Dilepton decay from excited states in 28Si populated via different isospin entrance channels.

Abstract

Nuclear states in $^{28}\mathrm{Si}$, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction $^{4}\mathrm{He}$${+}^{24}$Mg and the mixed-isospin $^{3}\mathrm{He}$${+}^{25}$Mg reaction. In both reactions the dilepton (${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ spectrum, over the converted photon yield, is observed in the energy region 17--22 MeV in the $^{3}\mathrm{He}$ induced reaction, possibly indicating an entrance channel effect. \textcopyright{} 1996 The American Physical Society.