Low-lying magnetic excitations of doubly-closed-shell nuclei and nucleon-nucleon effective interactions

We studied the low lying magnetic spectra of 12C, 16O, 40Ca, 48Ca and 208Pb nuclei within the Random Phase Approximation (RPA) theory. The description of low-lying magnetic states of doubly-closed-shell nuclei imposes severe constraints on the spin and tensor terms of the nucleon-nucleon effective interaction. We have first made an investigation by using four phenomenological effective interactions and we have obtained good agreement with the experimental magnetic spectra, and with the electron scattering responses. Then we made self-consistent RPA calculations to test the validity of the finite-range D1 Gogny interaction. For all the nuclei investigated we have found that this interaction inverts the energies of all the magnetic states forming isospin doublets