Time-dependent $$\gamma $$-ray emission after spontaneous fission of $$^{252}$$Cf with an experimental verification

Abstract The time-dependent emission of prompt fission $$\gamma $$ γ rays was measured at JRC-GEEL with the VESPA spectrometer, the VErsatile $$\gamma $$ 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 $$\gamma $$ 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 $$\gamma $$ γ -ray detectors and the fission detector was the key to extracting the time-dependent fission $$\gamma $$ 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 $$\gamma $$ γ -ray emission occurs well beyond 5  $$\mu $$ μ s after the instant of fission. Our new data suggests that about 1.5% of the fission $$\gamma $$ γ rays are still emitted later than those 5  $$\mu $$ μ s after fission. In conclusion, for $$^{252}$$ 252 Cf(sf) we find a total average fission $$\gamma $$ γ -ray multiplicity of $$\overline{M}_\mathrm {\gamma ,tot}$$ M ¯ γ , tot = 9.4 ± (0.2 $$_\textrm{stat}$$ stat + 0.4 $$_\textrm{syst}$$ syst ), with a late multiplicity $$\overline{M}_\mathrm {\gamma ,late}$$ M ¯ γ , late (t > 2.5 ns) = 0.70 ± (0.22 $$_\textrm{stat}$$ stat + 0.57 $$_\textrm{syst}$$ syst ), corresponding to (7.4 ± 1.6 $$_\textrm{stat}$$ stat ± 4.2 $$_\textrm{syst}$$ syst )%.