Abstract:The 239 Pu fission chambers are widely used to measure fission spectrum neutron flux due to a flat response to fast neutrons. However, in the mean time the resonance and thermal neutrons can cause a significant influence on the measurement if they are moderated, which could be eliminated by using 10 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 15mm thick 10 B cover, 0.5mm 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 20mm thick polythene covering fission chamber. The average 239 Pu fission cross-section of thermal and resonance neutrons is calculated to be 500b and 24.95b with the assumption of 1/v and 1/E spectra respectively, then thermal, resonance and fast neutron flux are achieved to be 2.30×10 6 , 2.24×10 6 and 1.04×10 8 cm -2 ·s -1 .