Use of Prompt Fission Neutrons for Measuring the Fission Efficiency

The pressing need for high-precision data on neutron-induced reactions of interest for emerging nuclear technologies and nuclear astrophysics has triggered the development of new methods and experiments, especially on capture cross-section measurements of fissile nuclei. In these measurements, radiative capture γ-rays are hidden in a large background of fission γ-rays. The latter has then to be subtracted via a veto method, by detecting fission events with a very good efficiency. This efficiency is limited by the backscattering and the self-absorption of the fission fragments (FF) in the target, and by the alpha radioactivity of the sample. An accurate determination of the fission efficiency is rather difficult. It is often determined by using known fission cross sections if the neutron flux is given. However, in some cases, the fission cross section is not well known and the neutron flux cannot be measured with sufficient accuracy. Another method to determine the fission efficiency is based on the detection of prompt fission neutrons in coincidence or anti-coincidence with the fission detector. Different examples are presented and advantages and disadvantages of this method will be discussed.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard