Chapter IV. Coupling Effects of Two-Particle and Alpha Two-Hole States inA= 18 Nuclei

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Abstract
Properties of low-lying positive-parity levels in A = 18 nuclei are shown to be well understood by considering admixtures of 2p states and 4p-2h states with α-cluster structure (α-2h states). In order to represent α-2h states, and α-14N(14C) cluster model is introduced. This model can describe the weak coupling feature of α-2h states and the coupling between 2p and α-2h states properly. It is shown that the α-2h states exist in the same energy region and also with the same level density as 2p states, and therefore couplings between these two types of states arise inevitably. Calculated couplings of 2p and α-2h states are found to be stronger in T = 0 than in T = 1 states. The binding energy of the ground 1+ (T = 0) state is reproduced by virtue of the coupling effect. Further, a selection rule for the couplings between T = 0 states is derived, which leads to a simple understanding of the breaking mechanism of the weak coupling structure in α-2h states. E2 transitions between α-2h states are enhanced owing to the α-clustering, in good agreement with experiments. Furthermore, by using additional charges δen2p = 0.5 e and δep2p = 0.2 e for 2p states, most of the known E2 transitions are well reproduced. E2 transitions between T = 1 analogue states show interesting coupling effects of 2p and α-2h states. The α-widths and single-nucleon spectroscopic factors are also reproduced nicely and are discussed from the viewpoint of couplings of 2p and α-2h states.

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