arXiv subjects

Toshihiko Kawano

Publications and source records attributed to Toshihiko Kawano.

At least 19 recordsLinked to original sources

arXiv2026

Calculation of tetraneutron-induced reaction cross sections with optical and Hauser-Feshbach statistical models

Interactions of tetraneutrons, which are assumed to be produced in the nuclear fission process, with nuclei are studied in the framework of optical and Hauser-Feshbach statistical models. It predicts a large probability of $^{89}\mathrm{Sr}$ production for the tetraneutron-induced reaction on $^{88}\mathrm{Sr}$ compared to other isotopes. The same technique is applied to the tetraneutron-induced reaction on $^{27}\mathrm{Al}$ and the hexaneutron-induced reaction on natural zinc to revisit two historical multi-neutron experiments performed in the past.

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arXiv2026

Fast-reactor neutron sources in evaluated nuclear data library validation

Two different neutron sources, based on $^{235}$U-fission neutrons with different average energies, provide integral benchmark data for validation of $γ$-ray production data in the evaluated nuclear data libraries corresponding to fast-neutron-induced inelastic-neutron scattering reactions. Firstly, we consider the IRT-M Research Reactor, formerly located at the Nuclear Research Institute just outside of Baghdad, Iraq, to demonstrate the validation methodology using the associated $γ$-ray data in the Evaluated Nuclear Data File, version VIII.0 (ENDF/B-VIII.0), for several $γ$-ray transitions over a wide range of nuclides including $^{28}$Si, $^{32}$S, $^{56}$Fe, and $^{186}$W. Using the characterized neutron flux of the Baghdad IRT-M Reactor, we find flux-weighted cross-sections deduced using the ENDF/B-VIII.0 $γ$-ray data to be in good agreement with the integral measurements performed at the Baghdad Research Reactor in addition to the corresponding results of different reaction-model calculations, {\tt CoH$_{3}$} and {\tt EMPIRE}. Given the excitation thresholds for the $γ$-ray transitions involved in this investigation, these observations lend further support to the characterization of the IRT-M flux in the fast-neutron energy region $0.862 \leq E_{n} \leq 5.0$ MeV. The additional detail devoted to the IRT-M source reflects the broader scope of the validation work carried out at that facility. A second neutron source considered for this validation work is the Forschungsreaktor M{ü}nich (FRM-II), Garching, Germany. Again, the flux-weighted $γ$-ray data from ENDF/B-VIII.0 for $^{56}$Fe compare well to the integral FRM-II measurement and reaction-model calculations.

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arXiv2025

Updating GEANIE $^{239}$Pu prompt $γ$-ray experimental data using modern Hauser-Feshbach fission fragment decay model

We calculate fission $γ$ rays for neutron-induced reactions on $^{239}$Pu with the Hauser-Feshbach fission fragment decay model. By applying the calculated fission $γ$ rays as a background contribution, the historical $^{239}$Pu(n,$x$n$γ$) reaction cross section data measured by the GEANIE (GErmanium Array for Neutron Induced Excitations) spectrometer are corrected. The correction also includes other (n,$x$n) reactions that have very similar energies to the $γ$ lines reported by GEANIE. In many cases, the original GEANIE data are strongly reduced and they become much closer to the statistical Hauser-Feshbach model predictions. The total inelastic scattering, (n,2n), and (n,3n) cross sections are inferred based on the corrected GEANIE data, and compared with available experimental data as well as the statistical model calculations. Expected $γ$-ray energy spectra for neutron-induced measurements on $^{239}$Pu are also discussed.

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arXiv2025

Modeling direct and pre-equilibrium processes of neutron-induced reactions with noniterative finite amplitude method and distorted-wave Born approximation

We develop a calculation method for describing the direct and pre-equilibrium processes in neutron-induced reactions based on the framework of noniterative finite amplitude method (FAM) and distorted-wave Born approximation (DWBA). The noniterative FAM is used to derive equations of quasiparticle random-phase approximation (QRPA) for neutron-induced inelastic scatterings to both the discrete and continuum states in a consistent manner. The Skyrme force is employed as an interaction between the projectile neutron with nucleons inside the target nucleus. We apply this method to the neutron-induced reaction on 208Pb. We demonstrate that the calculated differential inelastic scattering cross sections to low-lying states reproduce available experimental data without any phenomenological parameters that are often introduced in conventional DWBA calculations. The calculated double differential cross section to the continuum state also agrees with the experimental data in the energy region relevant to the direct and pre-equilibrium processes. These results are used to investigate the spin distribution of the populated states in the residual nucleus.

