arXiv subjects

Masaaki Kimura

Publications and source records attributed to Masaaki Kimura.

At least 55 records · Page 3Linked to original sources

arXiv2015

Quenching of N = 28 shell gap and a low-lying quadrupole mode in the vicinity of neutron-rich N = 28 isotones

We investigate the low-lying 2$^+$ states of $N=28$ isotones ($^{48}$Ca, $^{46}$Ar, $^{44}$S and $^{42}$Si) with using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory in which the pairing is taken into account self-consistently. The quadrupole mode with very small excitation energies emerges in $^{46}$Ar, $^{44}$S and $^{42}$Si due to the strong quadrupole correlations triggered by the quenching of the $N=28$ shell gap. The importance of the quadrupole correlations between the protons and neutrons, and the role of the pairing correlation to reinforce the lowest 2$^+$ mode are discussed. It is also shown that the $B$(E2$\uparrow$) values of $^{48}$Ca and $^{46}$Ar are plausibly explained by our calculations.

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arXiv2014

Low-lying 2$^+$ states generated by $pn$-quadrupole correlation and $N=28$ shell quenching

The quadrupole vibrational modes of neutron-rich $N$=28 isotones ($^{48}$Ca, $^{46}$Ar, $^{44}$S and $^{42}$Si) are investigated using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory with several choice of energy density functionals, including nuclear pairing correlation. It is found that the quenching of $N$=28 shell gap and the proton holes in the $sd$-shell trigger quadrupole correlation and increase the collectivity of the low-lying 2$^+$ state in $^{46}$Ar. It is also found that the pairing correlation plays an important role to increase the collectivity. We also demonstrate that the same mechanism to enhance the low-lying collectivity applies to other $N$=28 isotones $^{44}$S and $^{42}$Si, and it generates a couple of low-lying 2$^+$ states which can be associated with the observed $2^+$ states.

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arXiv2014

Ground-state properties of neutron-rich Mg isotopes

We analyze recently-measured total reaction cross sections for 24-38Mg isotopes incident on 12C targets at 240 MeV/nucleon by using the folding model and antisymmetrized molecular dynamics(AMD). The folding model well reproduces the measured reaction cross sections, when the projectile densities are evaluated by the deformed Woods-Saxon (def-WS) model with AMD deformation. Matter radii of 24-38Mg are then deduced from the measured reaction cross sections by fine-tuning the parameters of the def-WS model. The deduced matter radii are largely enhanced by nuclear deformation. Fully-microscopic AMD calculations with no free parameter well reproduce the deduced matter radii for 24-36Mg, but still considerably underestimate them for 37,38Mg. The large matter radii suggest that 37,38Mg are candidates for deformed halo nucleus. AMD also reproduces other existing measured ground-state properties (spin-parity, total binding energy, and one-neutron separation energy) of Mg isotopes. Neutron-number (N) dependence of deformation parameter is predicted by AMD. Large deformation is seen from 31Mg with N = 19 to a drip-line nucleus 40Mg with N = 28, indicating that both the N = 20 and 28 magicities disappear. N dependence of neutron skin thickness is also predicted by AMD.

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arXiv2014

Superdeformed $Λ$ hypernuclei with antisymmetrized molecular dynamics

The response to the addition of a $Λ$ hyperon is investigated for the deformed states such as superdeformation in $^{41}_Λ$Ca, $^{46}_Λ$Sc and $^{48}_Λ$Sc. In the present study, we use the antisymmetrized molecular dynamics (AMD) model. It is pointed out that many kinds of deformed bands appear in $^{45}$Sc and $^{47}$Sc. Especially, it is found that there exists superdeformed states in $^{45}$Sc. By the addition of a $Λ$ particle to $^{40}$Ca, $^{45}$Sc and $^{47}$Sc, it is predicted, for the first time, that the superdeformed states exist in the hypernuclei $^{41}_Λ$Ca and $^{46}_Λ$Sc. The manifestation of the dependence of the $Λ$-separation energy on nuclear deformation such as spherical, normal deformation and superdeformation is shown in the energy spectra of $^{41}_Λ$Ca, $^{46}_Λ$Sc and $^{48}_Λ$Sc hypernuclei.

