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In this paper we present results from measurements of prompt gamma rays from the thermal neutron induced fission of U-235. Photons were measured in coincidence with fission fragments with cerium-doped LaCl3 and LaBr3 as well as CeBr3 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 (nu) over bar (gamma) = (8.19 +/- 0.11) per fission, the average energy per photon to be epsilon(gamma) = (0.85 +/- 0.02) MeV, and the total energy to be E-gamma,E-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 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.

The production rates of ultracold neutrons (UCN) from cold neutrons on gaseous, liquid, and solid deuterium targets have been measured. The comparison of the measured and calculated UCN production on gaseous ${}^{2}{H}_{2}$ is used to calibrate the simulated target extraction and transport efficiencies of the experimental apparatus. The production cross section in solid ${}^{2}{H}_{2}$ at 8 K for UCN with energies between 0 and 250 neV is ${R}_{\mathrm{solid},8 K}={\ensuremath{\sigma}}_{\mathrm{solid},8 K}^{\mathrm{CN}\ensuremath{\rightarrow}\mathrm{UCN}} \ensuremath{\rho}=(1.11\ifmmode\pm\else\textpm\fi{}0.23)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This value is consistent with other experiments in which UCN had been extracted from ${}^{2}{H}_{2}$. The value also agrees with calculations using the incoherent approximation and a simple Debye model and corroborates predictions for UCN densities expected at the high-intensity UCN source at the Paul Scherrer Institut. The temperature dependence of the UCN production in solid ${}^{2}{H}_{2}$ down to 8 K can be explained within the same model when multiple-phonon excitation is included.