Measuring the energies and multiplicities of prompt gamma-ray emissions from neutron-induced fission of $^{235}$U using the STEFF spectrometer

Following a NEA high priority nuclear data request, an experimental campaign to measure the prompt $\gamma$-ray emissions from $^{235}$U has been performed. This has used the STEFF spectrometer at the new Experimental Area 2 (EAR2) within the neutron timeof-flight facility (n_TOF), a white neutron source facility at CERN with energies from thermal to approximately 1 GeV. Prior to the experimental campaign, STEFF has been optimised for the environment of EAR2. The experimental hall features a high background $\gamma$-ray rate, due to the nature of the spallation neutron source. Thus an investigation into reduction of the background $\gamma$-ray rate, encountered by the NaI(Tl) detector array of STEFF, has been carried out. This has been via simulations using the simulation package FLUKA. Various materials and shielding geometries have been investigated but the effects determined to be insufficient in reducing the background rate by a meaningful amount. The NaI(Tl) detectors have been modified to improve their performance in a high count rate environment, and their behaviour characterised to understand the response to higher count rates. Initial testing demonstrated that the modified detectors maintain a potential to measure $\gamma$-ray multiplicities up to 3 counts per microsecond. However, the energy resolution fails somewhere below 1.75 counts per microsecond. The experimental campaign has produced a large amount of data. The preliminary analysis of phase one data has considered incoming neutron energies ranging from thermal to an upper limit of 1 eV, with a minimum $\gamma$-ray energy threshold of 160 keV. Results have been achieved for the prompt fission $\gamma$-ray multiplicity and total energy of $M_{\gamma}$ = 6.3 $\pm$ 0:2 and $E_{\gamma}$ = 9.0 $\pm$ 0.1 MeV respectively. Further work is ongoing by the STEFF team at Manchester to improve upon these results and analyse the remainder of the data set at higher incoming neutron energies.