Improved values for the characteristics of prompt-fissionγ-ray spectra from the reaction235U(nth,

Andreas Oberstedt

¹

,

T. Belgya

²

,

R. Billnert

³

et al.

Abstract

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.

Properties of prompt-fissionγrays

“…The spin is mostly removed by the γ emissions, dominated by statistical dipole transitions at higher energies and quadrupole transitions at low energies. The agreement with the measured prompt γ spectrum [10][11][12][13][14][15][16] is in general good, including a reasonable description of the features observed experimentally at low energies [10,[12][13][14], which are attributed to transitions between discrete levels of specific fragments. We also present in the current paper results for the multiplicity dependent spectrum and compare them against a parameterized model of the γ-ray emissions [15,16].…”

“…These data have a threshold of 90 keV and 140 keV, respectively, with comparable time-coincidence windows from the fission event of 5 ns and 10 ns respectively. In particular, Pleasonton data [11] is not compatible with the Peelle [10] and Oberstedt [14] measured multiplicities. As we will show below, the multiplicity, and, hence, the multiplicity distribution, are very sensitive to the threshold energy.…”

Prompt fissionγ-ray characteristics from neutron-induced fission on 239Pu and the time-dependence of prompt-γray emission

“…Here, we report about our most recent measurement of prompt fission γ-ray spectral PFGS characteristics from neutron-induced fission on 239 Pu. Those results made it necessary to verify previous results obtained for prompt fission γ-ray characteristics from neutron-induced fission on 235 U [14]. In addition, we present first results on time-dependent PFGS emission.…”

“…This required a verification of our results on PFGS from thermal-neutron induced fission on 235 U [14]. Therefore, we added a γ-ray detector to the SCINTIA array [11], an instrument dedicated to measure prompt-neutron spectral data on 235 U at thermal and resolved-resonance energies.…”

“…Some years ago, a collaboration of scientists from JRC Geel (the former JRC IRMM) and other institutes took the lead in establishing a dedicated measurement programme on prompt fission neutron [10,11] and fission γ-ray characteristics [12][13][14][15][16][17] responding to a high-priority data request of the OECD NEA. Here, we report about our most recent measurement of prompt fission γ-ray spectral PFGS characteristics from neutron-induced fission on 239 Pu.…”