Photofission of235U and238U at intermediate energies: absolute cross sections and fragment mass distributions

Abstract

The total photofission cross section σγ,F for235U and238U has been measured in the energy range 50≤E γ≤800 MeV at the 855 MeV Mainz Microtron MAMI using energy and time tagged photons (Glasgow Tagger) and a 4π arrangement of position sensitive fragment detectors. Besides the absolute photofission cross section σγF , which almost completely exhausts the total photon absorption cross section for these nuclei, fragment mass distributions in this energy domain were determined via time of flight techniques (TOF). The results for the total photofission cross sections σγ,F normalized to the atomic numberA for both isotopes coincide, and agree in theΔ-resonance region, within the systematic errors, with the socalled“Universal Curve” σγ,T /A of the total photon absorption cross section σγ,T . At higher energies the cross sections exhibit a smooth behaviour. In particular, it is shown for the first time that there isno resonance-like shape near the D13 resonance (at ≈710 MeV) as observed for the free proton. This complete suppression of the D13 resonance in complex nuclei is not yet understood on a microscopic level. The fragment mass distributions show a predominantly mass symmetric fission. However, contributions from mass asymmetric fission at some photon energies may give a hint of an increased mass asymmetric fission after the onset of the pion and two pion channels.