Tetrahedral 4 alpha and C-12 + alpha cluster structures in O-16

摘要

I have investigated structures of the ground and excited states of O-16 with the method of variation after spin-parity projection in the antisymmetrized molecular dynamics model combined with the generator coordinate method of C-12 + alpha cluster. The calculation reasonably reproduces the experimental energy spectra; E2, E3, E4, and ISI transitions; and a-decay properties. The formation of 4 alpha clusters has been confirmed from nucleon degrees of freedom in the AMD model without assuming the existence of any clusters. They form "tetrahedral" 4 alpha- and C-12 + alpha cluster structures. The C-12 + alpha structure constructs the K-pi = 0(+) band consisting of the 0(2)(1), 2(1)(1), and 4(1)(+) states and the K-pi = 0(-) band of the 12(-)(,) 3(2)(-), and 5(1)(-) states. The 0(1)(+), 3(1)(-), and 4(2)(+) states are assigned to the ground band constructed from the tetrahedral 4a structure. The 0(1)(+) and 3(1)(-) are approximately interpreted as Td band members with the ideal tetrahedral configuration. The ground-state 4 alpha correlation plays an important role in the enhancement of the E3 transition strength to the 3(1)(-). The 4(2)(+) state is not the ideal T-d member but constructed from a distorted tetrahedral 4a structure. Moreover, significant state mixing of the tetrahedral 4 alpha and C-12 + a cluster structures occurs between 4(1)(+) and 4(2)(+) states, indicating that the T-d configuration of 4 alpha is rather fragile at J(pi) = 4(+).