Prompt fission gamma-ray characteristics from neutron-induced fission on (PU)-P-239 and the time-dependence of prompt gamma-ray emission

Recent years have seen an increased interest in prompt fission gamma-ray (PFG) measurements motivated by a high priority request of the OECD/NEA for high precision data, mainly for the nuclear fuel isotopes U-235 and Pu-239. 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 gamma-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 its after the instant of fission or later. To study these gamma-rays, the detector response to neutrons had to be determined and a correction had to be applied to the gamma-ray spectra. The latest results for PFG characteristics from the reaction Pu-239(nth,f) will be presented, together with an analysis of PFGs emitted up to 200 ns after fission in the spontaneous fission of Cf-252 as well as for thermal-neutron induced fission on U-235 and Pu-239. The results are compared with calculations in the framework of the Hauser-Feshbach Monte Carlo code CGMF and FIFRELIN.