arXiv subjects

Kazuyuki Ogata

Publications and source records attributed to Kazuyuki Ogata.

At least 37 records · Page 2Linked to original sources

arXiv2023

Effective polarization in proton-induced $α$ knockout reactions

The effective polarization of the residual nucleus in the proton-induced $α$ knockout reaction is investigated within the distorted wave impulse approximation framework. The strong absorption of the emitted $α$ particle results in strong selectivity on the reaction "position" depending on the third component of the single-particle orbital angular momentum of the $α$ particle inside a nucleus, hence on the spin direction of the reaction residue. This is caused by a mechanism that is similar to the Maris effect, the effective polarization in the proton-induced proton knockout reactions. However, as a distinct feature of the effective polarization in the $α$ knockout process, the spin degrees of freedom of the reacting particles play no role. The $α$ knockout process with complete kinematics can be a useful polarization technique for the residual nucleus, without actively controlling the spin of the proton.

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arXiv2022

Importance of the deuteron breakup in the deuteron knockout reaction

An isoscalar $pn$ pair is expected to emerge in nuclei that have similar proton and neutron numbers and it may be a candidate for a deuteron ``cluster.'' There is, however, no experimental evidence for it. The purpose of this paper is to construct a new reaction model for the ($p,pd$) reaction including the deuteron breakup in the elementary process and the deuteron reformation by the final-state interactions (FSIs). How these processes contribute to the observables of the reaction is investigated. The distorted wave impulse approximation is extended in twofold. The elementary processes of the ($p,pd$), i.e., the $p$-$d$ elastic scattering and $d(p,p)pn$ reaction, are described with an impulse picture employing a nucleon-nucleon effective interaction. The three-body scattering waves in the final state of the ($p,pd$) reaction are calculated with the continuum-discretized coupled-channels method. The triple differential cross section (TDX) of the ($p,pd$) reaction is calculated with the new model. The elementary processes are described reasonably well with the present model. As for the ($p,pd$) reaction, the deuteron reformation can either increase or decrease the TDX height depending on the interference between the elastic and breakup channel of deuteron, while the \textit{back-coupling} effect always decreases it. It is shown that the deuteron reformation significantly changes the TDX of the ($p,pd$) reaction through the interference. It is important to include this process to quantitatively discuss the ($p,pd$) cross sections in view of the deuteron formation in nuclei. For more quantitative discussion regarding the experimental data, further improvement will be necessary.

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arXiv2021

Ambiguities from nuclear interactions in the $^{12}$C($p,2p$)$^{11}$B reaction

We investigate the impact of ambiguities coming from the choice of optical potentials and nucleon-nucleon scattering cross sections on the spectroscopic factors extracted from the $^{12}$C($p,2p$)$^{11}$B reaction. These ambiguities are evaluated by analyzing the cross sections of the $^{12}$C($p,2p$)$^{11}$B reaction at 100 and 200 MeV within the framework of the distorted-wave impulse approximation with realistic choices of nuclear inputs. The results show that the studied ambiguities are considerably large in this energy region and careful choices of nuclear inputs used in the reaction calculations are required to extract reliable structure information.

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arXiv2021

Investigation of multi-step effects for proton inelastic scattering to the $2^{+}_{1}$ state in $^6$He

Multi-step effects between bound, resonant, and non-resonant states have been investigated by the continuum-discretized coupled-channels method (CDCC). In the CDCC, a resonant state is treated as multiple states fragmented in a resonance energy region, although it is described as a single state in usual coupled-channel calculations. For such the fragmented resonant states, one-step and multi-step contributions to the cross sections should be carefully discussed because the cross sections obtained by the one-step calculation depend on the number of those states, which corresponds to the size of the model space. To clarify the role of the multi-step effects, we propose the one-step calculation without model-space dependence for the fragmented resonant states. Furthermore, we also discuss the multi-step effects between the ground, $2^{+}_{1}$ resonant, and non-resonant states in $^6$He for proton inelastic scattering.

