Molecular Structure with Exotic Clusters in light Neutron-rich Nuclei

The excited states of Be-12, Be-14 and B-15 have been studied with a method of antisymmetrized molecular dynamics. In the predicted excited states we find novel molecule-like structures with very exotic clusters such as 6He+8He in Be-14 and 6He+9Li in B-15. The origin of the He-6 cluster development in the very neutron-rich nuclei is understood by the new-type correlation among 4 neutrons and 2 protons. In this paper we also present our recent challenge to study $sd$-shell nuclei. Shape coexistence problems in Ar-36 and Ca-40 are discussed.Comment: 10 pages, 5 figures. The proceedings of Yukawa International Seminar 2001 on "Physics of Unstable Nuclei". To appear in Prog. Theor. Phy

Shape coexistence in N=14 isotones: 19B, 24Ne and 28Si

Shape coexistence problems in N=14 isotones are studied with a microscopic method of antisymmetrized molecular dynamics. The present calculations reproduce features of deformation in 28Si, and predict possible shape coexistence of neutron density in neutron-rich nuclei(19B and 24Ne). We also systematically study the structures of the ground and excited states, and the molecular resonances of 28Si. Besides the shape coexistence in the low-energy region, the results indicate the high-lying levels with $\alpha$-cluster and 12C+16O molecular structures in 28Si, which are consistent with the observed spin-assigned resonances. The resonance states above the threshold energies are connected with the low-lying deformed states from a view point of molecular excitation by discussing inter-cluster wave functions.Comment: 4 pages, 3 figures, Proceedings of "Frontiers of Collective Motions (CM2002)" Nov. 6-9, 2002, Aizu-Wakamatsu, Japa

Deformation of C isotopes

Systematic analysis of the deformations of proton and neutron densities in even-even C isotopes was done based on the method of antisymmetrized molecular dynamics. The $E2$ transition strength was discussed in relation to the deformation. We analyze the $B(E2;2^+_1\to 0^+_1)$ in $^{16}$C, which has been recently measured to be abnormally small. The results suggest the difference of the deformations between proton and neutron densities in the neutron-rich C isotopes. It was found that stable proton structure in C isotopes plays an important role in the enhancement the neutron skin structure as well as in the systematics of $B(E2)$ in the neutron-rich C.Comment: submitted to Phys.Rev.

Negative parity states of 11^{11}B and 11^{11}C and the similarity with $^{12}C

The negative parity states of $^{11}$B and $^{11}$C were studied based on the calculations of antisymmetrized molecular dynamics(AMD). The calculations well reproduced the experimental strengths of Gamov-Teller(GT), $M1$ and monopole transitions. We, especially, focused on the $3/2^-_3$ and $5/2^-_2$ states, for which GT transition strengths were recently measured. The weak $M1$ and GT transitions for the $3/2^-_3$ in $^{11}$B and $^{11}$C are described by a well-developed cluster structure of $2\alpha$+$t$ and $2\alpha$+$^3$He, respectively, while the strong transitions for the $5/2^-_2$ is characterized by an intrinsic spin excitation with no cluster structure. It was found that the $3/2^-_3$ state is a dilute cluster state, and its features are similar to those of the $^{12}$C$(0^+_2)$ which is considered to be a gas state of three $\alpha$ clusters.Comment: 10 pages, 4 figures, submitterd to Physical Review

New effective nuclear forces with a finite-range three-body term and their application to AMD+GCM calculations

We propose new effective inter-nucleon forces with a finite-range three-body operator. The proposed forces are suitable for describing the nuclear structure properties over a wide mass number region, including the saturation point of nuclear matter. The forces are applied to microscopic calculations of $Z=N$ ($A\le 40$) nuclei and O isotopes with a method of antisymmetrized molecular dynamics. We present the characteristics of the forces and discuss the importance of the finite-range three-body term.Comment: 15 pages, 11 figures, submitted to Phys.Rev.

Structure of Light Unstable Nuclei Studied with Antisymmetrized Molecular Dynamics

Structures of light unstable nuclei, Li, Be, B, and C isotopes are systematically studied with a microscopic method of antisymmetrized molecular dynamics. The theoretical method is found to be very useful to study ground and excited states of various nuclei covering unstable nuclei. The calculations succeed to reproduce many experimental data for nuclear structures; energies, radii, magnetic dipole moments, electric quadrupole moments, transition strength. In the theoretical results it is found that various exotic phenomena in unstable nuclei such as molecular-like structures, neutron skin, and large deformations may appear in unstabel nuclei. We investigate the structure change with the increase of neutron number and with the increase of the excitation energies, and find the drastic changes between shell-model-like structures and clustering structures. The mechanism of clustering developments in unstable nuclei are discussed.Comment: 73 pages, Revtex, 42 postscript figures (using epsf.sty). to be published in Suppl. Prog. Theor. Phy

Structure of Excited States of 10Be studied with Antisymmetrized Molecular Dynamics

We study structure of excited states of 10Be with the method of variation after spin parity projection in the framework of antisymmetrized molecular dynamics. Present calculations describe many excited states and reproduce the experimental data of E2 and E1 transitions and the new data of the $\beta$ transition strength successfully. We make systematic discussions on the molecule-like structures of light unstable nuclei and the important role of the valence neutrons based on the results obtained with the framework which is free from such model assumptions as the existence of inert cores and clusters.Comment: 15 pages, RevTex, seven postscript figures (using epsf.sty

Dipole resonances in light neutron-rich nuclei studied with time-dependent calculations of antisymmetrized molecular dynamics

In order to study isovector dipole response of neutron-rich nuclei, we have applied a time-dependent method of antisymmetrized molecular dynamics. The dipole resonances in Be, B and C isotopes have been investigated. In $^{10}$Be, $^{15}$B, $^{16}$C, collective modes of the vibration between a core and valence neutrons cause soft resonances at the excitation energy $E_x=10-15$ MeV below the giant dipole resonance(GDR). In $^{16}$C, we found that a remarkable peak at $E_x=14$ MeV corresponds to coherent motion of four valence neutrons against a $^{12}$C core, while the GDR arises from the core vibration in the $E_x >20$ MeV region. In $^{17}$B and $^{18}$C, the dipole strengths in the low energy region decline compared with those in $^{15}$B and $^{16}$C. We also discuss the energy weighted sum rule for the $E1$ transitions.Comment: 12 figures, submitted to Phys. Rev.

Structure of excited states of Be-11 studied with Antisymmetrized Molecular Dynamics

The structures of the ground and excited states of Be-11 were studied with a microscopic method of antisymmetrized molecular dynamics. The theoretical results reproduce the abnormal parity of the ground state and predict various kinds of excited states. We suggest a new negative-parity band with a well-developed clustering structure which reaches high-spin states. Focusing on a $2\alpha$ clustering structure, we investigated structure of the ground and excited states. We point out that molecular orbits play important roles for the intruder ground state and the low-lying $2\hbar \omega$ states. The features of the breaking of $\alpha$ clusters were also studied with the help of data for Gamow-Teller transitions.Comment: 24 pages, 7 figures, to be submitted to Phys.Rev.