Dilepton decay from excited states in Si28 populated via different isospin entrance channels

Nuclear states in Si-28, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction He-4+Mg-24 and the mixed-isospin He-3+Mg-25 reaction. In both reactions the dilepton (e(+)e(-)) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the e(+)e(-) spectrum, over the converted photon yield, is observed in the energy region 17-22 MeV in the He-3 induced reaction, possibly indicating an entrance channel effect.