Eur. Phys. J. A (2025) 61: 38
https://doi.org/10.1140/epja/s10050-025-01508-6

Regular Article - Experimental Physics

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

¹ Nuclear Physics Division, Bhabha Atomic Research Centre, Trombay, 400085, Mumbai, India
² Homi Bhabha National Institute, Anushaktinagar, 400094, Mumbai, India

^a rgthomas@barc.gov.in

Received: 19 December 2024
Accepted: 9 February 2025
Published online: 27 February 2025

Abstract

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, $\overline{M_{\gamma }}$, is found to be $\sim 40\%$ lower and the average photon energy, ${ϵ}_{\gamma }$, 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.