Prompt fissionγ-ray characteristics from neutron-induced fission on ²³⁹Pu and the time-dependence of prompt-γray emission

Abstract

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 235 U and 239 Pu. 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 µs 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 239 Pu(n th ,f) will be presented, together with an analysis of PFGs emitted up to 200 ns after fission in the spontaneous fission of 252 Cf as well as for thermal-neutron induced fission on 235 U and 239 Pu. The results are compared with calculations in the framework of the Hauser-Feshbach Monte Carlo code CGMF and FIFRELIN.

Overview of two deterministic modelings for prompt emission in fission

“…A focus of the low energy part of the prompt γ-ray spectrum is illustrated in Fig. 8 where the calculated spectrum of 235 U(n th ,f) is compared with recent experimental data [24] (grey symbols). A slight underestimation of a part of experimental data, but remaining inside the error bars, is observed at low prompt γ-ray energies below 1 MeV.…”

Gamma-ray Spectroscopy in Low-Power Nuclear Research Reactors

“…The HPGe spectrum illustrates a substantial loss of spectral information during power operation compared to its shutdown counterpart. This may mostly be due to the pileup (estimated to be >60%) or the relative increase in fission gamma rays that follow an approximately exponential shape in energy [78]. Both plots show that our detectors receive much greater count rates at power than at shutdown.…”