The ²³⁵U(n,f) Prompt Fission Neutron Spectrum at 100 K Input Neutron Energy

A measurement of the ²³⁵U prompt fission neutron spectrum (PFNS) was performed at the Budapest Nuclear Research Reactor at 100 K incident neutron energy. The motivation for this investigation was to verify some literature data measured over the past 20 years that contradict the Los Alamos model, as well as integral data, benchmark (Keff) experiments, and recent spectral data taken at 0.5 MeV incident neutron energy. The measured spectra using three neutron detectors are in excellent agreement with each other. The average spectrum confirms literature data within the error bars in the neutron energy range of 0.7 to 10 MeV. However, the present PFNS shape cannot predict integral experimental data. It seems to be clear now that the disagreement between microscopic and macroscopic data is not connected with a systematic experimental error in the PFNS at low incident neutron energy.