In this paper we report on a new study of prompt $\gamma$-rays from the spontaneous fission of ${}^{252}\mathrm{Cf}$. 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 $\gamma$-ray multiplicity ${\overline{\nu }}_{\gamma }=(8.29\pm 0.13)$, the average energy per photon ${\epsilon }_{\gamma }=(0.80\pm 0.02)$ MeV, and the total $\gamma$-ray energy release per fission $E_{\gamma ,\text{tot}}=(6.65\pm 0.10)$ 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 $\gamma$-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 $\gamma$-ray multiplicity, either from a properly measured and normalized spectrum or a measured multiplicity distribution, give equivalent and consistent results.

Fast-neutron-induced fission of ${}^{238}\mathrm{U}$ at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency $\gamma$-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via $\gamma \text{−}\gamma$ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. This has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly.

The shape isomer in ${}^{235}\mathrm{U}$ has been searched for in a neutron-induced fission experiment on ${}^{234}\mathrm{U}$, which was performed at the isomer spectrometer NEPTUNE of the EC-JRC IRMM. A neutron source, with a tunable pulse frequency in the Hz to kHz range and its individually adjustable neutron pulse width in connection with an appropriate detector system turned out to be the ideal instrument to perform an isomer search, when decay half-lives above $100\mu \mathrm{s}$ are expected. From the delayed fission events observed for two different NEPTUNE settings and at mean incident neutron energies $E_n=0.95$ and 1.27 MeV the isomeric fission half-life could be determined to be $T_{1/2}=(3.6\pm 1.8)\mathrm{ms}$. The corresponding cross section was determined to ${\sigma }_{if}=(10\pm 8)\mu \mathrm{b}$. With these results an experimental confirmation for the existence of a superdeformed shape isomer in odd-uranium isotopes is given for the first time.

The energy dependent neutron-induced fission cross section of ${}_{}{}^{233}{}_{}{}^{}P\mathrm{a}$ has for the first time been measured directly with monoenergetic neutrons. This nuclide is an important intermediary in a thorium based fuel cycle, and its fission cross section is a key parameter in the modeling of future advanced fuel and reactor concepts. A first experiment resulted in four cross section values between 1.0 and 3.0 MeV, establishing a fission threshold in excess of 1 MeV. Significant discrepancies were found with a previous indirect experimental determination and with model estimates.