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arXiv2012

Antisymmetrized molecular dynamics and its applications to cluster phenomena

Structure and reaction studies with a method of antisymmetrized molecular dynamics (AMD) were reviewed. Applications of time-independent and time-dependent versions of the AMD were described. In applications of time-independent AMD to nuclear structure studies, structures of neutron-rich nuclei such as Be, C, Ne, and Mg isotopes were described focusing on cluster aspects. Important roles of valence neutrons were discussed. The results suggested a variety of cluster structures appear also in unstable nuclei as well as in stable nuclei. Deformation and cluster phenomena in $Z\sim N$ nuclei in $p$- and $sd$- shell regions were also discussed. Applications of time-dependent AMD contain various topics such as fragmentation in heavy-ion collisions as well as nuclear responses. The AMD calculations successfully describe multifragmentation which is one of the remarkable phenomena in heavy-ion collisions. The approach is able to link reactions and nuclear matter properties. The studies suggested the important balance between single-particle motions and correlations to form clusters and fragments.

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arXiv2012

Deformation of Ne isotopes in the island-of-inversion region

The deformation of Ne isotopes in the island-of-inversion region is determined by the double-folding model with the Melbourne $g$-matrix and the density calculated by the antisymmetrized molecular dynamics (AMD). The double-folding model reproduces, with no adjustable parameter, the measured reaction cross sections for the scattering of $^{28-32}$Ne from $^{12}$C at 240MeV/nucleon. The quadrupole deformation thus determined is around 0.4 in the island-of-inversion region and $^{31}$Ne is a halo nuclei with large deformation. We propose the Woods-Saxon model with a suitably chosen parameterization set and the deformation given by the AMD calculation as a convenient way of simulating the density calculated directly by the AMD. The deformed Woods-Saxon model provides the density with the proper asymptotic form. The pairing effect is investigated, and the importance of the angular momentum projection for obtaining the large deformation in the island-of-inversion region is pointed out.

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arXiv2011

Deformation effect on total reaction cross sections for neutron-rich Ne-isotopes

Isotope-dependence of measured reaction cross sections in scattering of $^{28-32}$Ne isotopes from $^{12}$C target at 240 MeV/nucleon is analyzed by the double-folding model with the Melbourne $g$-matrix. The density of projectile is calculated by the mean-field model with the deformed Wood-Saxon potential. The deformation is evaluated by the antisymmetrized molecular dynamics. The deformation of projectile enhances calculated reaction cross sections to the measured values.

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arXiv2011

Determination of the structure of $^{31}$Ne by full-microscopic framework

We perform the first quantitative analysis of the reaction cross sections of $^{28-32}$Ne by $^{12}$C at 240 MeV/nucleon, using the double-folding model (DFM) with the Melbourne $g$-matrix and the deformed projectile density calculated by the antisymmetrized molecular dynamics (AMD). To describe the tail of the last neutron of $^{31}$Ne, we adopt the resonating group method (RGM) combined with AMD. The theoretical prediction excellently reproduce the measured cross sections of $^{28-32}$Ne with no adjustable parameters. The ground state properties of $^{31}$Ne, i.e., strong deformation and a halo structure with spin-parity $3/2_{}^-$, are clarified.

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arXiv2011

Modification of triaxial deformation and change of spectrum in $^{25}_{\ Λ}$Mg caused by $Λ$ hyperon

The positive-parity states of $^{25}_{\ Λ}$Mg with a $Λ$ hyperon in $s$ orbit were studied with the antisymmetrized molecular dynamics for hypernuclei. We discuss two bands of $^{25}_{\ Λ}$Mg corresponding to the $K^π=0^+$ and $2^+$ bands of $^{24}$Mg. It is found that the energy of the $K^π= 2^+ \otimes Λ_s$ band is shifted up by about 200 keV compared to $^{24}$Mg. This is because the $Λ$ hyperon in $s$ orbit reduces the quadrupole deformation of the $K^π= 0^+ \otimes Λ_s$ band, while it does not change the deformation of the $K^π= 2^+ \otimes Λ_s$ band significantly.

