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.

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