arXiv subjects

K. Nomura

Publications and source records attributed to K. Nomura.

At least 19 recordsLinked to original sources

arXiv2026

Impacts of hexadecapole correlations in actinide nuclei

The impact of hexadecapole correlations on the low-energy spectroscopic properties of Th, U, and Pu nuclei, within the mass range $232 \le A \le 240$, is studied systematically using the mapped $sdg$-IBM model. Fermionic input is obtained via the quadrupole-hexadecapole constrained Hartree-Fock-Bogoliubov approximation, based on the parametrization D1S of the Gogny energy density functional. The $sdg$-IBM Hamiltonian parameters are determined by mapping the quadrupole-hexadecapole fermionic mean-field potential energy surfaces onto the corresponding bosonic surfaces. The low-energy spectra and transition strengths, obtained via the diagonalization of the $sdg$-IBM Hamiltonian, compare well with the available experimental data. It is shown that the effects of hexadecapole collectivity can be observed in high-spin yrast states with spins $J^π \geqslant 10^{+}$. The mapped $sdg$-IBM improves the excitation energies of those states, as compared with the simpler $sd$-IBM model. The $sdg$-IBM also improves the description of the $E2$ transition strengths between high-spin yrast states and predicts strong $E4$ transitions from nonyrast $4^+$ states to the $0^+$ ground state.

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arXiv2026

Hindered $ΔK=1$ Dipole Strength in octupole bands in $N=90$ $^{154}$Gd from Lifetime Measurements with $γ-γ$ fast timing technique

The lifetimes of the low-lying negative-parity $1^-$ state at 1414~keV and $2^-$ state at 1398~keV in $^{154}$Gd have been measured using the $γ$--$γ$ fast-timing technique with the VENTURE array at VECC, Kolkata. The states were populated through the $β$ decay of $^{154}$Tb, produced in proton-induced reactions at the K130 cyclotron. From the measured lifetimes, absolute $B(E1)$ transition strengths were deduced. The extracted $B(E1)$ values are compared with those of neighboring Gd isotopes and with Gogny-HFB-based $sdf$-IBM calculations. The results show that the measured $E1$ strengths from these states are strongly hindered compared with the corresponding $ΔK=0$ transitions, providing evidence for weak $ΔK=1$ dipole strength in $^{154}$Gd.

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arXiv2026

Microscopic formulation of the interacting boson-fermion model using the nuclear energy density functional

Microscopic modeling of low-energy spectroscopy in medium-heavy and heavy odd-$A$ nuclei is an outstanding open problem in nuclear physics. We propose a novel spectra-generating collective model for odd-$A$ nuclei constructed by means of the nuclear energy density functional theory and the interacting boson-fermion model. The bosonic Hamiltonian for an even-even nucleus, which is treated as a core, and the strength parameters for the interactions between the core and an odd nucleon are completely determined by using as microscopic inputs the potential energy curves and deformed single-particle spectra obtained from the self-consistent mean-field calculations. In applications to odd-$A$ Eu, Sm, La, and Ba isotopes, we demonstrate the validity of the proposed method in reproducing reasonably the observed low-energy spectra and shape phase transitions in the general cases of the quadrupole collective states, that is, nearly spherical, strongly deformed, and $γ$-soft shapes, in the presence of an odd nucleon in a single-$j$ orbit.

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arXiv2026

Microscopic derivation of the interacting boson model parameters with machine learning

Machine learning is applied to derive microscopically parameters of the interacting boson model for nuclear spectroscopy. A physics-guided neural network is proposed, which is trained to map the potential energy landscapes that are calculated within the nuclear density functional theory onto the bosonic parameter space. To incorporate the underlying nuclear structure information and mitigate parameter degeneracy, the network integrates a global quadrupole collectivity indicator and valence nucleon numbers as key input features. In its applications to rare-earth nuclei, by reproducing the microscopic energy landscapes without any manual parameter tuning, the trained network is shown to provide a set of the model parameters and energy spectra that reflect the nuclear structural evolution, offering a robust alternative microscopic description of nuclear collectivity.

