Admixtures of Shell and Cluster States in 18O

Abstract

The properties of the low-lying T=O positive-parity levels in F are shown to be well understood by considering admixtures of 2p shell-model states and states with alpha-cluster structures. In order to represent the states, a-N cluster model is introduced. By this model, weak coupling features and coupling between shell and cluster states are well described. The binding energies of the ground P and the lowest s+ levels are reproduced by the couplings with the cluster states. On the other hand, weak coupling features of cluster states are disturbed to some extent. As a result, the energy spectrum, E2-transition rates and reduced a-widths of all T=O positive-parity levels below 7 MeV excitation energy are systematically reproduced. and is a basic two-body system in nuclear many-body problem. Many efforts have been made to formulate the effective two-body interaction in 18F from the nucleon-nucleon force. And it has been shown that the 3p-1h core-renormalization plays an impor­ tant role.v For the T=O states, however, the 3P-1h renormalization has small effects and the energy of the ground 1 + state cannot be reproduced. It seems that the above situation is not so affected for the T = 0 states even if the higher­ order effects are taken into account.'l.sl Kuo1l has shown by the 2nd order pertur­ bation calculation that a 4p-2h renormalization selectively lowers the T = 0, J = 1 + state. W eidenmueller et al.,'l however, have pointed out that for the 4p-2h renormali­ zation the convergence of perturbation expansion cannot be expected because of the intruder states. This suggests that it is necessary to treat the 4p-2h states in the explicit manner together with 2p states, not by the perturbation expansion. Arima et al. have originally pointed out that the 1.7 MeV 1 7 level is not understood in the 2p shell-model,') and have proposed the weak coupling model6} in order to understand the level structures of 160* -'0N e nuclei; the above level in 18F is assigned as such a 4p-2h state. There have been deformed Hartree-Fock7}