Spectra of sd-shell nuclei and three-nucleon forces

A three-nucleon force, to be used in sd-shell-modell calculations has been evaluated microscopically. One part of this three-nucleon force arises from the fact that non-nucleonic degrees of freedom, with the possibility of exciting nucleons to the Δ(3, 3) resonance, are neglected in a shell-model calculation. Besides this component of a 'real' three-nucleon force also the effective three-body interaction is included in second-order perturbation theory which accounts for the restriction to sd-shell-model configurations. A shell-model computer code has been set up which allows the treatment of a general three-body force in a jj-coupled basis. The three-body force has a very small influence on the spectra of the systems with three valence nucleons (A=19). Larger effects emerge for A=20 and A=21. (orig.)