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arXiv2011

Deformation of hypernuclei studied with antisymmetirzed molecular dynamics

An extended version of the antisymmetrized molecular dynamics to study structure of $p$-$sd$ shell hypernuclei is developed. By using an effective $ΛN$ interaction, we investigate energy curves of $^9_Λ$Be, $^{13}_Λ$C and $^{20,21}_Λ$Ne as function of nuclear quadrupole deformation. Change of nuclear deformation caused by $Λ$ particle is discussed. It is found that the $Λ$ in p-wave enhances nuclear deformation, while that in s-wave reduces it. This effect is most prominent in $^{13}_Λ$C. The possibility of the parity inversion in $^{20}_Λ$Ne is also examined.

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arXiv2010

Triaxial superdeformation in $^{40}$Ar

Superdeformed (SD) states in $^{40}$Ar have been studied using the deformed-basis antisymmetrized molecular dynamics. Low energy states were calculated by the parity and angular momentum projection (AMP) and the generator coordinate method (GCM). Basis wave functions were obtained by the energy variation with a constraint on the quadrupole deformation parameter $β$, while other quantities such as triaxiality $γ$ were optimized by the energy variation. By the GCM calculation, an SD band was obtained just above the ground state (GS) band. The SD band involves a $K^π= 2^+$ side band due to the triaxiality. The calculated electric quadrupole transition strengths of the SD band reproduce the experimental values appropriately. Triaxiality is significant for understanding low-lying states.

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arXiv2009

Cluster structures and superdeformation in $^{28}$Si

We have studied positive-parity states of $^{28}$Si using antisymmetrized molecular dynamics (AMD) and multi-configuration mixing (MCM) with constrained variation. Applying constraints to the cluster distance and the quadrupole deformation of the variational calculation, we have obtained basis wave functions that have various structures such as $α$-$^{24}$Mg and $^{12}$C-$^{16}$O cluster structures as well as deformed structures. Superposing those basis wave functions, we have obtained a oblate ground state band, a $β$ vibration band, a normal-deformed prolate band, and a superdeformed band. It is found that the normal-deformed and superdeformed bands contain large amounts of the $^{12}$C-$^{16}$O and $α$-$^{24}$Mg cluster components, respectively. The results also suggest the presence of two excited bands with the developed $α$-$^{24}$Mg cluster structure, where the inter-cluster motion and the $^{24}$Mg-cluster deformation play important roles.

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arXiv2008

Deformations and clustering correlations in $p$- and $sd$-shell nuclei using the Gogny and Skyrme interactions

Structures of $p$- and $sd$-shell nuclei are studied with the deformed-basis antisymmetrized molecular dynamics method using the Gogny D1S and Skyrme SLy7 forces as effective interactions. By the energy variation with a constraint, energy curves as functions of quadrupole deformation parameter $β$ are obtained. The energy curves for $sd$-shell nuclei show structure change as a function of $β$, and suggest shape coexistence. Nuclear structures in the deformed region are discussed focusing on deformations and clustering. It is found that the deformations often involve cluster structures. Effective-interaction dependence is also discussed comparing the results obtained with the two effective interactions. Although the two forces give similar results when cluster structures do not develop clearly, they give different energies in the largely deformed region when an $α$ cluster develops.

