Low Flux Measurements of ²³⁹Pu and ²³⁵U Capture-to-Fission Ratios in a Fast Reactor Spectrum

The effective capture-to-fission ratio has been measured for ²³⁹Pu and ²³⁵U in a low flux fast-reactor spectrum by the reactivity-reaction-rate technique. A ²⁵²Cf source and a ⁶Li absorber were used to measure the relative importance of fission and absorbed neutrons, respectively. The measurements were made in Assembly 24 of ZPR-9 which was designed to produce a neutron spectrum that emphasized fission and capture events in the 0.1 to 25 keV range. For ²³⁹Pu and ²³⁵U the measured values, corrected to infinite sample dilution, were 0.516 ± 0.040 and 0.337 ± 0.029, respectively. Corresponding integral alpha values calculated from ENDF/B were 0.352 and 0.346. The experimental value for ²³⁹ is in reasonable agreement with calculations based on recent differential data obtained by both the Oak Ridge-Rensselaer and the United Kingdom groups. These calculations yielded ²³⁹Pu alpha values of 0.466 (ORNL-RPI) and 0.471 (UK).