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arXiv2025

Mapped $spdf$ interacting boson model for quadrupole-octupole collective states in nuclei

Dipole bosons are introduced in the interacting boson model (IBM) by means of the self-consistent mean-field method. The constrained mean-field calculations employing a given nuclear energy density functional yield the potential energy surfaces in terms of the axially-symmetric quadrupole-octupole, dipole-quadrupole, and dipole-octupole deformations. By mapping these energy surfaces onto the expectation values of the IBM Hamiltonian in the coherent state of the interacting $s$, $p$, $d$, and $f$ bosons, strength parameters of the $spdf$-IBM Hamiltonian are determined. In an illustrative application to octupole-deformed actinides $^{218-230}$Ra and $^{220-232}$Th, it is shown that effects of including $p$ bosons in the IBM mapping are to lower significantly negative-parity yrast levels, and to improve descriptions of observed energy-level systematic in nearly spherical and transitional nuclei, and of the behaviors of the reduced electric dipole transitions and intrinsic dipole moments with neutron number.

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arXiv2025

Microscopic determination of the interacting boson-fermion model Hamiltonian from the nuclear energy density functional

A microscopic formulation of the interacting boson-fermion model for odd-$A$ nuclei is made using the nuclear energy density functional framework. Strength parameters for the bosonic Hamiltonian and boson-fermion interactions are shown to be determined completely so that energy surfaces and deformed single-particle energies of the Bose-Fermi systems should match the corresponding self-consistent mean-field solutions for fermionic systems. In an illustrative application to axially symmetric odd-$A$ Eu, this procedure is shown to be valid in describing spherical-to-deformed shape phase transitions in odd-$A$ and even-even systems.

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arXiv2025

Sensitivity analysis of $β$-decay half-life predictions for Ge, As, Zr and Mo nuclei within the mapped interacting boson model

We analyze parameter sensitivities of the mapped interacting boson model (IBM) and boson-fermion-fermion model (IBFFM) in the description of $β$-decay properties of the even-mass neutron-deficient Ge and As, and neutron-rich Zr and Mo isotopes. Based on the self-consistent mean-field calculations with a given energy density functional and a pairing interaction, the IBM Hamiltonian for even-even nuclei, single-particle energies, and occupation probabilities for unpaired nucleons, which are necessary building blocks of the IBFFM Hamiltonian and Gamow-Teller and Fermi transition operators, are completely determined. A few coupling constants of the boson-fermion and residual neutron-proton interactions are only phenomenological parameters fitted to reproduce low-energy spectra of odd-mass and odd-odd nuclei. It is found that the calculated $\log{}_{10}ft$ values for the $β^+$ decays $^{68}$As$\to^{68}$Ge are particularly sensitive to the quadrupole-quadrupole boson interaction strength used for the parent ($^{68}$As) nucleus. We further incorporate higher-order terms in the one-nucleon transfer operators in the boson system, and find that, while their effects are non-negligible, they do not significantly alter qualitative features of $β$-decay properties. We report a novel application of the mapped IBM framework to compute $β$-decay half-lives, and show that the observed trend along isotopic chains are reasonably reproduced.

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arXiv2025

Nuclear size, electric monopole transitions, and the location of $0^+_2$ states

The work addresses the isotopic shift of nuclear radii for the even-even $^{36-52}$Ca isotopes using the interacting boson model (IBM) that includes the mixing from normal and intruder configurations. We obtain a good agreement between the calculated and experimental data, particularly for the dip at $^{48}$Ca. A direct correlation between nuclear size and electric monopole transitions is established to compute the electric monopole transition strengths, $ρ^2(E0)$. We further study the isotopic shift for the even-even $^{32-46}$Ar and $^{44-50}$Ti isotopes.

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arXiv2025

Quadrupole-hexadecapole correlations in neutron-rich samarium and gadolinium isotopes

We present an extensive study of quadrupole-hexadecapole correlation effects in even-even Sm and Gd isotopes with neutron number $N=88-106$. The calculations are performed in the framework of the Gogny energy density functional (EDF) with the D1S parametrization and the $sdg$ interacting boson model (IBM). The quadrupole-hexadecapole constrained self-consistent mean-field potential energy surface is mapped onto the expectation value of the $sdg$-boson Hamiltonian. This procedure determines the parameters of the $sdg$-IBM Hamiltonian microscopically. Calculated excitation energies and transition strengths are compared to the ones obtained with a simpler $sd$-IBM, as well as with the experimental data. The Gogny-EDF mapped $sdg$-IBM reproduces spectroscopic properties of the studied nuclei as reasonably as in the case of the previous $sdg$-boson mapping calculations that were based on the relativistic EDF, indicating that the axial quadrupole-hexadecapole method is sound regardless of whether relativistic or nonrelativistic EDF is employed. The mapped $sdg$-IBM improves some of the results in lighter Sm and Gd isotopes compared to the mapped $sd$-IBM, implying the existence of significant hexadecapole correlations in those nuclei. For those nuclei with $N \geq 94$, hexadecapole effects are minor, and the only significant difference between the two boson models can be found in the description of $E0$ monopole transitions.

