We present results from new spectral prompt γ-ray measurements from the spontaneous fission of 252 Cf. Apart from one recent experiment, about four decades have passed since the last dedicated experiments were reported in the literature. Hence, there was a need for a revision. We have measured prompt fission γ rays with both cerium-doped LaBr 3 and CeBr 3 scintillation detectors, both of which exhibit excellent timing and good energy resolution. The emission yield was determined to be ν γ = (8.30 ± 0.08)/fission and ν γ = (8.31 ± 0.10)/fission, with the average energy of γ = (0.80 ± 0.01) MeV and γ = (0.80 ± 0.01) MeV and total energy of E γ,tot = (6.64 ± 0.08) MeV and E γ,tot = (6.65 ± 0.12) MeV, with the LaBr 3 and CeBr 3 detectors, respectively. Since the results from the two detectors are in excellent agreement and confirm the historical data, but not those in the present evaluated nuclear data files, we strongly recommend an update.
In this paper we present results from measurements of prompt γ rays from the thermal neutron induced fission of 235 U. Photons were measured in coincidence with fission fragments with cerium-doped LaCl 3 and LaBr 3 as well as CeBr 3 scintillation detectors, which offer an intriguing combination of excellent timing resolution and good resolving power. The spectra measured with all employed detectors are in excellent agreement with respect to their shapes. Characteristic parameters were extracted for a γ-energy range from 0.1 to 6.0 MeV and the results obtained with several detectors were averaged. From that, the average emission yield of prompt-fission γ rays was determined to be ν γ = (8.19 ± 0.11) per fission, the average energy per photon to be γ = (0.85 ± 0.02) MeV, and the total energy to be E γ,tot = (6.92 ± 0.09) MeV. The uncertainties are much lower than the 7.5% requested for the modeling of advanced nuclear reactor cores. Estimating the influence of γ rays with energies between 6 and 10 MeV on the values determined in this work revealed a negligible deviation of the order of the found uncertainties.
In this paper we report on a new study of prompt γ-rays from the spontaneous fission of 252 Cf. Photons were measured in coincidence with fission fragments by employing four different lanthanide halide scintillation detectors. Together with results from a previous work of ours, we determined characteristic parameters with high precision, such as the average γ-ray multiplicity ν γ = (8.29 ± 0.13), the average energy per photon γ = (0.80 ± 0.02) MeV, and the total γ-ray energy release per fission E γ,tot = (6.65 ± 0.10) MeV. The excellent agreement between the individual results obtained in all six measurements proves the good repeatability of the applied experimental technique. The impact of low-energy photons, i.e., below 500 keV, on prompt fission γ-ray spectra characteristics has been investigated as well by comparing our results with those taken with the DANCE detector system, which appears to suffer from absorption effects in the low-energy region. Correction factors for this effect were estimated, giving results comparable to ours as well as to historical ones. From this we demonstrate that the different techniques of determining the average γ-ray multiplicity, either from a properly measured and normalized spectrum or a measured multiplicity distribution, give equivalent and consistent results.
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.
Abstract. In recent years the measurement of prompt fission γ-ray spectra (PFGS) has gained renewed interest, after about forty years since the first comprehensive studies of the reactions 235 U(n th , f), 239 Pu(n th , f) and 252 Cf(sf). The renaissance was initiated by requests for new values especially for γ-ray multiplicity and average total energy release per fission in neutron-induced fission of 235 U and 239 Pu. Both isotopes are considered the most important ones with respect to the modeling of innovative cores required for the Generation-IV reactors, the majority working with fast neutrons. During the last 5 years we have conducted a systematic study of spectral data for thermal-neutron-induced fission on 235 U and 241 Pu as well as for the spontaneous fission of 252 Cf with unprecedented accuracy. From the new data we conclude that those reactions do not considerably contribute to the observed heat excess and suspect other reactions playing a significant role. Possible contributions may originate from fast-neutron-induced reactions on 238 U, which is largely present in the fuel, or from γ-induced fission from neutron capture in the construction material. A first experiment campaign on prompt γ-ray emission from fast-neutron-induced fission on 235,238 U was successfully performed in order to test our assumptions. In the following we attempt to summarize, what has been done in the field to date, and to motivate future measurement campaigns exploiting dedicated neutron and photon beams as well as upcoming highly efficient detector assemblies.
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