Influence of thermal and resonance neutron on fast neutron flux measurement by ²³⁹Pu fission chamber

Abstract: The ²³⁹Pu fission chambers are widely used to measure fission spectrum neutron flux due to a flat response to fast neutrons. However, in the meantime the resonance and thermal neutrons can cause a significant influence on the measurement if they are moderated, which could be eliminated by using ¹⁰B and Cd covers. At a column enriched uranium fast neutron critical assembly, the fission reaction rates of ²³⁹Pu are measured as 1.791× 10^-16, 2.350×10^-16 and 1.385× 10^-15 per second for 15 mm thick ¹⁰B cover, 0.5 mm thick Cd cover, and no cover respectively, while the fission reaction rate of ²³⁹Pu is rapidly increased to 2.569× 10^-14 for a 20 mm thick polythene covering fission chamber. The average ²³⁹Pu fission cross-section of thermal and resonance neutrons is calculated to be 500 b and 24.95 b with the assumption of 1/v and 1/E spectra respectively, then thermal, resonance and fast neutron flux are achieved to be 2.30× 10⁶, 2.24× 10⁶ and 1.04× 10⁸ cm^-2·^-1.