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

Prompt fission \(\gamma \)-ray spectra (PFGS) extending to very low photon energies have been measured in the \(^{232}\)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 \(^{232}\)Th foils, and the coincident prompt fission \(\gamma \)-rays were detected using two CeBr\(_{3}\) scintillation detectors. The measured spectra were unfolded using the corresponding response matrices of the detectors, simulated using Geant4, to obtain the PFGS in the \(\gamma \)-ray energy region of 0.1–1.2 MeV. Surprisingly, significant reduction in photon intensity is observed for the \(^{232}\)Th(n,f) reaction below the photon energy of 0.4 MeV when compared with the available data of \(^{238}\)U(n,f) and \(^{239}\)Pu(n,f) reactions at similar excitation energies. Moreover, the average \(\gamma \)-ray multiplicity, \(\bar{M_\gamma }\), is found to be \(\sim 40\%\) lower and the average photon energy, \(\epsilon _{\gamma }\), is found to be \(\sim 50\%\) higher for \(^{232}\)Th(n,f) reaction as compared to \(^{238}\)U(n,f) and \(^{239}\)Pu(n,f) reactions in the \(\gamma \)-ray energy region of \(0.1-1.2\) MeV. It is found that the GEF model fails to predict the observed features of the PFGS in the \(^{232}\)Th(n,f) reaction. The present experimental results may provide important insight in improving the current models addressing fast neutron induced fission of actinides.