First experimental prompt #gamma#-ray spectra in fast-neutron-induced fission of U-238

The knowledge of prompt fission #gamma#-ray emission has been of major interest in reactor physics for a few years. Only few experimental spectra were published until now for fast-neutron-induced fission, and measurements would be also valuable in order to improve our understanding of the fission process. A simple experimental method was used to measure the first prompt fission #gamma#-ray spectra up to 10 MeV. In this approach, the #gamma# rays are measured with a bismuth germanate (BGO) detector which offers two significant advantages with respect to other #gamma#-ray detectors: a high peak-to-total ratio and a high efficiency. The prompt fission neutrons are rejected by the time-of-flight technique between the BGO detector and a fission trigger given by a fission chamber. Prompt fission #gamma#-ray spectra were measured for 1.6 #+-# 0.1, 5.1 #+-# 0.2 and 15.0 #+-# 0.6 MeV neutron-induced fission on U-238 at the CEA, DAM, DIF Van de Graaff accelerator; average multiplicity and mean photon energy per fission were deduced from the spectra. (authors)