In this paper we present results from the first high-precision prompt-γ-ray spectral measurements from the reaction 241 Pu(n th , f). Apart from one recent experiment, no data are reported in the literature for this fissioning system, which motivated a new dedicated experiment. We have measured prompt-fission γ rays with three cerium-doped LaBr 3 (two 5.08 cm × 5.08 cm and one 7.62 cm × 7.62 cm) and one CeBr 3 (5.08 cm × 5.08 cm) scintillation detectors, which all exhibit excellent timing and good energy resolution. The average γ-ray multiplicity was determined to be ν γ = (8.21 ± 0.09) per fission, the average energy to be γ = (0.78 ± 0.01) MeV, and the total energy to be E γ,tot = (6.41 ± 0.06) MeV as the weighted average from all detectors. Since the results from all detectors are in excellent agreement, and the total released γ energy is modestly higher than the one in the present evaluated nuclear data files, we suspect that the underestimation of the prompt-γ heating in nuclear reactors is due to fast-neutron-induced fission on 238 U or rather from fission induced by γ rays from neutron capture in the construction material.

In this paper we present new results for prompt fission γ-ray spectral characteristics from the thermal neutron induced fission of 240 Pu *. The measured spectra were unfolded by using the detectors' response functions, simulated with GEANT4. We obtained in average per fission a γ-ray multiplicityM γ = (7.35 ± 0.12), a mean photon energy¯ γ = (0.85 ± 0.02) MeV, and an average total energy released in fissionĒ γ,tot = (6.27 ± 0.11) MeV. Our results are in good agreement with historical data measured in the 1970s by Verbinski et al. and results from recent calculations in the framework of Monte Carlo Hauser-Feshbach models. Our measured average total energy is slightly smaller than the one deduced previously and present in evaluated data. From this we conclude that the 239 Pu(n th ,f) reaction may be ruled out as possible source of γ heating underestimation, when compared with benchmark calculations based on existing nuclear data.

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