Abstract
Recent years have seen an increased interest in prompt fission γ-ray (PFG) measurements motivated by a high priority request of the OECD/NEA for high precision data, mainly for the nuclear fuel isotopes 235U and 239Pu. Our group has conducted a PFG measurement campaign using state-of-the-art lanthanum halide detectors for all the main actinides to a precision better than 3%. The experiments were performed in a coincidence setup between a fission trigger and γ-ray detectors. The time-of-flight technique was used to discriminate photons, traveling at the speed of light, and prompt fission neutrons. For a full rejection of all neutrons below 20 MeV, the PFG time window should not be wider than a few nanoseconds. This window includes most PFG, provided that no isomeric states were populated during the de-excitation process. When isomeric states are populated, PFGs can still be emitted up to 1 yus after the instant of fission or later. To study these γ-rays, the detector response to neutrons had to be determined and a correction had to be applied to the γ-ray spectra. The latest results for PFG characteristics from the reaction 239Pu(nth,f) will be presented, together with an analysis of PFGs emitted up to 200 ns after fission in the spontaneous fission of 252Cf as well as for thermal-neutron induced fission on 235U and 239Pu. The results are compared with calculations in the framework of the Hauser-Feshbach Monte Carlo code CGMF and FIFRELIN.
Highlights
After almost 80 years since the discovery of nuclear fission the underlying process is still not understood in sufficient details
Recent years have seen an increased interest in prompt fission γ-ray (PFG) measurements motivated by a high priority request of the OECD/Nuclear Energy Agency (NEA) for high precision data, mainly for the nuclear fuel isotopes 235U and 239Pu
The first community is represented by the OECD Nuclear Energy Agency (NEA) [1], who expressed the need for more accurate fission crosssection and fragment yield data for safety assessments of Generation-IV reactor systems [2]
Summary
After almost 80 years since the discovery of nuclear fission the underlying process is still not understood in sufficient details. The advances made in recent editions of various fission models presently on the market [3,4,5,6,7,8,9] require precise experimental data on prompt fission neutron and γ-ray emission as input parameters. Average energy per particle and total dissipated energy per fission, preferably as function of fission-fragment mass and total kinetic energy, are key input to benchmark nuclear fission models attempting to describe the competition between prompt neutron and γ-ray emission. In the following we recall the essential information about our experimental technique and the applied data analysis, before we present and discuss our experimental results
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