The Determination of the Fast-Fission Cross Section of Protactinium-233 with Fission Neutrons

The fast-fission cross section of ²³³Pa has been determined for a fission-neutron spectrum originating from a ²³³U source that was exposed to thermal reactor neutrons. The o_f was measured by comparing the fission rate of a known quantity of ²³³Pa to that of a known thorium sample, using back-to-back fission counters. An average above-threshold value of 775 ± 190 mb has been found, based on a fission cross section of 142 mb for ²³²Th and an assumed threshold of 0.9 MeV for ²³³Pa. This value is in very good agreement with published estimated values. The characteristics of the fission counter in the presence of the high beta background resulting from 0.3 to 1.8 Ci of ²³³Pa were investigated and are presented in the paper.