arXiv subjects

Toshio Suzuki

Publications and source records attributed to Toshio Suzuki.

At least 37 records · Page 2Linked to original sources

arXiv2022

Systematic shell-model study on spectroscopic properties in the south region of $^{208}$Pb

We aim to study the properties of nuclei in the south region of $^{208}$Pb systematically, including the binding and excitation energies and electromagnetic properties, in order to predict unknown properties of these nuclei, such as isomerism, utilizing a theoretical model which describes the experimentally known properties precisely. We also address whether the $N=126$ shell closure is robust or not when the proton number decreases from $^{208}$Pb. We performed large-scale shell-model calculations with a new Hamiltonian suggested in the present work. The model space is taken as the five proton orbits within $50<Z\leqslant82$ and the thirteen neutron orbits within $82<N\leqslant184$. And one-particle one-hole excitation is allowed across the $N=126$ gap. The Hamiltonian is constructed by combining the existing Hamiltonians, KHHE (with adjustment of its proton-proton part) and KHPE, and the monopole based universal interaction. The shell-model results well reproduce the experimentally observed binding energies and spectroscopic properties, such as isomerism, core excitation, and electromagnetic properties. Some possible isomeric states in neutron-rich Pb, Tl, and Hg isotopes are predicted with transition energies and half-lives. We also examine the effective charges and the quenching of the $g$ factors suitable for this region by systematic comparisons between observed and calculated electromagnetic properties. A new Hamiltonian is constructed for nuclei in the south region of $^{208}$Pb, mainly including Pb, Tl, Hg, Au, Pt, Ir, Os, Re, and W isotopes around $N=126$, and provides them reasonable descriptions on nuclear properties including binding energies, excitation energies and electromagnetic properties through comprehensive and systematic studies.

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arXiv2022

Nuclear Weak Rates and Nuclear Weak Processes in Stars

Nuclear weak rates in stellar environments are obtained by shell-model calculations including Gamow-Teller (GT) and spin-dipole transitions, and applied to nuclear weak processes in stars. The important roles of accurate weak rates for the study of astrophysical processes are pointed out. The weak rates in $sd$-shell are used to study the evolution of ONeMg cores in stars with 8-10 M$_{\odot}$. Cooling of the core by nuclear Urca processes, and the heating by double e-captures on $^{20}$Ne are studied. Especially, the e-capture rates for a second-forbidden transition in $^{20}$Ne are evaluated with the multipole expansion method of Walecka and Behrens-B$\ddot{\mbox{u}}$hring, and the final fate of the cores, core-collapse or thermonuclear explosion, are discussed. The weak rates in $pf$-shell are applied to nucleosynthesis of iron-group elements in Type Ia supernovae. The over-production problem of neutron-rich iron isotopes compared with the solar abundances is now reduced to be within a factor of two. The weak rates for nuclear Urca pair with $A$=31 in the island of inversion are evaluated with the effective interaction obtained by the extended Kuo-Krenciglowa method. The transition strengths and e-capture rates in $^{78}$Ni, important for core-collapse processes, are evaluated with the $pf$-$sdg$ shell, and compared with those obtained by the random-phase-approximation and an effective rate formula. $β$-decay rates of $N$ =126 isotones are evaluated with both the GT and first-forbidden transitions. The half-lives are found to be shorter than those obtained by standard models. Neutrino-nucleus reaction cross sections on $^{13}$C, $^{16}$O and $^{40}$Ar are obtained with new shell-model Hamiltonians. Implications on nucleosynthesis, neutrino detection, neutrino oscillations and neutrino mass hierarchy are discussed.

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arXiv2022

The relationship of the neutron-skin thickness to the symmetry energy and its slope

The neutron-skin thickness of asymmetric semi-infinite nuclear matter is shown to be a function of Coulomb energy, the asymmetry-energy coefficient($J$), the slope($L$) of the asymmetry energy, and the incompressibility coefficient, in addition to he Fermi momentum and the asymmetry parameter. The relational formula is derived on the basis of the Hugenholtz-Van Hove theorem in the mean-field approximation for nuclear matter. Using the formula as a guide, the neutron-skin thickness($δR$) in $^{208}$Pb is examined. The linear correlation between $L$ and $δR$ appears as a kind of spurious ones through the model-dependent correlation of $L$ with $J$ which is included in the main components of the formula.

