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

Abstract The time-dependent emission of prompt fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> rays was measured at JRC-GEEL with the VESPA spectrometer, the VErsatile $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> SPectrometer Array, coupled to a position-sensitive fission detector. To be able to do so, we developed a method to assess and correct for $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -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 $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray detectors and the fission detector was the key to extracting the time-dependent fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -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 $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray emission occurs well beyond 5 $$\mu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> s after the instant of fission. Our new data suggests that about 1.5% of the fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> rays are still emitted later than those 5 $$\mu $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> s after fission. In conclusion, for $$^{252}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>252</mml:mn> </mml:mmultiscripts> </mml:math> Cf(sf) we find a total average fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray multiplicity of $$\overline{M}_\mathrm {\gamma ,tot}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mover> <mml:mi>M</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>,</mml:mo> <mml:mi>tot</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 9.4 ± (0.2 $$_\textrm{stat}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mtext>stat</mml:mtext> <mml:mrow/> </mml:mmultiscripts> </mml:math> + 0.4 $$_\textrm{syst}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mtext>syst</mml:mtext> <mml:mrow/> </mml:mmultiscripts> </mml:math> ), with a late multiplicity $$\overline{M}_\mathrm {\gamma ,late}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mover> <mml:mi>M</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>,</mml:mo> <mml:mi>late</mml:mi> </mml:mrow> </mml:msub> </mml:math> (t &gt; 2.5 ns) = 0.70 ± (0.22 $$_\textrm{stat}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mtext>stat</mml:mtext> <mml:mrow/> </mml:mmultiscripts> </mml:math> + 0.57 $$_\textrm{syst}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mtext>sy