arXiv subjects

Yohei Chiba

Publications and source records attributed to Yohei Chiba.

8 recordsLinked to original sources

arXiv2022

Role of Tensor Interaction as Salvation of Cluster Structure in $^{44}$Ti

Background: The $^{44}$Ti nucleus has been known to have a $^{40}$Ca+$α$ cluster structure, and inversion doublet structure has been observed; however, $α$ cluster structure tends to be washed out when the breaking of the $α$ cluster is allowed due to the spin-orbit interaction. Nevertheless, $α$ clustering in medium-heavy nuclei is quite a hot subject recently. Purpose: The tensor interaction has been known to play an essential role in the strong binding of the $^4$He nucleus, which induces the two-particle-two-hole (2p2h) excitation. Since this excitation is blocked when another nucleus approaches, it is worthwhile to show whether the tensor effect works to keep the distance between $^4$He and $^{40}$Ca and becomes the salvation of the clustering in $^{44}$Ti. Methods: The spin-orbit effect is included in the cluster model by using the antisymmetrized quasi cluster model (AQCM) developed by the authors. We have also developed an improved version of the simplified method to include the tensor contribution ($i$SMT), which allows us to estimate the tensor effect within the cluster model. The competition of these two is investigated in the medium-heavy mass region for the first time. Results: According to AQCM, the spin-orbit interaction completely breaks the $α$ cluster and restores the symmetry of $jj$-coupling shell model when the $α$ cluster approaches the $^{40}$Ca core. On the other hand, $i$SMT gives a large distance between $α$ and $^{40}$Ca due to the tensor effect. Conclusions: In $^{44}$Ti, because of the strong spin-orbit and tensor contributions, two completely different configurations ($jj$-coupling shell model and cluster states) almost degenerate, and their mixing becomes important.

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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

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

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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arXiv2019

Cluster correlation and nuclear vorticity in low-lying $1^-$ states of $^{24}$Mg

We investigated cluster correlation and nuclear voricity in low-lying $1^-$ states of $^{24}$Mg within antisymmetrized molecular dynamics framework. We found that the toroidal and compressional dipole modes separately appear as the $K=1$ and $K=0$ states. The $1^-$ ($K=1$) state is the toroidal dipole state with the strong nuclear vorticity but no prominent cluster structure, and the $1^-$ ($K=0$) state is the compressional dipole state having enhanced cluster structure but has the weaker vorticity.

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arXiv2019

Quantitative description of the $^{20}$Ne($p$,$pα$)$^{16}$O cross section as a means of probing the surface $α$ amplitude

The proton-induced $α$ knockout reaction has been utilized for decades to investigate the $α$ cluster states of nuclei, of the ground state in particular. However, even in recent years, it is reported that the deduced $α$ spectroscopic factors from $α$ knockout experiments and reaction analyses with a phenomenological $α$ cluster wave function diverge depending on the kinematical condition of the reaction. In the present study we examine the theoretical description of the $^{20}$Ne($p$,$pα$)$^{16}$O cross section based on the antisymmetrized molecular dynamics and the distorted wave impulse approximation by comparing with existing experimental data. We also investigate the correspondence between the $α$ cluster wave function and the $α$ knockout cross section. The existing $^{20}$Ne($p$,$pα$)$^{16}$O data at 101.5 MeV is well reproduced by the present framework. Due to the peripherality of the reaction, the surface region of the cluster wave function is selectively reflected to the knockout cross section. A quantitatively reliable $α$ cluster wave function, $p$-$α$ cross section, and distorting potentials between scattering particles, $α$-$^{16}$O in particular, are crucial for the quantitative description of the ($p$,$pα$) cross section. Due to the peripherality of the reaction, the ($p$,$pα$) cross section is a good probe for the surface $α$ amplitude.

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arXiv2017

Laplace expansion method for the calculation of the reduced width amplitudes

We derive the equations to calculate the reduced width amplitudes (RWA) of the different size clusters and deformed clusters without any approximation. These equations named Laplace expansion method are applicable to the nuclear models which uses the Gaussian wave packets. The advantage of the method is demonstrated by the numerical calculations of the ${}^{16}{\rm O}+α$ and ${}^{24}{\rm Mg}+α$ RWAs in $^{20}{\rm Ne}$ and $^{28}{\rm Si}$.

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