the fission cross-sections of 235U and 239Pu for Na---Be, La---Be, Na---D and Ga---D photoneutrons have been measured absolutely. The neutron source strength was measured using a manganese bath to compare the photoneutron yield from the sources with the standard source NBS-II. Fission counts were accumulated with the source positioned symmetrically between two identical fission foils in an experiment package suspended in a low-albedo laboratory. Fission fragments passing through limited solid angle apertures were recorded on polyester track-etch films. The masses of the foil deposits were determined by microbalance weighings and confirmed by thermal neutron fission and alpha counting. After making a correction for the calculated energy distribution of the source neutrons, values of 1.471 +/- 0.029, 1.274 +/- 0.026, 1.162 +/- 0.025 and 1.195 +/- 0.026 barns were obtained for the 235U fission cross-section at the source median energies of 140, 265, 770 and 964 keV, respectively. Corresponding values of 1.469 +/- 0.045, 1.515 +/- 0.038, 1.670 +/- 0.039 and 1.643 +/- 0.038 barns were determined for 239Pu.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22757/1/0000312.pd