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arXiv2024

Shape transition and coexistence in Te isotopes studied with the quadrupole collective Hamiltonian based on a relativistic energy density functional

Evolution and coexistence of shape and the related spectroscopic properties of even-even Te isotopes are investigated within the quadrupole collective model that is based on the nuclear density functional theory. By means of the constrained self-consistent mean-field calculations performed within the relativistic Hartree-Bogoliubov method with a choice of the energy density functional and pairing interaction, the deformation-dependent mass parameters and moments of inertia as well as collective potential of the triaxial quadrupole collective Hamiltonian are completely determined. The collective model produces for the near mid-shell nuclei, e.g., $^{116}$Te and $^{118}$Te, the low-energy $0^+_2$ state, which can be interpreted as the intruder state originating from the strongly deformed prolate minimum in the potential energy surface, along with the $0^+_1$ ground state that is attributed to the normal state based on a weakly oblate deformed global minimum. The collective model calculation suggests a parabolic behavior of the $0^+_2$ energy level near the neutron mid-shell $N=66$, as observed experimentally. Sensitivities of the calculated low-energy spectra to the pairing strength and collective mass parameters are analyzed.

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arXiv2024

Role of octupole shape degree of freedom in neutron-rich odd-mass xenon isotopes

Influences of the octupole shape degree of freedom on low-energy spectra of neutron-rich odd-mass xenon isotopes are studied within the interacting boson-fermion model that is based on the nuclear density functional theory. The interacting-boson Hamiltonian describing low-energy quadrupole and octupole collective states of the even-even nuclei $^{140,142,144}$Xe, single-particle energies, and occupation probabilities for an unpaired neutron in the odd-mass nuclei $^{141,143,145}$Xe, are determined based on the axially symmetric quadrupole-octupole deformation-constrained self-consistent mean-field calculations with a choice of the energy density functional and pairing interaction. Strength parameters of the boson-fermion interactions are empirically determined to reproduce a few low-lying levels of each odd-mass nucleus. The mean-field calculation predicts for $^{142}$Xe a potential energy surface that is notably soft in the octupole deformation with a non-zero octupole global minimum. The octupole correlations are shown to be relevant in positive-parity excited states of $^{143,145}$Xe.

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arXiv2024

Parameter dependence of the $β$-decay properties of neutron-rich Zr isotopes within the interacting boson model

We investigate parameter dependence of the calculated $β$-decay properties, as well as low-lying states for the neutron-rich Zr isotopes within the neutron-proton interacting boson model (IBM-2) and interacting boson-fermion-fermion model (IBFFM-2). It is shown that the calculated $\log_{10}ft$ values for the transitions of the $0^+_1$ ground states of the parent even-even nuclei $^{96-102}$Zr into the $1^+_1$ states of the daughter odd-odd nuclei $^{96-102}$Nb consistently exhibit a strong dependence on those parameters associated with the quadrupole-quadrupole boson interaction, and with the residual interaction between an unpaired neutron and an unpaired proton in the IBFFM-2 Hamiltonian for the odd-odd Nb nuclei. By the reduction in magnitude of the quadrupole-quadrupole interaction strength by approximately a factor of 2, the calculated $\log_{10}ft$ values for the Zr$(0^+_1)\to$Nb$(1^+_1)$ transitions increase and agree with the experimental values. This points to a significant improvement over the previous study performed in the same mass region, that consists of the mapping from a relativistic energy density functional calculation onto the IBM-2 Hamiltonian.

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arXiv2024

Microscopic description of hexadecapole collectivity in even-even rare-earth nuclei near $N=90$

We present an extensive study of hexadecapole correlations in the rare-earth region near $N=90$ and the effects these correlations have on various nuclear properties, such as the low-energy spectra, as well as quadrupole, hexadecapole, and monopole transition strengths. In order to examine hexadecapole correlations, we employ a mapped $sdg$ interacting boson model, with parameters derived from a self-consistent mean-field calculations with a relativistic energy density functional. We apply this model to even-even isotopes of Nd, Sm, Gd, Dy, and Er ($Z=60 - 68$) with neutron numbers $N=84-96$. The obtained results show a good agreement with the experiment. By comparing the results with the ones obtained from a simpler mapped $sd$ interacting boson model, we show that the inclusion of the hexadecapole degree of freedom via $g$ boson is necessary to improve the results of the $J^π \geq 6^{+}$ yrast energies in the nuclei with $N=84$ and 86, being near the neutron shell closure. The $sdg$ interacting boson model increases the quadrupole transition strengths between yrast states in the $N=90$ and 92 well deformed nuclei, which is in good agreement with the experiment for most of those isotopes. The presence of $g$ bosons does have an important effect on hexadecapole transition strengths, although experimental data for such transitions are limited. The obtained monopole transition strengths do not differ significantly from the ones obtained from the simpler $sd$ model.

