Nuclear wave functions of microscopic a-particle model, which were obtained in the generator coordinate calculation by the present authors, are found to reproduce properties of low energy levels of 12 C nucleus, i.e., a-decay widths, electric transitions and charge form factors for electron scattering. Especially calculated widths of the Oz +and 10.3 MeV levels are found to be in excellent agreement with experiment, assuming that the spin of the latter is 2. The monopole transition from the o, + state to the ground state is also well reproduced as well as the E2 transition to the 2, + state. Furthermore analysis of the reduced width amplitudes is made which clarifies characters of the levels. It is found that in the o, + and 10.3 MeV states an a-particle is coupled mainly to the 'Be ground state with orbital angular momenta l equal to 0 and 2 respectively. In the predicted higher positive-parity states O, +, 2, + and 4, +, an a-particle is coupled mainly to the first excited state of 'Be with l equal to 2. § 1. IntroductionIn 12 C nucleus the 0/ level at 7.7 MeV and the next positive-parity level at 10.3 MeV are well known as anomalous states due to their low excitation energies and extremely large a-reduced widths. v From the viewpoint of nuclear shell model, low-lying o+ states in 12 C and 16 0 are considered to come from four-particle excitations. But the experimental excitation energies are too low to be reproduced by simple four-particle four-hole shell-model configurations. Morinaga suggested that the 0, + and 10.3 MeV levels form a rotational band with a linear chain structure of 3 a-particles.') According to Ikeda's diagram,') we can expect the appearance of nuclear states with some cluster structure near the threshold energies for decay into a-particles. The above-noted excited states in 12 C are expected to be such states, that is, to have distinct cluster structure which is associated with the 8 Be+a channel open at 7.4 MeV. Ikeda et al. 4 l analyzed a-decay widths of 3 a-cluster configurations by using the Bloch-Brink wave function 5 J and suggested a structure of loosely coupled 3 a-particles for the excited states. Horiuchi succeeded in obtaining a shell-like ground state and an excited state with a well-developed cluster structure by extending the orthogonality condition model to three-cluster system.At the same time, full microscopic calculations were made on this nucleus by several authors. 6 l, 7 l They, however, restricted freedom of 3 a-particle motion to by guest on April 10, 2015 http://ptp.oxfordjournals.org/ Downloaded from 1622 E. Uegaki, Y. Abe, S. Okabe and I-I. Tanalw the linear chain configuration 61 or to the hinge-angle vibration.n As a result they obtained only moderate success for the excited states.We recently investigated the structure of 12 C nucleus by using the microscopic ct-particle model of Bloch-Brink and by taking full account of motion of :1 a-clusters (Ref. 8), hereafter referred to as I). vVe succeeded in reproducing all the low energy levels of 12 C nucleus (except f...
