Fission of 238U from 100 MeV to 1000 MeV by a Coherent Photon Beam

The cross-section per equivalent quantum in the fission induced in238U by a quasi-monochromatic photon beam has been measured. The photon beam was obtained by coherent bremsstrahlung from 1000 MeV electrons striking a diamond single crystal. In the experiment the energies of the main peak of the photon spectrum assumed sixteen different values in the range from 220 MeV to 500 MeV. The fission fragments have been detected by means of glass sandwiches containing thin targets of UF4. The cross-section per photon from 100 MeV to 1000 MeV has been deduced from the experimental yields applying an appropriate unfolding method after having subtracted from the yields the contribution due to photons with energies below 100 MeV. The obtained cross-section clearly shows a resonance centred at a photon energy 320 MeV with a FWHM {reversed tilde equals}125 MeV and a peak value of 70 mb. This resonance is in good agreement with the first baryon resonance in pion photoproduction. The energy dependence of the photofission cross-section for238U can be explained by both assuming that the photomesonic absorption mechanism prevails over the quasi-deuteron interaction in producing fission above 150 MeV and that the nuclear fissility increases with the energy.