Absolute Neutron-Flux Measurement at Fast Pulse Reactors with Calibration against Californium-252

A new flux-transfer technique has been used to measure the total neutron flux to an accuracy of 5% at the Army Pulse Radiation Facility (APRF) fast pulse reactor. Use is made of 239Pu-loaded fission chambers developed by the National Bureau of Standards (NBS) and calibrated against a 252Cf source at NBS. Advantages of the method are that it uses a standard source for calibration, that it is independent of errors in fission foil masses and that the effects of cross-section errors are minimized by the need to use only cross-section ratios. Therefore, the method can serve as a calibration procedure that is readily amenable to interlaboratory comparison. The results show that previous dosimetry is 18% too high in the APRF in-core irradiation facilities, and 21% too high at core surface. Flux calibrations have also been made by using 237Np, several uranium isotopes, and the 32S(n,p)32P reaction. Fluxes from all measurements agree to 7%. The (>10 keV/>3 Mev) flux ratios determined for the glory holes and leakage spectra are 8.4 ± 0.6 and 7.3 ± 0.5, respectively.