Electrodisintegration of Nuclei by Positrons and Electrons

The ratio $\frac{{\sigma }^{-}}{{\sigma }^{+}}$ of the cross sections for electrodisintegration of nuclei by electrons and positrons has been measured at 27 MeV for target nuclei of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$, ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$. For ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{107}{}_{}{}^{}\mathrm{Ag}$ the ratio was measured as a function of energy in the region of the giant resonance. The ratio $\frac{{\sigma }^{-}}{{\sigma }^{+}}$ was determined from activities induced in thin foils which were bombarded by electrons and positrons from an electron linear accelerator. The measured ratio $\frac{{\sigma }^{-}}{{\sigma }^{+}}$ appears to vary linearly with $Z$ and to be independent of energy in the range covered. The results may be used to estimate the extent to which the plane-wave theory of electro-disintegration fails because of the Coulomb distortion of the electron wave function.