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arXiv2022

The mean square radius of the neutron distribution in the relativistic and non-relativistic mean field models

It is investigated why the root mean square radius of the point neutron distribution is smaller by about 0.1 fm in non-relativistic mean field models than in relativistic ones. The difference is shown to stem from the different values of the product of the effective mass and the strength of the one-body potential in the two frameworks. The values of those quantities are constrained by the Hugenholtz-Van Hove theorem. The neutron skin is not a simple function of the symmetry potential, but depends on the nucleon effective mass.

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arXiv2021

Structure of $^{46,47}$Ca from the $β^-$ decay of $^{46,47}$K in the framework of nuclear shell model

In the present work, we report a comprehensive theoretical study of the $\log ft$ values for the allowed and forbidden $β^-$ decay transitions of $^{46}$K and $^{47}$K corresponding to recently available experimental data from the GRIFFIN spectrometer at TRIUMF-ISAC [Phys. Rev. C {\bf 100}, 054327 (2019); Phys. Rev. C {\bf 102}, 054314 (2020)]. We perform the nuclear shell-model calculation in $sdpf$-valence space, with the SDPF-MU interaction, to calculate low-lying energy spectra of $^{46,47}$K and $^{46,47}$Ca. For further investigation, we also calculate spectroscopic properties of $^{46,47}$K, $^{46,47}$Ca and compare the results with the experimental data. We suggest spin-parity of several levels, which were previously tentative, in both the calcium isotopes. Based on the energy and $\log ft$ values, we conclude that the state at 3.984 MeV in $^{46}$Ca could be either $3^{-}$ or $2^{+}$ and a state at 4.432 MeV might be $3^{-}$. The spin-parity of the states at 2.875, 3.951 and 4.453 MeV are predicted to be $1/2^-$, $3/2^-$ and $1/2^-$ or $1/2^+$, respectively, in $^{47}$Ca. We also give the confirmation to spin-parity of 2.850, 3.889 and 4.606 MeV states for $^{47}$Ca. Our results of level schemes of $^{46,47}$K are in a reasonable agreement with the experimental data, while the agreement of calculated $\log ft$ values for the $β^-$ decay with the experimental data is limited.

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arXiv2020

Screening Effects on Electron Capture Rates and Type Ia Supernova Nucleosynthesis

Type Ia supernovae (SNe Ia) are believed to be a thermonuclear explosion of a white dwarf, but the mass of their progenitors is still an open problem. In near-Chandrasekhar-mass (near-M_Ch) models of SNe Ia, the central density reaches >10^9 g cm^{-3}. The electron chemical potential becomes higher than the Q-values of electron capture (EC) transitions between fp-shell nuclei, so a portion of the available electrons is captured by iron group elements and thus neutron-rich isotopes are formed. Since EC reaction rates are sensitive to the density, the degree of neutronization is a key to distinguish near- and sub-M_Ch models. In order to compare observations and theoretical models, an accurate treatment of EC reactions is necessary. In previous theoretical works, however, effects of electron screening on ECs are ignored. Screening lowers EC rates and thus leads to a higher electron fraction. We implement electron screening on ECs to calculate explosive SN Ia nucleosynthesis in a near-M_Ch single degenerate model. It is found that some of neutron-rich nuclear abundances, namely those of 46,48Ca, 50Ti, 54Cr, 58Fe, 64Ni and 67,70Zn, decrease when screening effects on ECs are considered. Of these, 50Ti, 54Cr and 58Fe are particularly interesting because a significant portion of the solar abundance of these nuclei is presumed to originate from SNe Ia. We conclude that implementing the screening effect on ECs in modern SN Ia models is desirable to precisely calculate abundances of neutron-rich nuclides.

