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

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 using10 B and Cd covers. At a column enriched uranium fast neutron critical assembly,the fission reaction rates of 239 Pu are measured as 1.791×10-16,2.350×10-16 and 1.385×10-15 per second for 15 mm thick10 B cover,0.5 mm thick Cd cover,and no cover respectively,while the fission reaction rate of 239 Pu is rapidly increased to 2.569×10-14 for a 20 mm thick polythene covering fission chamber. The average 239 Pu fission cross-section of thermal and resonance neutrons is calculated to be 500 b and24.95 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 1.04×108cm-2·s-1.