Fission Cross-Section of Re, W and Ta by a Coherent Photon Beam from 1000 MeV Electrons

The photofission yields of Re, W and Ta induced by a coherent bremsstrahlung beam from 1000 MeV electrons striking a diamond single crystal have been measured. The experiment has been performed at eighteen different energies of the main peak of the photon spectrum, in the energy range between 220 MeV and 550 MeV, by detecting the fission fragments with glass sandwiches. The behaviour of the photo-fission cross-section has been deduced from the experimental yields by using an appropriate unfolding method. The obtained curves clearly show a first resonance centred at a photon energy k≃350 MeV with a FWHM≃145 MeV, while there is a hint of a second resonance at k≃750 MeV. Information on the energy dependence of the nuclear fissility from 100 MeV to 1000 MeV has been deduced from the comparison of the estimated photofission cross-section with the total photon interaction cross-section. It has been found that the photomesonic model of the fission process permits to explain the energy dependence of our photofission cross-sections if a nuclear fissility increasing with photon energy is assumed