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arXiv2025

Time Evolution of Prompt Gamma-Ray Emission in $^{252}$Cf(sf) and $^{233,235}$U($n$,f) Reactions

We investigate the time evolution of prompt fission $γ$ emission due to the presence of ns to ms isomers in the fragments produced in the neutron-induced fission of $^{233,235}$U and in the spontaneous fission of $^{252}$Cf. Calculations performed with the CGMF fission event generator are compared with recent experimental data on $^{252}$Cf(sf) and $^{235}$U($n$,f) obtained with the DANCE+NEUANCE setup at the Los Alamos Neutron Science Center. Of particular interest are the average $γ$-ray energy spectrum as a function of time since scission, $ϕ(ε_γ,t)$, the increase of the average $γ$-ray multiplicity over time, $N_γ(t)$, and its evolution in time as a function of the fission fragment mass, $N_γ(A,t)$. The time evolution of isomeric ratios in post-neutron emission fission fragments can be defined and used to test and reveal some deficiencies in our knowledge of the low-lying levels of neutron-rich nuclei produced in fission reactions.

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arXiv2023

Solving one-dimensional penetration problem for fission channel in the statistical Hauser-Feshbach theory

We solve the Schrödinger equation for an arbitrary one-dimensional potential energy to calculate the transmission coefficient in the fission channel of compound nucleus reactions. We incorporate the calculated transmission coefficients into the statistical Hauser-Feshbach model calculation for neutron-induced reactions on $^{235,238}$U and $^{239}$Pu. The one-dimensional model reproduces the evaluated fission cross section data reasonably well considering the limited number of model parameters involved. A resonance-like structure appears in the transmission coefficient for a double-humped fission barrier shape that includes an intermediate well, which is understood to be a quantum mechanical effect in the fission channel. The calculated fission cross sections for the neutron-induced reactions on $^{235,238}$U and $^{239}$Pu all exhibit a similar structure.

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arXiv2022

Consideration of memory of spin and parity in the fissioning compound nucleus by applying the Hauser-Feshbach fission fragment decay model to photonuclear reactions

Prompt and $β$-delayed fission observables, such as the average number of prompt and delayed neutrons, the independent and cumulative fission product yields, and the prompt $γ$-ray energy spectra for the photonuclear reactions on $^{235,238}$U and $^{239}$Pu are calculated with the Hauser-Feshbach Fission Fragment Decay (HF$^3$D) model and compared with available experimental data. In the analysis of neutron-induced fission reactions to the case of photo-induced fission, an excellent reproduction of the delayed neutron yields supports a traditional assumption that the photo-fission might be similar to the neutron-induced fission at the same excitation energies regardless of the spin and parity of the fissioning systems.

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arXiv2022

QRPA calculations for M1 transitions with the noniterative finite amplitude method and the application to neutron radiative capture cross sections

We derive the equations of quasiparticle random-phase approximation (QRPA) based on the finite amplitude method (FAM) with the Hartree-Fock+BCS (HF+BCS) single-particle states, and calculate the magnetic dipole (M1) transition for deformed gadolinium isotopes. Our QRPA calculation shows both large spin-flip transitions in the 5 to 10 MeV excitation energy and the low energy orbital transition that would correspond to the M1 scissors mode observed in nuclear experiments. Then, we calculate neutron capture reactions based on the statistical Hauser-Feshbach theory with the photoabsorption cross sections given by QRPA. We find that the capture cross section is enhanced due to the contribution from the low energy M1 transition although the calculated capture cross section still underestimates the experimental data. This issue in the calculated capture cross section could be improved by uncertainties of low energy E1 transition neglected in our QRPA calculation.