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arXiv2007

Clustering and Triaxial Deformations of $^{40}$Ca

We have studied the positive-parity states of $^{40}$Ca using antisymmetrized molecular dynamics (AMD) and the generator coordinate method (GCM). Imposing two different kinds of constraints on the variational calculation, we have found various kinds of $^{40}{\rm Ca}$ structures such as a deformed-shell structure, as well as $α$-$^{36}$Ar and $^{12}$C-$^{28}$Si cluster structures. After the GCM calculation, we obtained a normal-deformed band and a superdeformed band together with their side bands associated with triaxial deformation. The calculated $B(E2)$ values agreed well with empirical data. It was also found that the normal-deformed and superdeformed bands have a non-negligible $α$-$^{36}$Ar cluster component and $^{12}$C-$^{28}$Si cluster component, respectively. This leads to the presence of an $α$-$^{36}$Ar higher-nodal band occurring above the normal-deformed band.

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arXiv2007

Largely deformed states of $^{13}$B

The excited states of $^{13}$B were studied with a method of antisymmetrized molecular dynamics(AMD). The theoretical results suggest that the intruder states with large deformations construct the rotational bands, $K^π=3/2^-$ and $K^π=1/2^+$, starting from 5 MeV and 8 MeV, respectively. The neutron structure of the $K^π=3/2^-$ is analogous to the intruder ground state of $^{12}$Be. In the predicted $K^π=1/2^+$ band, we found very exotic structure with a proton intruder configuration. This proton intruder state has a larger deformation than superdeformation. The band-head $1/2^+$ state is assigned to the $1/2^+$(4.83 MeV), which was experimentally suggested to be the proton intruder state because of the strong production via the proton-transfer to the $^{12}$Be($0^+$) state in the $^4$He($^{12}$Be,$^{13}$B$γ$)$X$ experiments.

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arXiv2007

The intruder feature of 31Mg and the coexistence of many particle and many hole states

The low-lying level structure of $^{31}{\rm Mg}$ has been investigated by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM) with the Gogny D1S force. It is shown that the N=20 magic number is broken and the ground state has the pure neutron $2p3h$ configuration. The coexistence of many particle and many hole states at very low excitation energy is discussed.

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arXiv2004

New Constraint of Clustering for AMD and Its Application to the Study of $2α$-$^{12}$C Structure of $^{20}$Ne

A new constraint of clustering for the AMD calculation is proposed. This new constraint gives us large improvement in studying the cluster structure by AMD which sometimes meets difficulty in giving rise to some specific cluster configurations. The usefulness of this new constraint is verified by applying it to the the study of the third $K^π= 0^+$ band of $^{20}$Ne which has been discussed to have $2α$-$^{12}$C structure. This band has not been easy even to construct by AMD. We see that the AMD+GCM calculation by the use of the new constraint gives rise to the third $K^π= 0^+$ band which contains the $2α$-$^{12}$C structure as an important component.

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arXiv2004

Coexistence of cluster structure and superdeformation in $^{44}$Ti studied with deformed-basis antisymmetrized molecular dynamics

The nucleus $^{44}$Ti has low-lying levels of various kinds of mutually very different nuclear structure displaying the richness of the nuclear many-body dynamics. It is shown that the deformed-basis antisymmetrized molecular dynamics by the use of the Gogny D1S force reproduces successfully and unifiedly two types of coexistence phenomena in $^{44}$Ti. Namely, on one hand, the coexistence of the mean field structure and the cluster structure is confirmed by verifying the normally deformed structure of the $K^π$=$3^-_1$ band with a 1-particle-jump intrinsic configuration and the $α+ ^{40}$Ca cluster structure of the $K^π$=$0^-_2$ band. The mixed character of the mean-field-like structure and the $α+ ^{40}$Ca cluster structure of the ground band is also shown. On the other hand, the coexistence of the normal deformed mean field and the superdeformed mean field is confirmed by verifying the triaxial superdeformation of the $K^π$=$0_2^+$ band and the $K^π$=$2^+_1$ band which has a 4-particle-jump intrinsic configuration. Good reproduction of the experimental data is shown for many kinds of quantities including the energy spectra, electric transition rates, alpha spectroscopic factors. Preliminary discussions are given on the existence of hyperdeformed excited states, the relation between superdeformation and clustering and so on.

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