, octupole shapes in nuclei are a prominent aspect of nuclear structure, and have been recurrently studied over the decades. Recent experiments using radioactive-ion beams have provided evidence for stable octupole shapes. A variety of nuclear models have been employed for the related theoretical analyses. We review recent studies on the nuclear octupole shapes and collective excitations within the interacting boson model. A special focus is placed on the microscopic formulation of this model by using the mean-field method that is based on the framework of nuclear density functional theory. As an illustrative example, a stable octupole deformation, and a shape phase transition as a function of nucleon number that involves both quadrupole and octupole degrees of freedom are shown to occur in light actinides. Systematic spectroscopic studies indicate enhancement of the octupole collectivity in a wide mass region. Couplings between the octupole and additional degrees of freedom are incorporated in a microscopic manner in the boson system, and shown to play a crucial role in the description of the related intriguing nuclear structure phenomena such as the shape coexistence.

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arXiv2022

Effect of configuration mixing on quadrupole and octupole collective states of transitional nuclei

A model is presented that simultaneously describes shape coexistence and quadrupole and octupole collective excitations within a theoretical framework based on the nuclear density functional theory and the interacting boson model. An optimal interacting-boson Hamiltonian that incorporates the configuration mixing between normal and intruder states, as well as the octupole degrees of freedom, is identified by means of self-consistent mean-field calculations using a universal energy density functional and a pairing interaction, with constraints on the triaxial quadrupole and the axially-symmetric quadrupole and octupole shape degrees of freedom. An illustrative application to the transitional nuclei $^{72}$Ge, $^{74}$Se, $^{74}$Kr, and $^{76}$Kr shows that the inclusion of the intruder states and the configuration mixing significantly lower the energy levels of the excited $0^+$ states, and that the predicted low-lying positive-parity states are characterized by the strong admixture of nearly spherical, weakly deformed oblate, and strongly deformed prolate shapes. The low-lying negative-parity states are shown to be dominated by the deformed intruder configurations.

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arXiv2022

Octupole correlations in collective excitations of neutron-rich $N\approx56$ nuclei

Octupole correlations in the low-energy collective states of neutron-rich nuclei with the neutron number $N\approx56$ are studied within the interacting boson model (IBM) that is based on the nuclear density functional theory. The constrained self-consistent mean-field (SCMF) calculations using a universal energy density functional and a pairing interaction provide the potential energy surfaces in terms of the axially-symmetric quadrupole and octupole deformations for the even-even nuclei $^{86-94}$Se, $^{88-96}$Kr, $^{90-98}$Sr, $^{92-100}$Zr, and $^{94-102}$Mo. The SCMF energy surface is then mapped onto the energy expectation value of a version of the IBM in the boson condensate state, which consists of the neutron and proton monopole $s$, quadrupole $d$, and octupole $f$ bosons. This procedure determines the strength parameters of the IBM Hamiltonian, which is used to compute relevant spectroscopic properties. At the SCMF level, no octupole deformed ground state is obtained, while the energy surface is generally soft in the octupole deformation at $N\approx56$. The predicted negative-parity yrast bands with the bandhead state $3^-_1$ weakly depend on $N$ and become lowest in energy at $N=56$ in each of the considered isotopic chains. The model further predicts finite electric octupole transition rates between the lowest negative- and the positive-parity ground-state bands.

