Measurement of fission rate ratios in fresh UO2 fuel utilising short-lived high-energy gamma activity

Determining fission rates via neutron activation is an established technique used in fresh fuel investigations at research reactors. Measuring fission rates in burnt fuel, however, requires the development of new techniques which are able to discriminate against the intense intrinsic gamma activity of the fuel reaching energies up to approximately 2 MeV. The presented study approaches the problem by focussing on the delayed gamma activity in the higher energy region (2-5 MeV). In order to identify suitable gamma-ray lines for this new measurement technique, fresh UO2 pellets have been irradiated and their gamma activity shortly after irradiation has been recorded with an HPGe-detector. It was possible to identify four gamma-ray lines above 2 MeV from which fission rate ratios could be derived with a satisfactory accuracy. Several additional promising gamma-ray lines could be identified but were not further considered because of too poor statistics. Low counting rates in the higher energy region were noted as a major challenge for future experiments. With regard to the envisaged burnt fuel experiments, 138Cs, 142La and 95Y appear to be very promising candidates, showing several prominent gamma-ray lines above 2 MeV and similar high fission yields for both uranium and plutonium fissions.