Measurement of Fissions Produced in Bulk Plutonium-239 by 2-eV to 10-keV Neutrons

Abstract

This experiment was conducted to obtain data to be used in calculating the number of fissions produced by neutrons in bulk 239Pu as a function of neutron energy. The data provide a consistent set of group-averaged cross sections and self-shielding factors. Although self-shielding factors have been calculated from cross-section data, no previous experiments to measure the energy dependence of 239 Pu self shielding exist. A consistent set of cross sections is possible because of the wide neutron energy range over which this experiment was done. No attempt was made to determine resonance parameters, since in this experiment poor energy resolution was used to improve statistics. (Resonance parameters are, in fact, unnecessary to determine group-averaged cross sections and room-temperature self-shielding factors.) Good-geometry self-shielding factors were measured by a plutonium fission counter shielded by various thicknesses of plutonium. Average fission cross sections, total cross sections, and self-shielding factors have been determined in 11 energy groups whose end points are in the ratio of 2.15-to-1. The energy range was 2.15 eV to 10 keV. The LRL Linac neutron time-of-flight facility was used, with a neutron resolution of 0.18 μsec/m. The detector consisted of a spark chamber that was sensitive to fission fragments, facing a 0.4 mg/cm2 plutonium-239 foil. Seven Pu absorber foils ranging from 0.06 to 3 g/cm2 were used in the self-shielding measurements. This range of absorber thickness yields an adequate description of the resonance-produced surface-absorption effect throughout the above energy region.