Trinucleon cluster states in 18F and 18O

Abstract

We propose that a number of low-lying negative-parity states in 18F and 18O can be understood as ( A = 15) + ( A = 3) cluster states. We introduce a cluster-core interaction consisting of a local central part which gives rise to the usual L( L + 1) rotational spacing, and three spin-dependent terms: spin-spin, tensor-spin and spin-orbit. These interactions will stagger the levels obtained by coupling the total intrinsic spin S = 0 or 1 to the orbital angular momentum L and we show that the strengths of these interactions can be chosen to yield good fits to the low-lying energy spectra. We note that a relation exists between the 18F and 18O spectra, and we point out that this can be understood if an isospin-exchange factor is introduced in the spin-spin interaction. In fact, both the 18F and 18O energy spectra can be generated from essentially the same cluster-core interaction, the rotational and spin-orbit components of which are closely related to those of the “parent” nucleus 19F. A number of high-spin states in both nuclei are predicted to lie quite close to where strongly populated states have been observed in heavy-ion three-nucleon transfer reactions. Electromagnetic transition strengths between cluster states are also calculated and are in good agreement with the available experimental data.