Measurements of the 14 Mev Fission Cross-Sections for Uranium-235 and Plutonium-239.

Abstract

The fission cross-sections of ('235)U and ('239)Pu have been measured at a neutron energy of 14.63 MeV. A 150 kV accelerator, installed on a low mass floor at the center of a large laboratory, provided a source of D-T neutrons under conditions of low scattering. Fission events were accumulated with the target foils and detectors positioned inside a vacuum chamber and at various distances from the neutron source. Fission fragments fragments passing through a limited solid angle aperture were registered on polyester track-etch films. The masses of the foil deposits were previously determined by microbalance weighings and confirmed by thermal fission and alpha counting. The angular distribution of fission fragments for ('235)U and ('239)Pu were measured in a separate experiment. In this case the fission fragments were detected by a polyester track-etch film which was shaped to cover a 110(DEGREES) arc on a sphere of 5.08 cm radius centered on the deposit. The neutron flux was measured relative to ('56)Fe(n,p)('56)Mn and ('27)Al(n,(alpha))('24)Na reaction cross-sections. ('56)Mn and ('24)Na activities were measured in a 4(pi)(beta) gas flow proportional counter. The absolute efficiencies were separately measured using a 4(pi)(beta)-(gamma) coincidence counting technique. The room scattered flux contribution to the total flux was determined from different source-detector spacing runs. The precise neutron energy was directly measured using a silicon surface barrier detector. The cross-sections obtained for ('235)U(n,f) and ('239)Pu(n,f) reactions were 2.070 (+OR-) .046 and 2.44 (+OR-) .092 respectively and a ('239)Pu(n,f)/('235)U(n,f) cross-section ratio of 1.179 (+OR-) .028.