Statistical study of the prompt-fission γ-ray spectrum for ²³⁸U(n,f) in the fast-neutron region

Prompt-fission gamma-ray spectra (PFGS) have been measured for the ²³⁸U(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 γ rays were measured using a number of LaBr₃ scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission 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 ²³⁸U(n,f) and also for ²⁵²Cf(sf) as a reference. Spectral characteristics of PFG emission, such as mean γ multiplicity and average total γ-ray energy per fission, as well as the average γ-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 MeV were found to be the mean gamma multiplicity M-bar_γ = 6.54 ± 0.19, total released energy per fission E_γ,tot = 5.25 ± 0.20 MeV, and the average γ-ray energy ε_γ = 0.80 ± 0.04 MeV. Under similar conditions, the values at E_n = 4.8 MeV were measured to be M-bar_γ = 7.31 ± 0.46, E_γ,tot = 6.18 ± 0.65 MeV, and ε_γ = 0.84 ± 0.11 MeV. (authors)