THE SLOW NEUTRON FISSION CROSS SECTIONS OF THE COMMON FISSILE NUCLIDES

The fission cross sections of U/sup 233/, U/aup 235/, Pu/sup 239/, and Pu/sup 241/ were measured, relative to gold, in a Maxwellian neutron spectrum. The techniques employed are entirely conventional, but by careful attention to detail the overall error was reduced to less than one per cent. The masses of fissile material used were determined by alpha counting which was done in the same chamber as the fission counting without disturbing the samples. Two types of measurements were made. In the first, precise ratios of the fission cross sections were obtained. The ratios of the cross sections of Pu/sup 239/ and Pu/ sup 241/ and of U/sup 233/, and U/sup 235/ were obtained independent of alpha half lives by using an isotopes dilution technique. In the other mneasurement, the fission cross section of U/sup 233/ was determined relative to the activation cross section of gold. The absolute activity of the gold foils was measured in a 4 pi gamma scintillation counter by comparison with Au/sup 198/ samples standardized in a 4 pi BETA - gamma coincidence system. (auth)