astro-ph.HE↗

arXiv2020

The mean square radius of the neutron distribution and the skin thickness derived from electron scattering

The second-order moment of the nuclear charge density($R^2_c$) is dominated by the mean square radius(msr) of the point proton distribution($R_p^2$), while the fourth-order moment($Q^4_c$) depends on the msr of the point neutron one($R_n^2$) also. Moreover, $R^2_n$ is strongly correlated to $R^2_c$ in nuclear models. According to these facts, the linear relationship between various moments in the nuclear mean field models are investigated with use of the least squares method for $^{40}$Ca, $^{48}$Ca and $^{208}$Pb. From the intersection points of the obtained straight lines with those of the experimental values for $R^2_c$ and $Q^4_c$ determined through electron scattering, the values of $R_p$ and $R_n$ are estimated. Since relativistic and non-relativistic models provide different lines, the obtained values of $R_n$ and the skin thickness($R_n-R_p$) differ from each other in the two frameworks.

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arXiv2020

Shell model results for nuclear $β^-$-decay properties of $sd$ shell nuclei

We evaluate the allowed $β^-$-decay properties of nuclei with $Z = 8 - 15$ systematically under the framework of the nuclear shell model with the use of the valence space Hamiltonians derived from modern $ab~intio$ methods, such as in-medium similarity renormalization group and coupled-cluster theory. For comparison we also show results obtained with fitted interaction derived from the chiral effective field theory and phenomenological USDB interaction. We have performed calculations for O $\rightarrow$ F, F $\rightarrow$ Ne, Ne $\rightarrow$ Na, Na $\rightarrow$ Mg, Mg $\rightarrow$ Al, Al $\rightarrow$ Si, Si $\rightarrow$ P and P $\rightarrow$ S transitions. Theoretical results of $B(GT)$, log$ft$ values and half-lives, are discussed and compared with the available experimental data.

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arXiv2020

Evolution of shell structure in exotic nuclei

The atomic nucleus is a quantum many-body system whose constituent nucleons (protons and neutrons) are subject to complex nucleon-nucleon interactions that include spin- and isospin-dependent components. For stable nuclei, already several decades ago, emerging seemingly regular patterns in some observables could be described successfully within a shell-model picture that results in particularly stable nuclei at certain magic fillings of the shells with protons and/or neutrons: N,Z = 8, 20, 28, 50, 82, 126. However, in short-lived, so-called exotic nuclei or rare isotopes, characterized by a large N/Z asymmetry and located far away from the valley of beta stability on the nuclear chart, these magic numbers, viewed through observables, were shown to change. These changes in the regime of exotic nuclei offer an unprecedented view at the roles of the various components of the nuclear force when theoretical descriptions are confronted with experimental data on exotic nuclei where certain effects are enhanced. This article reviews the driving forces behind shell evolution from a theoretical point of view and connects this to experimental signatures.

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arXiv2019

Effects of the Metallicity on Li and B Production in Supernova Neutrino Process

The neutrino process ($ν$-process) for the production of 7Li and 11B in core-collapse supernovae (SNe) is extensively investigated. Initial abundances of s-nuclei and other physical conditions are derived from an updated calculation of the SN 1987A progenitor. The nuclear reaction network including neutrino reactions is constructed with the variable order Bader-Deuflhard integration method. We find that yields of 7Li and 11B significantly depend on the stellar metallicity while they are independent of the weak s-process during the stellar evolution. When the metallicity is high, there are more neutron absorbers, i.e., 56Fe, 14N (from initial CNO nuclei), and 54Fe, and the neutron abundance is small during the $ν$-process. Since 7Be is predominantly destroyed via 7Be(n,p)7Li, a change in the neutron abundance results in different 7Be yields. Then, the calculated yield ratio 7Li/11B=0.93 for the solar metallicity is larger than that for the SN 1987A 7Li/11B=0.80 by 16 % in the inverted mass hierarchy case. We analyze contributions of respective reactions as well as abundance evolution, and clarify the $ν$-process of 7Li and 11B.

