Fast neutron flux measurement uncertainty due to the variation of neutron spectra distribution

As the fission cross-section of ~(239)Pu is flat while the neutron energy is larger than 10keV, the ~(239)Pu fission chamber is often used to measure the absolute fast neutron flux in the fast reactors or assemblies. Due to a little fraction in the fast neutron assemblies, having a relative large cross-section, the thermal neutron and resonance region neutron make the measured reaction rate varied abruptly when some moderators are near the detector. The ~(239)Pu fission rate with and without ~(10)B capsule, the Cadmium rate of ~(197)Au and ~(115)In, and the ~(32)S reaction rate are measured to analyze the neutron spectra variation out of the CFBR-II reactor. The fast neutron flux measurement methods are concluded in the end.