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arXiv2022

Comprehensive Analyses of the Neutrino-Process in the Core-collapsing Supernova

We investigate the neutrino flavor change effects due to neutrino self-interaction, shock wave propagation as well as matter effect on the neutrino-process of the core-collapsing supernova (CCSN). For the hydrodynamics, we use two models: a simple thermal bomb model and a specified hydrodynamic model for SN1987A. As a pre-supernova model, we take an updated model adjusted to explain the SN1987A employing recent development of the $(n,γ)$ reaction rates for nuclei near the stability line $(A \sim 100)$. As for the neutrino luminosity, we adopt two different models: equivalent neutrino luminosity and non-equivalent luminosity models. The latter is taken from the synthetic analyses of the CCSN simulation data which involved quantitatively the results obtained by various neutrino transport models. Relevant neutrino-induced reaction rates are calculated by a shell model for light nuclei and a quasi-particle random phase approximation model for heavy nuclei. For each model, we present abundances of the light nuclei ($^7$Li, $^7$Be, $^{11}$B and $^{11}$C) and heavy nuclei ($^{92}$Nb, $^{98}$Tc, $^{138}$La and $^{180}$Ta) produced by the neutrino-process. The light nuclei abundances turn out to be sensitive to the Mikheyev-Smirnov-Wolfenstein (MSW) region around O-Ne-Mg region while the heavy nuclei are mainly produced prior to the MSW region. Through the detailed analyses, we find that neutrino self-interaction becomes a key ingredient in addition to the MSW effect for understanding the neutrino-process and the relevant nuclear abundances. The normal mass hierarchy is shown to be more compatible with the meteorite data. Main nuclear reactions for each nucleus are also investigated in detail.

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arXiv2022

Non-iterative finite amplitude methods for E1 and M1 giant resonances

The finite amplitude method (FAM) is a very efficient approach for solving the fully self-consistent random-phase approximation (RPA) equations. We use FAM to rederive the RPA matrices for general Skyrme-like functionals, calculate the electric dipole (E1) and the magnetic dipole (M1) giant resonances, and compare the results with available experimental and evaluated data. For the E1 transitions in heavy nuclei, the calculations reproduce well the resonance energy of the photoabsorption cross sections. In the case of M1 transitions, we show that the residual interaction does not affect the transition strength of double-magic nuclei, which suggests that the spin terms in the Skyrme force currently neglected in the present computation could improve the agreement between FAM and experimental data.

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arXiv2021

Influence of non-statistical properties in nuclear structure on emission of prompt fission neutrons

The Hauser-Feshbach Fission Fragment Decay (HF$^3$D) model is extended to calculate the prompt fission neutron spectrum (PFNS) for the thermal neutron induced fission on $^{235}$U, where the evaporated neutrons from all possible fission fragment pairs are aggregated. By studying model parameter sensitivities on the calculated PFNS, as well as non-statistical behavior of low-lying discrete level spin distribution, we conclude that discrepancies between the aggregation calculation and the experimental PFNS seen at higher neutron emission energies can be attributed to both the primary fission fragment yield distribution and the possible high spin states that are not predicted by the statistical theory of nuclear structure.

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arXiv2020

Unified description of the coupled-channels and statistical Hauser-Feshbach nuclear reaction theories for low energy neutron incident reactions

We incorporate the coupled-channels optical model into the statistical Hauser-Feshbach nuclear reaction theory, where the scattering matrix is diagonalized by performing the Engelbrecht-Weidenmüller transformation. This technique has been implemented in the coupled-channels optical model code ECIS by J. Raynal, and we extend this method so that all the open channels in a nucleon-induced reaction on a deformed nucleus can be calculated consistently.

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arXiv2020

Number of Particles in Fission Fragments

In current simulations of fission, the number of protons and neutrons in a given fission fragment is almost always obtained by integrating the total density of particles in the sector of space that contains the fragment. Because of the antisymmetry of the many-body wave function of the whole nucleus, this procedure systematically gives noninteger numbers of particles in the fragments. We introduce a novel sampling method to estimate rigorously the probability of finding $Z$ protons and $N$ neutrons in a fission fragment without resorting to projectors, which can sometimes give unwieldy results. When applied on standard Hartree-Fock-Bogoliubov many-body states, we show that our approach reproduces indeed the results of full particle-number projection. We then estimate the charge and mass number dispersion of several scission configurations in $^{240}$Pu with and without pairing correlations included. We show that odd-even effects in the charge probability naturally occur within our approach, which could explain the well-known odd-even staggering of charge distributions. Our method is applicable either in static calculations of scission configurations such as, e.g., in the macroscopic-microscopic approach or energy density functional theory, but also in explicitly time-dependent density functional theory simulations of fission.