astro-ph.SR↗

arXiv2019

Evolution of ONeMg Core in Super-AGB Stars towards Electron-Capture Supernovae: Effects of Updated Electron-Capture Rate

Stars with $\sim 8-10~{M}_{\odot}$ evolve to form a strongly degenerate ONeMg core. When the core mass becomes close to the Chandrasekhar mass, the core undergoes electron captures on $^{24}$Mg and $^{20}$Ne, which induce the electron-capture supernova (ECSN). In order to clarify whether the ECSN leads to a collapse or thermonuclear explosion, we calculate the evolution of an $8.4~M_\odot$ star from the main sequence until the oxygen ignition in the ONeMg core. We apply the latest electron-capture rate on $^{20}$Ne including the second forbidden transition, and investigate how the location of the oxygen ignition (center or off-center) and the $Y_e$ distribution depend on the input physics and the treatment of the semiconvection and convection. The central density when the oxygen deflagration is initiated, $ρ_{\rm c,def}$, can be significantly higher than that of the oxygen ignition thanks to the convection, and we estimate $\log_{10}(ρ_{\rm c,def}/\mathrm{g~cm^{-3}})>10.10$. We perform two-dimensional simulations of the flame propagation to examine how the final fate of the ONeMg core depends on the $Y_e$ distribution and $ρ_{\rm c,def}$. We find that the deflagration starting from $\log_{10}(ρ_{\rm c,def}/\mathrm{g~cm^{-3}})>10.01 (< 10.01)$ leads to a collapse (thermonuclear explosion). Since our estimate of $ρ_{\rm c,def}$ exceeds this critical value, the ONeMg core is likely to collapse, although further studies of the convection and semiconvection before the deflagration are important.

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arXiv2019

Electron-capture Rates in $^{20}$Ne for a Forbidden Transition to the Ground State of $^{20}$F Relevant to Final Evolution of High-density O-Ne-Mg Cores

Electron capture on $^{20}$Ne is critically important for the final stage of evolution of stars with the initial masses of 8 - 10 $M_{\odot}$. In the present paper, we evaluate electron capture rates for a forbidden transition $^{20}$Ne (0$_{g.s.}^{+}$) $\rightarrow$ $^{20}$F (2$_{g.s.}^{+}$) in stellar environments by the multipole expansion method with the use of shell-model Hamiltonians. These rates have not been accurately determined in theory as well as in experiments. Our newly evaluated rates are compared with those obtained by a prescription that treats the transition as an allowed Gamow-Teller (GT) transition with the strength determined from a recent $β$-decay experiment for $^{20}$F (2$_{g.s.}^{+}$) $\rightarrow$ $^{20}$Ne (0$_{g.s.}^{+}$) \citep{Kirsebom}. We find that different electron energy dependence of the transition strengths between the two methods leads to sizable differences in the weak rates of the two methods. We also find that the Coulomb effects, that is, the effects of screening on ions and electrons are non-negligible. We apply our e-capture rates on $^{20}$Ne to the calculation of the evolution of high-density O-Ne-Mg cores of 8 - 10 $M_{\odot}$ stars. We find that our new rates affect the abundance distribution and the central density at the final stage of evolution.

astro-ph.SR↗

arXiv2019

The n-th order Moment of the Nuclear Charge Density and Contribution from the Neutrons

The relativistic expression for the $n$-th order moment of the nuclear charge density is presented. For the mean square radius(msr) of the nuclear charge density, the non-relativistic expression, which is equivalent to the relativistic one, is also derived consistently up to $1/M^2$ with use of the Foldy-Wouthuysen transformation. The difference between the relativistic and non-relativistic expressions for the msr of the point proton density is also discussed. The $n(\ge 4)$-th order moment of the nuclear charge density depends on the point neutron density. The 4-th order moment yields a useful information on the msr of the point neutron density, and is expected to play an important role in electron scattering off neutron-rich nuclei.

