Magnetic dipole moments as a strong signature for alpha -clustering in even-even self-conjugate nuclei

Abstract

We investigate the magnetic dipole moments in even-even self-conjugate nuclei from ^{12}C to ^{48}Cr. For these nuclei, the measured gyromagnetic factors of excited states turn out to assume the same value of g approx + 0.5 within statistical errors. This peculiar feature can be interpreted on the basis of collective excitations of alpha -clusters. Analogously, the behaviour of the same observable is studied for all isotopes obtained by adding one or two neutrons to the considered self-conjugate nuclei. It is found that for the N = Z + 1 isotopes the alpha -cluster structure hardly contributes to the observed negative g- factor value, corroborating molecular alpha -cluster models. The addition of a further neutron, however, restores the original alpha -cluster g-factors, except for the semi-magic isotopes, in which the deviations from g approx + 0.5 can be associated with the relevant shell closures. Secondly, we analyze the same observable in the framework of a macroscopic alpha -cluster model on a finite lattice of side length L. We focus on the discretization effects induced in the magnetic dipole moments of the 2_1^+ and the 3_1^- states of ^{12}C at different values of the lattice spacing a.