Statistical and pre-equilibrium (γ,α) cross sections ofZr90and their multipolarities via theZr90(e,α) reaction

The ($e$,$\alpha$) cross section in ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ has been measured at incident electron energies from 13.5 MeV to 66.5 MeV for $\alpha$ particles between 6.9 and 16.8 MeV. The ($\gamma$,$\alpha$) cross section was deduced from it assuming both $E1$ interaction and $E2\mathrm{}$ interaction. The angular distribution of the ($\gamma$,${\alpha }_0$) cross section and an experiment using the bremsstrahlung plus electron beam make it clear that the $E1$ interaction is dominant over all the present energy range. The ($\gamma$,$\alpha$) cross section extracted by using $E1$ virtual photon spectra has a large bump above the excitation energy of 30 MeV in addition to a bump in the giant dipole resonance region. The ($\gamma$,${\alpha }_0$) cross section also has a bump at the giant dipole resonance which exhausts most of the ($\gamma$,$\alpha$) cross section in that region. The compound nucleus model was used successfully to explain the bump at the giant dipole resonance. The cross section above 30 MeV is discussed in terms of the pre-equilibrium $\alpha$ emission process combined with the quasi-deuteron model.