Statistical study of the prompt-fission γ -ray spectrum for U238(n, f ) in the fast-neutron region

Abstract

Prompt-fission $\ensuremath{\gamma}$-ray spectra (PFGS) have been measured for the $^{238}\mathrm{U}(n,f$) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission $\ensuremath{\gamma}$ rays were measured using a number of ${\mathrm{LaBr}}_{3}$ scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission $\ensuremath{\gamma}$ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for $^{238}\mathrm{U}(n,f$) and also for $^{252}\mathrm{Cf}(sf$) as a reference. Spectral characteristics of PFG emission, such as mean $\ensuremath{\gamma}$ multiplicity and average total $\ensuremath{\gamma}$-ray energy per fission, as well as the average $\ensuremath{\gamma}$-ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at ${E}_{n}=1.9\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were found to be the mean $\ensuremath{\gamma}$ multiplicity ${\overline{M}}_{\ensuremath{\gamma}}=6.54\ifmmode\pm\else\textpm\fi{}0.19$, total released energy per fission ${E}_{\ensuremath{\gamma},\mathrm{tot}}=5.25\ifmmode\pm\else\textpm\fi{}0.20$ MeV, and the average $\ensuremath{\gamma}$-ray energy ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.80\ifmmode\pm\else\textpm\fi{}0.04$ MeV. Under similar conditions, the values at ${E}_{n}=4.8\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were measured to be ${\overline{M}}_{\ensuremath{\gamma}}=7.31\ifmmode\pm\else\textpm\fi{}0.46, {E}_{\ensuremath{\gamma},\mathrm{tot}}=6.18\ifmmode\pm\else\textpm\fi{}0.65$ MeV, and ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.84\ifmmode\pm\else\textpm\fi{}0.11$ MeV.