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arXiv2021

Three-body description of $^9$C: Role of low-lying resonances in breakup reactions

The $^9$C nucleus and related capture reaction, ${^8\mathrm{B}}(p,γ){^9\mathrm{C}}$, have been intensively studied with an astrophysical interest. Due to the weakly-bound nature of $^9$C, its structure is likely to be described as the three-body (${^7\mathrm{Be}}+p+p$). Its continuum structure is also important to describe reaction processes of $^9$C, with which the reaction rate of the ${^8\mathrm{B}}(p,γ){^9\mathrm{C}}$ process have been extracted indirectly. We perform three-body calculations on $^9$C and discuss properties of its ground and low-lying states via breakup reactions. We employ the three-body model of $^9$C using the Gaussian-expansion method combined with the complex-scaling method. This model is implemented in the four-body version of the continuum-discretized coupled-channels method, by which breakup reactions of $^9$C are studied. The intrinsic spin of $^7$Be is disregarded. By tuning a three-body interaction in the Hamiltonian of $^9$C, we obtain the low-lying $2^+$ state with the resonant energy 0.781 MeV and the decay width 0.137 MeV, which is consistent with the available experimental information and a relatively high-lying second $2^+$ wider resonant state. Our calculation predicts also sole $0^+$ and three $1^-$ resonant states. We discuss the role of these resonances in the elastic breakup cross section of $^9$C on $^{208}$Pb at 65 and 160 MeV/A. The low-lying 2$^+$ state is probed as a sharp peak of the breakup cross section, while the 1$^-$ states enhance the cross section around 3 MeV. Our calculations will further support the future and ongoing experimental campaigns for extracting astrophysical information and evaluating the two-proton removal cross-sections.

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arXiv2021

Correspondence between isoscalar monopole strengths and $α$ inelastic cross sections on $^{24}$Mg

The correspondence between the isoscalar monopole (IS0) transition strengths and $α$ inelastic cross sections, the $B({\rm IS0})$-$(α,α')$ correspondence, is investigated for $^{24}$Mg($α,α'$) at 130 and 386 MeV. We adopt a microscopic coupled-channel reaction framework to link structural inputs, diagonal and transition densities, for $^{24}$Mg obtained with antisymmetrized molecular dynamics to the ($α,α'$) cross sections. We aim at clarifying how the $B({\rm IS0})$-$(α,α')$ correspondence is affected by the nuclear distortion, the in-medium modification to the nucleon-nucleon effective interaction in the scattering process, and the coupled-channels effect. It is found that these effects are significant and the explanation of the $B({\rm IS0})$-$(α,α')$ correspondence in the plane wave limit with the long-wavelength approximation, which is often used, makes no sense. Nevertheless, the $B({\rm IS0})$-$(α,α')$ correspondence tends to remain because of a strong constraint on the transition densities between the ground state and the $0^+$ excited states. The correspondence is found to hold at 386 MeV with an error of about 20%-30%, while it is seriously stained at 130 MeV mainly by the strong nuclear distortion. It is also found that when a $0^+$ state that has a different structure from a simple $α$ cluster state is considered, the $B({\rm IS0})$-$(α,α')$ correspondence becomes less valid. For a quantitative discussion on the $α$ clustering in $0^+$ excited states of nuclei, a microscopic description of both the structure and reaction parts will be necessary.

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arXiv2021

Manifestation of the divergence between antisymmetrized-molecular-dynamics and container pictures of $^{9}$Be via ${}^{9}$Be($p,pn$)${}^{8}$Be knockout reaction

We propose a new approach to probe the spatial extension of the valence neutron orbital in the $^{9}$Be nucleus via the ${}^{9}$Be($p,pn$)${}^{8}$Be knockout reaction. This property of the nuclear molecular orbital has not been established in previous experimental studies and divergence exists between the theoretical descriptions of ${}^{9}$Be from different perspectives, \textit{i.e.}, the antisymmetrized molecular dynamics and the container pictures of cluster dynamics. These pictures are represented by two different well-proven microscopic models, the antisymmetrized molecular dynamics (AMD) and Tohsaki-Horiuchi-Schuck-Röpke (THSR) wave functions. The corresponding reduced width amplitudes (RWAs) in the $^{8}$Be$+n$ channel are extracted from both the AMD and THSR wave functions, and they are found to describe drastically different valence-nucleon motion, which shows the theoretical ambiguity in describing the $π$-orbitals in $^{9}$Be. Using the RWAs as input, the physical observables of the ${}^{9}$Be($p,pn$)${}^{8}$Be knockout reaction are predicted by the distorted-wave impulse approximation (DWIA) framework. The magnitudes of the triple-differential cross sections (TDX) are found to be highly sensitive to the RWA input. It is concluded that the ${}^{9}$Be($p,pn$)${}^{8}$Be knockout reaction could provide a feasible probing for the subtle differences between several structure models manifesting through the spatial extension of the $π$-orbital in the $^{9}$Be nucleus.