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arXiv2022

$β$ decay and evolution of low-lying structure in Ge and As nuclei

A simultaneous calculation for the shape evolution and the related spectroscopic properties of the low-lying states, and the $β$-decay properties in the even- and odd-mass Ge and As nuclei in the mass $A\approx70-80$ region, within the framework of the nuclear density functional theory and the particle-core coupling scheme, is presented. The constrained self-consistent mean-field calculations using a universal energy density functional (EDF) and a pairing interaction determines the interacting-boson Hamiltonian for the even-even core nuclei, and the essential ingredients of the particle-boson interactions for the odd-nucleon systems, and of the Gamow-Teller and Fermi transition operators. A rapid structural evolution from $γ$-soft oblate to prolate shapes, as well as the spherical-oblate shape coexistence around the neutron sub-shell closure $N=40$, is suggested to occur in the even-even Ge nuclei. The predicted low-energy spectra, electromagnetic transition rates, and $β$-decay $\log{ft}$ values are in a reasonable agreement with experiment. The predicted $\log{ft}$ values reflect the structures of the wave functions for the initial and final nuclei of $β$ decay, which are, to a large extent, determined by the microscopic input provided by the underlying EDF calculation.

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arXiv2022

Two-neutrino double-$β$ decay in the mapped interacting boson model

A calculation of two-neutrino double-$β$ ($2νββ$) decay matrix elements within the interacting boson model (IBM) that is based on the nuclear density functional theory is presented. The constrained self-consistent mean-field (SCMF) calculation using a universal energy density functional (EDF) and a pairing interaction provides potential energy surfaces with triaxial quadrupole degrees of freedom for even-even nuclei corresponding to the initial and final states of the $2νββ$ decays of interest. The SCMF energy surface is then mapped onto the bosonic one, and this procedure determines the IBM Hamiltonian for the even-even nuclei. The same SCMF calculation provides the essential ingredients of the interacting boson fermion-fermion model (IBFFM) for the intermediate odd-odd nuclei and the Gamow-Teller and Fermi transition operators. The EDF-based IBM and IBFFM provide a simultaneous description of excitation spectra and electromagnetic transition rates for each nucleus, and single-$β$ and $ββ$ decay properties. The calculated $2νββ$-decay nuclear matrix elements are compared with experiment and with those from earlier theoretical calculations.

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arXiv2019

Beyond mean-field boson-fermion description of odd nuclei

We develop a novel theoretical method for calculating spectroscopic properties of those nuclei with odd number of nucleons, that is based on the nuclear density functional theory and the particle-boson coupling scheme. Self-consistent mean-field calculation based on the DFT is performed to provide microscopic inputs to build the Hamiltonian of the interacting boson-fermion systems, which gives excitation spectra and transition rates of odd-mass nuclei. The method is successfully applied to identify the quantum shape phase transitions and the role of octupole correlations in odd-mass nuclei, and is extended further to odd-odd nuclear systems.

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arXiv2019

Signatures of octupole shape phase transitions in radioactive nuclei

We analyze the octupole deformations and the related collective excitations in medium-heavy and heavy nuclei based on the microscopic framework of the nuclear energy density functional theory. Constrained self-consistent mean-field calculation with a given energy density functional is performed to provide for each nucleus a potential energy surface with axial quadrupole and octupole shape degrees of freedom. Spectroscopic properties are computed by means of the interacting-boson Hamiltonian, which is determined by mapping the fermionic potential energy surface onto the bosonic counterpart. The overall systematics of the calculated spectroscopic observables exhibit phase transitional behaviors between stable octupole deformation and octupole vibration characteristic of the octupole-soft potential within the set of nuclei in light actinide and rare-earth regions, Th, Ra, Sm, Gd, and Ba isotopes, where octupole shapes are most likely to occur.

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arXiv2019

Coexistence and evolution of shapes: mean-field-based interacting boson model

A method of deriving the Hamiltonian of the interacting boson model, that is based on the microscopic framework of the nuclear energy density functional, is presented. The constrained self-consistent mean-field calculation with a given energy density functional provides potential energy surface within the relevant collective coordinates, which is subsequently mapped onto the expectation value of the interacting-boson Hamiltonian in the boson condensate state. This procedure completely determines the strength parameters of the IBM, and the diagonalization of the mapped Hamiltonian yields excitation spectra and transition rates for a given nucleus. Two recent applications of the method are discussed, that is, the descriptions of the intruder states in Cadmium isotopes and the octupole correlations in neutron-rich odd-mass Barium isotopes.

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