Photofission of 238U induced by a brilliant, quasi-monochromatic, Compton-backscattered γ beam

The photofission cross-section of 238U was measured at sub-barrier energies as a function of the γ energy using, for the first time, a monochromatic, high-brilliance, Compton-backscattered γ beam. This prototype experiment was performed at the HIGS γ-beam facility, using the 238U(γ,f) reaction at a beam energy varied between E=4.7-6.0 MeV and with an energy resolution of dE=150-200 keV. Clear indications of predicted resonance structures have been observed at excitation energies of E=5.1 and 5.6 MeV. With the significantly higher intensity of the beam, when comparing to a tagged-photon facility, the cross-section could be measured at deep sub-barrier energies. The triple-humped fission barrier parameters of 238U have been determined by fitting EMPIRE3.1 nuclear reaction code calculations to the experimental photofission cross-section. Our present results on the fission barrier of 238U support a deep third minimum (EIII=3.6 MeV) and a low inner barrier (EA=4.3 MeV).