Abstract The time-dependent emission of prompt fission γ rays was measured at JRC-GEEL with the VESPA spectrometer, the VErsatile γ SPectrometer Array, coupled to a position-sensitive fission detector. To be able to do so, we developed a method to assess and correct for γ -rays induced by scattered prompt fission neutrons. This research follows measurements performed in the 1970 s; after that, no more experimental work was performed until about the past decade. The close geometry of γ -ray detectors and the fission detector was the key to extracting the time-dependent fission γ -ray spectra. In addition to the measurements, model calculations with the Hauser-Feshbach Monte-Carlo code FIFRELIN were performed. These calculations can very well confirm the observed trend. The results show for the first time that late fission γ -ray emission occurs well beyond 5 μ s after the instant of fission. Our new data suggests that about 1. 5% of the fission γ rays are still emitted later than those 5 μ s after fission. In conclusion, for ^252 252 Cf (sf) we find a total average fission γ -ray multiplicity of M_, tot M ¯ γ, tot = 9. 4 ± (0. 2 ₛtat stat + 0. 4 ₛyst syst), with a late multiplicity M_, late M ¯ γ, late (t > 2. 5 ns) = 0. 70 ± (0. 22 ₛtat stat + 0. 57 ₛyst sy
Oberstedt et al. (Tue,) studied this question.