THE EFFECT OF THE DELTA-EXCITATION ON THE NUCLEON-NUCLEON EFFECTIVE INTERACTIONS IN THE NUCLEAR SHELL-MODEL

The effect of Delta excitation on the shell-model effective NN interac tions is investigated by evaluating the Delta-particle nucleon-hole co re polarization diagrams within the folded-diagram formulation. The ca lculation has been performed using the NN<->N Delta transition G-matri x generated from a coupled-channel pi NN model which is constrained by the NN data up to 1 GeV and is based on a Delta-subtracted Paris pote ntial. Satisfactory convergence of the calculation is reached by inclu ding the Delta excitations up to 20 oscillator shells. The Delta-parti cle nucleon-hole core-polarization diagrams are found to be small for the sd-shell valence nucleons.