Improved values for the characteristics of prompt-fission $\gamma$-ray spectra from the reaction ${}^{235}$U($n_{\mathrm{th}}$,$f$)

In this paper we present results from measurements of prompt $\gamma$ 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 $\gamma$-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 $\gamma$ rays was determined to be ${\overline{\nu }}_{\gamma }=(8.19\pm 0.11)$ per fission, the average energy per photon to be ${\epsilon }_{\gamma }=(0.85\pm 0.02)$ MeV, and the total energy to be $E_{\gamma ,\mathrm{tot}}=(6.92\pm 0.09)$ MeV. The uncertainties are much lower than the 7.5$\%$ requested for the modeling of advanced nuclear reactor cores. Estimating the influence of $\gamma$ 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.