Intriguing features of the prompt fission $$\gamma $$-ray spectra in the fast neutron induced fission of $$^{232}$$Th

Abstract Prompt fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray spectra (PFGS) extending to very low photon energies have been measured in the $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>232</mml:mn> </mml:mmultiscripts> </mml:math> Th(n,f) reaction over a range of incident neutron energies below the second chance fission threshold, for which no data are available. Fission events were detected with an ionization chamber containing $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>232</mml:mn> </mml:mmultiscripts> </mml:math> Th foils, and the coincident prompt fission $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -rays were detected using two CeBr $$_{3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> scintillation detectors. The measured spectra were unfolded using the corresponding response matrices of the detectors, simulated using Geant4, to obtain the PFGS in the $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray energy region of 0.1–1.2 MeV. Surprisingly, significant reduction in photon intensity is observed for the $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>232</mml:mn> </mml:mmultiscripts> </mml:math> Th(n,f) reaction below the photon energy of 0.4 MeV when compared with the available data of $$^{238}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>238</mml:mn> </mml:mmultiscripts> </mml:math> U(n,f) and $$^{239}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>239</mml:mn> </mml:mmultiscripts> </mml:math> Pu(n,f) reactions at similar excitation energies. Moreover, the average $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray multiplicity, $$\bar{M_\gamma }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>γ</mml:mi> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> , is found to be $$\sim 40\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>40</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> lower and the average photon energy, $$\epsilon _{\gamma }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>ϵ</mml:mi> <mml:mi>γ</mml:mi> </mml:msub> </mml:math> , is found to be $$\sim 50\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>50</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> higher for $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>232</mml:mn> </mml:mmultiscripts> </mml:math> Th(n,f) reaction as compared to $$^{238}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>238</mml:mn> </mml:mmultiscripts> </mml:math> U(n,f) and $$^{239}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>239</mml:mn> </mml:mmultiscripts> </mml:math> Pu(n,f) reactions in the $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -ray energy region of $$0.1-1.2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0.1</mml:mn> <mml:mo>-</mml:mo> <mml:mn>1.2</mml:mn> </mml:mrow> </mml:math> MeV. It is found that the GEF model fails to predict the observed features of the PFGS in the $$^{232}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>232</mml:mn> </mml:mmultiscripts> </mml:math> Th(n,f) reaction. The present experi