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Hadrons and Nuclei

Eur. Phys. J. A 13, 43-47 (2002)

Molecular-orbital structure in neutron-rich Be and C isotopes

N. Itagaki1, S. Okabe2, K. Ikeda3 and I. Tanihata3

1  Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2  Center for Information and Multimedia Studies, Hokkaido University, Japan
3  RIB Science Laboratory, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan

(Received: 1 May 2001)

The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In 10Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0+ state of 10Be has a large $\alpha$- $\alpha$ cluster structure, and this is characterized by a $(1/2^+_\sigma)^2$ configuration. An enlargement of the $\alpha$- $\alpha$ distance due to two-valence neutrons along the $\alpha$- $\alpha$ axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the Sz=0 and the Sz=1 configurations, is important. In the ground state of 12Be, the calculated energy exhibits similar characteristics, that the remarkable $\alpha$ clustering and the contribution of the spin-orbit interaction make the binding of the state with $(3/2^-_\pi)^2(1/2^+_\sigma)^2$ configuration properly stronger in comparison with the closed p-shell $(3/2^-_\pi)^2(1/2^-_\pi)^2$ configuration. This is related to the breaking of the N=8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic $\alpha$+ $\alpha$+ $\alpha$+ n+ n+ $\cdot \cdot \cdot$ model. The combination of the valence neutrons in the $\pi$- and the $\sigma$-orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of 16C are the most promising candidates for such structure.

21.10.-k - Properties of nuclei; nuclear energy levels.
21.60.Gx - Cluster models.

© Società Italiana di Fisica, Springer-Verlag 2002