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arXiv2021

Effect of deuteron breakup on the deuteron-$Ξ$ correlation function

The hadron-deuteron correlation function has attracted many interests as a potential method to access the three-hadron interactions. However, the weakly-bound nature of deuteron has not been considered in the preceding studies. In this study, the breakup effect of deuteron on the deuteron-$Ξ^-$ ($d$-$Ξ^-$) correlation function $C_{dΞ^-}$ is investigated. The $d$-$Ξ^-$ scattering is described by a nucleon-nucleon-$Ξ$ three-body reaction model. The continuum-discretized coupled-channels method, which is a fully quantum-mechanical and non-perturbative reaction model, is adopted. $C_{dΞ^-}$ turns out to be sensitive to the strong interaction and enhanced by the deuteron breakup effect by 6--8 % for the $d$-$Ξ^-$ relative momentum below about 70 MeV/$c$. Low-lying neutron-neutron continuum states are responsible for this enhancement. Within the adopted model, the deuteron breakup effect on $C_{dΞ^-}$ is found to be appreciable but not very significant. Except for the enhancement by several percent, studies on $C_{dΞ^-}$ without the deuteron breakup effect can be justified.

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arXiv2021

Systematic study on the role of various higher-order processes in the breakup of weakly-bound projectiles

The virtual photon theory (VPT), which is based on first-order Coulomb dissociation restricted to the electric dipole ($E1$), has been successfully used to explain the breakup data for several cases. Our aim is to study the role of various higher-order processes that are ignored in the VPT, such as the nuclear breakup, interference between nuclear and Coulomb amplitudes, and multistep breakup processes mainly due to strong continuum-continuum couplings in the breakup of two-body projectiles on a heavy target at both intermediate and higher incident energies. For the purpose of numerical calculations, we employed eikonal version of three-body continuum-discretized coupled-channels (CDCC) reaction model. Our results for the breakup of $^{11}$Be and $^{17}$F on $^{208}$Pb target at 100, 250, and 520 MeV/A, show the importance of nuclear breakup contribution, and its significant role in the multistep processes. The multistep effect on Coulomb breakup for core-neutron projectile was found to be negligible, whereas it was important for core-proton projectile. Coulomb-nuclear interference (CNI) effect was also found to be non-negligible. Quantitatively, the multistep effects due to the nuclear breakup was found to depend on the incident energy through the energy dependence of the core-target and nucleon-target nuclear potentials. The nuclear breakup component, the CNI effect, and the multistep breakup processes are all found to be non-negligible; hence, the assumptions adopted in the VPT for the accurate description of breakup cross sections are not valid.

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arXiv2021

Practical method for decomposing discretized breakup cross sections into components of each channel

Background: In the continuum-discretized coupled-channel method, a breakup cross section (BUX) is obtained as an admixture of several components of different channels in multi-channel scattering. Purpose: Our goal is to propose an approximate way of decomposing the discretized BUX into components of each channel. This approximation is referred to as the "probability separation (P-separation)". Method: As an example, we consider $^{11}$Be scattering by using the three-body model with core excitation ($^{10}\mathrm{Be}+n+\mathrm{T}$, where T is a target). The structural part is constructed by the particle-rotor model and the reaction part is described by the distorted wave Born approximation (DWBA). Results: The validity of the P-separation is tested by comparing with the exact calculation. The approximate way reproduces the exact BUXs well regardless of the configurations and/or the resonance positions of $^{11}$Be. Conclusion: The method proposed here can be an alternative approach for decomposing discretized BUXs into components in four- or five-body scattering where the strict decomposition is hard to perform.

