Medium effects in electric form factors and transition strengths.

Abstract

Electric transition strengths are calculated in a relativistic model. It is found that for electric transitions there is, in addition to the spin-orbit current, an extra medium-induced current coming from the Darwin term in the Hamiltonian. A medium-modified transition strength is defined and the effects of both medium-induced currents are studied. The application for the first ${3}^{\mathrm{\ensuremath{-}}}$ excitation in $^{40}\mathrm{Ca}$ yields unrealistic results. However, when vacuum polarization corrections are included the resulting B(E3) value comes close to the nonrelativistic shell-model one.