Time-dependent γ-ray emission after spontaneous fission of ²⁵²Cf with an experimental verification

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