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arXiv2024

Impacts of hexadecapole deformations on the collective energy spectra of axially deformed nuclei

The hexadecapole deformation, as well as the quadrupole one, influences the low-lying states of finite nuclei. The hexadecapole correlations are often overshadowed by the large quadrupole effects, and hence have not been much investigated. Here we address the relevance of hexadecapole deformations in the calculations of low-energy collective states of heavy nuclei, by using the theoretical framework of the self-consistent mean-field method and the interacting-boson approximation. The interacting-boson Hamiltonian that explicitly includes the quadrupole and hexadecapole collective degrees of freedom is specified by a choice of the energy density functional and pairing interaction. In an illustrative application to axially deformed Gd isotopes, it is shown that the inclusion of the hexadecapole degree of freedom does not affect most of the low-spin and low-lying states qualitatively, but that has notable effects in that it significantly improves the description of high-spin states of the ground-state bands of nearly spherical vibrational nuclei and gives rise to $K^π=4^+$ bands exhibiting strong $E4$ transitions in strongly deformed nuclei.

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arXiv2023

Quadrupole-octupole coupling and the onset of octupole collectivity

Octupole deformation and collective excitations are studied within the interacting boson model. By using the results of the self-consistent mean-field calculations with a universal energy density functional, the Hamiltonian of the interacting $s$, $d$, and $f$ boson system is completely determined. A global systematic study confirms that significant octupole effects are present in actinide, lanthanide, and rare-earth nuclei corresponding to particular nucleon numbers for which octupole correlations are empirically suggested to be enhanced.

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arXiv2023

Classical and Bayesian error analysis of the relativistic mean-field model for doubly magic nuclei

The information-geometric statistical analysis on the stability of model reductions, reported previously [Imbrišak and Nomura, Phys. Rev. C 107, 034304 (2023)] with a focus on the manifold boundary approximation method in the application to the nuclear density-dependent point-coupling model of infinite nuclear matter, is extended to the numerically more challenging case of finite nuclei. A simple procedure is presented for determining the binding energies of doubly magic nuclei within the relativistic mean-field framework using the Woods-Saxon potential. The proposed procedure, employing the Fisher information matrix combined with algorithmic differentiation, is shown to provide reliable estimates of parameter uncertainties of the nuclear energy density functional for finite nuclei, while reducing the time-consuming sampling of the parameter space, which would be required in the numerically more involved Bayesian statistical techniques.

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arXiv2023

Stability of the manifold boundary approximation method for reductions of nuclear structure models

The framework of nuclear energy density functionals has been employed to describe nuclear structure phenomena for a wide range of nuclei. Recently, statistical properties of a given nuclear model, such as parameter confidence intervals and correlations, have received much attention, particularly when one tries to fit complex models. We apply information-theoretic methods to investigate stability of model reductions by the manifold boundary approximation method (MBAM). In an illustrative example of the density-dependent point-coupling model of the relativistic energy density functional, utilizing Monte Carlo simulations, it is found that main conclusions obtained from the MBAM procedure are stable under variation of the model parameters. Furthermore, we find that the end of the geodesic occurs when the determinant of the Fisher information metric vanishes, thus effectively separating the parameter space into two disconnected regions.

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arXiv2022

Collective-model description of shape coexistence and intruder states in cadmium isotopes based on a relativistic energy density functional

Low-energy structure of even-even $^{108-116}$Cd isotopes is analyzed using a collective model that is based on the nuclear density functional theory. Spectroscopic properties are computed by solving the triaxial quadrupole collective Hamiltonian, with parameters determined by the constrained self-consistent mean-field calculations within the relativistic Hartree-Bogoliubov method employing a universal energy density functional and a pairing force. The collective Hamiltonian reproduces the observed quadrupole phonon states of vibrational character, which are based on the moderately deformed equilibrium minimum in the mean-field potential energy surface. In addition, the calculation yields a low-lying excited $0^+$ band and a $γ$-vibrational band that are associated with a deformed local minimum close in energy to the ground state, consistently with the empirical interpretation of these bands as intruder bands. Observed energy spectra, $B(E2)$, and $ρ^2(E0)$ values are, in general, reproduced reasonably well.

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