Measurement ofE2−E0strength in the isovector giant resonance (GQR-GMR) region ofSi28nuclei through the(e,e'n)reaction

We have measured the $^{28}\mathrm{Si}$$(e,{e}^{'}n)$ reaction in the excitation energy range 21.5\char21{}40.5 MeV at three effective momentum transfers, 0.38, 0.49, and 0.60 fm${}^{\ensuremath{-}1}$. The $E1$ and $E2\text{\ensuremath{-}}E0$ components were separated based on their different momentum-transfer dependences. The $E1$ strength obtained was found to agree with that of photoreactions in shape and strength. The strength of the $E2\text{\ensuremath{-}}E0$ component at low excitation energies was very small when compared with that of the $(e,{e}^{'}p)$ reaction. At higher excitation energies, the $E2\text{\ensuremath{-}}E0$ component has a bump structure at about 26\char21{}30 MeV and it is suggested that it has an isovector character by comparison with $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{'})$ reaction data. This is supported by results of a $^{28}\mathrm{Si}$($^{7}\mathrm{Li}$,$^{7}\mathrm{Be}$)$^{28}\mathrm{Al}$ experiment. The $E2\text{\ensuremath{-}}E0$ strength of 37.9(\ifmmode\pm\else\textpm\fi{}4.7)% in the isovector $E2$ energy-weighted sum rule is exhausted in the excitation energy range 22.5\char21{}40.5 MeV.