Distribution of E2 excitations in sd-shell nuclei

The distribution of E2 excitations from the ground state is studied for sd-shell even-even nuclei by a realistic shell-model. It is shown that the Q-phonon scheme for the 21+ state is quite reasonable in this region. The origin of the deviations from the pure Q-phonon scheme can be explained in terms of a Hamiltonian with broken SU(3) symmetry. The symmetry breaking is generated by non-degenerate single particle energies.