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arXiv2019

Charged-current scattering off $^{16}$O nucleus as a detection channel for supernova neutrinos

Event spectra of the neutrino-$^{16}$O charged-current reactions in Super-Kamiokande are evaluated for a future supernova neutrino burst. Since these channels are expected to be useful for diagnosing a neutrino spectrum with high average energy, the evaluations are performed not only for an ordinary supernova neutrino model but also for a model of neutrino emission from a black-hole-forming collapse. Using shell model results, whose excitation energies are consistent with the experimental data, the cross sections of the $^{16}$O($ν_e, e^-$)X and $^{16}$O($\barν_e, e^+$)X reactions for each nuclear state with a different excitation energy are employed in this study. It is found that, owing to the components of the reaction with higher excitation energy, the event spectrum becomes 4-7 MeV softer than that in the case without considering the excitation energies. In addition, a simplified approach to evaluate the event spectra is proposed for convenience and its validity is examined.

astro-ph.HE↗

arXiv2019

Solovay reduction and continuity

The objective of this study is a better understanding of the relationships between reduction and continuity. Solovay reduction is a variation of Turing reduction based on the distance of two real numbers. We characterize Solovay reduction by the existence of a certain real function that is computable (in the sense of computable analysis) and Lipschitz continuous. We ask whether there exists a reducibility concept that corresponds to Hölder continuity. The answer is affirmative. We introduce quasi Solovay reduction and characterize this new reduction via Hölder continuity. In addition, we separate it from Solovay reduction and Turing reduction and investigate the relationships between complete sets and partial randomness.

math.LO↗

arXiv2019

Supernova Neutrino Process of Li and B Revisited

We reinvestigate effects of neutrino oscillations on the production of 7Li and 11B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto-neutron star, their flavors change and the neutrino reaction rates for spallation of 12C and 4He are affected. In this work corrected neutrino spallation cross sections for 4He and 12C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of 7Li and 11B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged current reaction rates of electron neutrinos are enhanced, and yields of proton-rich nuclei, along with 7Be and 11C, are increased. In the inverted hierarchy case, the charged current reaction rates of electron antineutrinos are enhanced, and yields of neutron-rich nuclei, along with 7Li and 11B, are increased. We find that variation of the metallicity modifies the yields of 7Li, 7Be, 11B, and 11C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars.

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arXiv2018

Nucleosynthesis Constraints on the Explosion Mechanism for Type Ia Supernovae

Observations of type Ia supernovae include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most-realistic recent treatment of electron captures. The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed detonation models) based upon new electron capture rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D-3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SNIa. We also note that observational constraints on the yields of $^{54}$Cr and $^{54}$Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SNIa explosions.

astro-ph.HE↗

arXiv2018

Neutrino-nucleus reactions on $^{16}$O based on new shell-model Hamiltonians

Neutrino-induced reactions on $^{16}$O are investigated by shell-model calculaions with new shell-model Hamiltonians, which can describe well the structure of $p$-shell and $p$-$sd$ shell nuclei. Distribution of the spin-dipole strengths in $^{16}$O, which give major contributions to the $ν$-$^{16}$O reaction cross sections, is studied with the new Hamiltonians. Muon-capture reaction rates on $^{16}$O are also studied to discuss the quenching of the axial-vector coupling in nuclear medium. Charged-current and neutral-current reaction cross sections are evaluated in various particle and $γ$ emission channels as well as the total ones at neutrino energies up to $E_ν\approx$ 100 MeV. Branching ratios for the various channels are obtained by the Hauser-Feshbach statistical model calculations, and partial cross sections for single- and multi-particle emission channels are evaluated. The cross sections updated are compared with previous continuum random phase approximation (CRPA) calculations. Effects of multi-particle emission channels on nucleosynthesis in core-collapse supernova (SN) explosions are investigated. Inclusion of $α$p emission channels is found to lead to an enhancement of production yields of $^{11}$B and $^{11}$C through $^{16}$O ($ν$, $ν$' $α$p) $^{11}$B and $^{16}$O ($ν$, e$^{-}$ $α$p) $^{11}$C reactions, respectively.

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