Nuclear excitation by electrons and positrons

Existing measurements of the ratio $\text{σ}{}^{\mathrm{?}}\text{σ}{}^{\mathrm{+}}$, the nuclear excitation cross sections produced by electrons and positrons, are compared with the results of distorted wave calculations. The behaviour as a function of atomic number Z and electron energy E₀ is shown to be principally due to Coulomb distortion. The value of $\text{σ}{}^{\mathrm{?}}\text{σ}{}^{\mathrm{+}}$ displays no clear structure corresponding to resonances in the photoabsorption cross section, except that it should be quite sensitive to the presence of quadrupole strength.