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

Abstract

The (e,..cap alpha..) cross section in /sup 90/Zr has been measured at incident electron energies from 13.5 MeV to 66.5 MeV for ..cap alpha.. particles between 6.9 and 16.8 MeV. The (..gamma..,..cap alpha..) cross section was deduced from it assuming both E1 interaction and E2 interaction. The angular distribution of the (..gamma..,..cap alpha../sub 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..,..cap 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..,..cap alpha../sub 0/) cross section also has a bump at the giant dipole resonance which exhausts most of the (..gamma..,..cap 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 ..cap alpha.. emission process combined with the quasi-deuteron model.