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arXiv2020

Characterizing $r$-Process Sites through Actinide Production

Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in $r$-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an $r$-process event may be. Furthermore, if an $r$-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of $r$-process site is needed to explain all levels of observed relative actinide enhancements.

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arXiv2019

Neutrino self-interaction and MSW effects on the supernova neutrino-process

We calculate the abundances of $^{7}$Li, $^{11}$B, $^{92}$Nb, $^{98}$Tc, $^{138}$La, and $^{180}$Ta produced by neutrino $(ν)$ induced reactions in a core-collapse supernova explosion. We consider the modification by $ν$ self-interaction ($ν$-SI) near the neutrinosphere and the Mikheyev-Smirnov-Wolfenstein effect in outer layers for time-dependent neutrino energy spectra. Abundances of $^{7}$Li and heavy isotopes $^{92}$Nb, $^{98}$Tc and $^{138}$La are reduced by a factor of 1.5-2.0 by the $ν$-SI. In contrast, $^{11}$B is relatively insensitive to the $ν$-SI. We find that the abundance ratio of heavy to light nucleus, $^{138}$La/$^{11}$B, is sensitive to the neutrino mass hierarchy, and the normal mass hierarchy is more likely to be consistent with the solar abundances.

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arXiv2019

Unified Coupled-Channels and Hauser-Feshbach Model Calculation for Nuclear Data Evaluation

We present an overview of the coupled-channels optical model and the Hauser-Feshbach theory code CoH$_3$, which focuses on the nuclear reaction calculations in the keV to tens of MeV region with special attention to the nuclear deformation. The code consists of three major sections that undertake the one-body potential mean-field theory, the coupled-channels optical model, and the Hauser-Feshbach statistical decay. There are other complementary segments to perform the whole nuclear reaction calculations, such as the direct/semidirect radiative capture process, pre-equilibrium process, and prompt fission neutron emission.

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arXiv2018

Actinide Production in Neutron-Rich Ejecta of a Neutron Star Merger

The rapid-neutron-capture ("r") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far fail to account for the observed over-enhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger and explore if varying levels of neutron richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, beta-decay, and mass model. For most cases, the actinides are over-produced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of a neutron star merger is a likely production site for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

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arXiv2018

Excitation functions for (p,x) reactions of niobium in the energy range of E$_{\text{p}}$ = 40-90 MeV

A stack of thin Nb foils was irradiated with the 100 MeV proton beam at Los Alamos National Laboratory's Isotope Production Facility, to investigate the $^{93}$Nb(p,4n)$^{90}$Mo nuclear reaction as a monitor for intermediate energy proton experiments and to benchmark state-of-the-art reaction model codes. A set of 38 measured cross sections for $^{\text{nat}}$Nb(p,x) and $^{\text{nat}}$Cu(p,x) reactions between 40-90 MeV, as well as 5 independent measurements of isomer branching ratios, are reported. These are useful in medical and basic science radionuclide productions at intermediate energies. The $^{\text{nat}}$Cu(p,x)$^{56}$Co, $^{\text{nat}}$Cu(p,x)$^{62}$Zn, and $^{\text{nat}}$Cu(p,x)$^{65}$Zn reactions were used to determine proton fluence, and all activities were quantified using HPGe spectrometry. Variance minimization techniques were employed to reduce systematic uncertainties in proton energy and fluence, improving the reliability of these measurements. The measured cross sections are shown to be in excellent agreement with literature values, and have been measured with improved precision compared with previous measurements. This work also reports the first measurement of the $^{\text{nat}}$Nb(p,x)$^{82\text{m}}$Rb reaction, and of the independent cross sections for $^{\text{nat}}$Cu(p,x)$^{52\text{g}}$Mn and $^{\text{nat}}$Nb(p,x)$^{85\text{g}}$Y in the 40-90 MeV region. The effects of $^{\text{nat}}$Si(p,x)$^{22,24}$Na contamination, arising from silicone adhesive in the Kapton tape used to encapsulate the aluminum monitor foils, is also discussed as a cautionary note to future stacked-target cross section measurements. \emph{A priori} predictions of the reaction modeling codes CoH, EMPIRE, and TALYS are compared with experimentally measured values and used to explore the differences between codes for the $^{\text{nat}}$Nb(p,x) and $^{\text{nat}}$Cu(p,x) reactions.

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