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arXiv2021

Unexpectedly enhanced $α$-particle preformation in $^{48}$Ti probed by the $(p,pα)$ reaction

The formation of $α$ particle on nuclear surface has been a fundamental problem since the early age of nuclear physics. It strongly affects the $α$ decay lifetime of heavy and superheavy elements, level scheme of light nuclei, and the synthesis of the elements in stars. However, the $α$-particle formation in medium-mass nuclei has been poorly known despite its importance. Here, based on the $^{48}{\rm Ti}(p,pα)^{44}{\rm Ca}$ reaction analysis, we report that the $α$-particle formation in a medium-mass nucleus $^{48}{\rm Ti}$ is much stronger than that expected from a mean-field approximation, and the estimated average distance between $α$ particle and the residue is as large as 4.5 fm. This new result poses a challenge of describing four nucleon correlations by microscopic nuclear models.

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arXiv2020

Proton induced deuteron knockout reaction as a probe of an isoscalar proton-neutron pair in nuclei

The isoscalar $pn$ pair is expected to emerge in nuclei having the similar proton and neutron numbers but there is no clear experimental evidence for it. We aim to clarify the correspondence between the $pn$ pairing strength in many-body calculation and the triple differential cross section (TDX) of proton-induced deuteron knockout ($p,pd$) reaction on $^{16}$O. The radial wave function of the isoscalar $pn$ pair with respect to the center of $^{16}$O is calculated with the energy density functional (EDF) approach and is implemented in the distorted wave impulse approximation (DWIA) framework. The $pn$ pairing strength $V_0$ in the EDF calculation is varied and the corresponding change in the TDX is investigated. A clear $V_0$ dependence of the TDX is found for the $^{16}$O($p,pd$)$^{14}$N($1_2^+$) at $101.3$ MeV. The nuclear distortion is found to make the $V_0$ dependence stronger. Because of the clear $V_0$-TDX correspondence, the ($p,pd$) reaction will be a promising probe for the isoscalar $pn$ pair in nuclei. For quantitative discussion, further modification of the description of the reaction process will be necessary.

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arXiv2020

Microscopic coupled-channel calculation of proton and alpha inelastic scattering to the $4^+_1$ and $4^+_2$ states of $^{24}$Mg

Background: The triaxial and hexadecapole deformations of the K=0+ and K=2+ bands of 24Mg have been investigated by the inelastic scatterings of various probes, including electrons, protons, and alpha particles, for a prolonged time. However, it has been challenging to explain the unique properties of the scatterings observed for the $4^+_1$ state through reaction calculations. Purpose: To investigate the structure and transition properties of the K=0+ and K=2+ bands of 24Mg employing the microscopic structure and reaction calculations via inelastic proton and alpha-scattering. Particularly, the E4 transitions to the $4^+_1$ and $4^+_2$ states were reexamined. Method: The structure of 24Mg was calculated employing the variation after the parity and total-angular momentum projections in the framework of the antisymmetrized molecular dynamics(AMD). The inelastic proton and alpha reactions were calculated by the microscopic coupled-channel (MCC) approach by folding the Melbourne g-matrix NN interaction with the AMD densities of 24Mg. Results: Reasonable results were obtained on the properties of the structure, including the energy spectra and E2 and E4 transitions of the K=0+ and K=2+ bands owing to the enhanced collectivity of triaxial deformation. The MCC+AMD calculation successfully reproduced the angular distributions of the $4^+_1$ and $4^+_2$ cross sections of proton scattering at incident energies of $E_p$=40--100MeV and alpha-scattering at $E_α$=100--400MeV. Conclusions: This is the first microscopic calculation that described the unique properties of the $0^+_1\to 4^+_1$ transition. In the inelastic scattering to the $4^+_1$ state, the dominant two-step process of the $0^+_1\to 2^+_1\to 4^+_1$ transitions and the deconstructive interference is the weak one-step process were essential.

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arXiv2020

Probing negative-parity states of $^{24}$Mg probed via proton and alpha inelastic scattering

[Background:] The band structure of the negative-parity states of $^{24}$Mg has not yet been clarified. The $K^π=0^-$, $K^π=1^-$, and $K^π=3^-$ bands have been suggested, but the assignments have been inconsistent between experiments and theories. [Purpose:] Negative-parity states of $^{24}$Mg are investigated by microscopic structure and reaction calculations via proton and alpha inelastic scattering to clarify the band assignment for the observed negative-parity spectra. [Method:] The structure of $^{24}$Mg was calculated using the antisymmetrized molecular dynamics~(AMD). Proton and alpha inelastic reactions were calculated using microscopic coupled-channel (MCC) calculations by folding the Melbourne $g$-matrix $NN$ interaction with the AMD densities of $^{24}$Mg. [Results:] The member states of the $K^π=0^+$, $K^π=2^+$, $K^π=0^-$, $K^π=1^-$, and $K^π=3^-$ bands of $^{24}$Mg were obtained through the AMD result. In the MCC+AMD results for proton and alpha elastic and inelastic cross sections, reasonable agreements were obtained with existing data, except in the case of the $4^+_1$ state. [Conclusions:] The $3^-$ state of the $K^π=3^-$ band and the $1^-$ and $3^-$ states of the $K^π=0^-$ bands were assigned to the $3^-_1$(7.62 MeV), $1^-_1$(7.56 MeV), and $3^-_2$(8.36 MeV) states, respectively. The present AMD calculation is the first microscopic structure calculation to reproduce the energy ordering of the $K^π=0^-$, $K^π=1^-$, and $K^π=3^-$ bands of $^{24}$Mg.

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arXiv2020

Nuclear medium effect on neutron capture reactions during neutron star mergers

The discovery of gravitational waves has confirmed old theoretical predictions that binary systems formed with compact stars play a crucial role not only for cosmology and nuclear astrophysics. As a byproduct of these and subsequent observations, it is now clear that neutron-star mergers can be a competitive site for the production of half of the elements heavier than iron in the universe following a sequence of fast neutron capture reactions known as the r process. In this article we discuss an effect which has been so far neglected in calculations of r-process nucleosynthesis in neutron star mergers. We show that the corrections due to the neutron environment even at relatively small neutron densities, within the bounds of numerical hydrodynamical simulations of neutron star mergers and after the onset of the r process, are non-negligible and need to be taken into account to accurately describe the elemental abundance as determined by observations.

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arXiv2020

Microscopic calculation of proton and alpha-particle inelastic scattering to study the excited states of $^{6}$He and $^{8}$He

Elastic and inelastic cross sections of the $p+{}^6\textrm{He}$, $p+{}^8\textrm{He}$, and $α+{}^8\textrm{He}$ reactions were investigated using the Melbourne $g$-matrix folding approach with the theoretical densities of ${}^6\textrm{He}$ and ${}^8\textrm{He}$ obtained by a microscopic structure model of antisymmetrized molecular dynamics (AMD). Microscopic coupled-channel (MCC) calculations of the $p+{}^6\textrm{He}$ and $p+{}^8\textrm{He}$ reactions were performed to investigate transition properties of the ${}^6\textrm{He}(2^+_1)$ and ${}^8\textrm{He}(2^+_1)$ states. The MCC+AMD calculations reproduced elastic cross sections of the $p+{}^6\textrm{He}$ reaction at $E=40.9$ MeV/A and of the $p+{}^8\textrm{He}$ reaction at $E=32.5$ and 72 MeV/A, which have both been measured by inverse kinematics experiments. For $p+{}^6\textrm{He}$ inelastic scattering to the $2^+_1$ state, the calculated result was in reasonable agreement with the $(p,p')$ data at $E=24.5$ and 40.9 MeV/A and supported the AMD prediction of the neutron transition matrix element $M_n=7.9$ fm$^2$. For the $p+{}^8\textrm{He}$ inelastic scattering to ${}^8\textrm{He}(2^+_1)$, the MCC+AMD calculation overshot the $(p,p')$ cross sections at $E=72$ MeV/A by a factor of three. According to a phenomenological model analysis, $M_n$ values in the range of 4--6 fm$^2$ were favored to reproduce the ${}^8\textrm{He}(2^+_1)$ cross sections of the $p+{}^8\textrm{He}$ reaction at $E=72$ MeV/A. For the $α+{}^8\textrm{He}$ reaction, the MCC+AMD calculation reproduced the elastic cross sections at $E=26$ MeV/A. Theoretical predictions of the ($p,p'$) and ($α,α'$) cross sections to the ${}^8\textrm{He}(0^+_2)$, ${}^8\textrm{He}(1^-_1)$, ${}^8\textrm{He}(2^+_3)$ and ${}^8\textrm{He}(3^-_1)$ states are also given.

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arXiv2020

Neutron dominance in excited states of $^{26}$Mg and $^{10}$Be probed by proton and alpha inelastic scattering

Isospin characters of nuclear excitations in $^{26}$Mg and $^{10}$Be are investigated via proton($p$) and alpha($α$) inelastic scattering. A structure model of antisymmetrized molecular dynamics (AMD) is applied to calculate the ground and excited states of $^{26}$Mg and $^{10}$Be. The calculation describes the isoscalar feature of the ground-band $2^+_1$($K^π=0^+_1$) excitation and predicts the neutron dominance of the side-band $2^+_2$($K^π=2^+$) excitation in $^{26}$Mg and $^{10}$Be. The $p$ and $α$ inelastic scattering off $^{26}$Mg and $^{10}$Be is calculated by microscopic coupled-channel (MCC) calculations with a $g$-matrix folding approach by using the matter and transition densities of the target nuclei calculated with AMD. The calculation reasonably reproduces the observed $0^+_1$, $2^+_1$, and $2^+_2$ cross sections of $^{26}$Mg+$p$ scattering at incident energies $E_p=$24 and 40 MeV and of $^{26}$Mg+$α$ scattering at $E_α=$104 and 120 MeV. For $^{10}$Be+$p$ and $^{10}$Be+$α$ scattering, inelastic cross sections to the excited states in the $K^π=0^+_1$ ground-, $K^π=2^+$ side-, $K^π=0^+_2$ cluster-, and $K^π=1^-$ cluster-bands are investigated. The isospin characters of excitations are investigated via inelastic scattering processes by comparison of the production rates in the $^{10}$Be+$p$, $^{10}$Be+$α$, and $^{10}$C+$p$ reactions. The result predicts that the $2^+_2$ state is selectively produced by the $^{10}$Be+$p$ reaction because of the neutron dominance in the $2^+_2$ excitation as in the case of the $^{26}$Mg+$p$ scattering to the $2^+_2$ state, whereas its production is significantly suppressed in the $^{10}$C+$p$ reaction.

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arXiv2020

Properties of $K^π=0^+_1$, $K^π=2^-$, and $K^π=0^-_1$ bands of $^{20}$Ne probed via proton and alpha inelastic scattering

The $K^π=0^+_1$, $K^π=2^-$, and $K^π=0^-_1$ bands of $^{20}$Ne are investigated with microscopic structure and reaction calculations via proton and $α$ inelastic scattering off $^{20}$Ne. Structures of $^{20}$Ne are calculated with variation after parity and total angular momentum projections in the antisymmetrized molecular dynamics(AMD). The $K^π=0^+_1$ and $K^π=0^-_1$ bands have $^{16}\textrm{O}+α$ cluster structures, whereas the $K^π=2^-$ band shows a $^{12}\textrm{C}+2α$-like structure. Microscopic coupled-channel calculations of proton and $α$ scattering off $^{20}$Ne are performed by using the proton-nucleus and $α$-nucleus potentials, which are derived by folding the Melbourne $g$-matrix $NN$ interaction with AMD densities of $^{20}$Ne. The calculation reasonably reproduces the observed cross sections of proton scattering at $E_p=25$--35 MeV and $α$ scattering at $E_α=104$--386 MeV. Transition properties from the ground to excited states are discussed by reaction analyses of proton and $α$ inelastic processes. Mixing of the $K^π=2^-$ and $K^π=0^-_1$ bands is discussed by detailed analysis of the $0^+_1\to 3^-_1$ and $0^+_1\to 3^-_2$ transitions. For the $3^-_1$ state, mixing of the $K^π=0^-_1$ cluster component in the $K^π=2^-$ band plays an important role in the transition properties.

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