The Low Energy Cross Sections of Fissile Nuclides

Abstract

A survey has been made of the experimental data for the cross sections and fission parameters of the fissile nuclides: U 233 , U 235 , and Pu 239 for the Neutron Physics Symposium at Rensselaer Polytechnic Institute, May 5-6, 1961. The best available data have been used to construct the most probable behavior of the low energy cross sections which are presented graphically. It is shown that the most probable 2200-meter/sec parameters of U 233 and U 235 form a set of values which are consistent with the results of ratio measurements. The values which are provisionally derived are for U 233 : σ nT = 587 ± 3, σ nn = 12 ± 3, σ nx = 575 ± 4, σ nγ = 49 ± 2, σ nf = 526 ± 4, α = 0.0935 ± 0.0038, ν = 2.502 ± 0.014, and η = 2.288 ± 0.010, and for U 235 : σ nT = 694 ± 4, σ nn = 16 ± 3, σ nx = 678 ± 5, σ nγ = 99 ± 4, σ nf = 579 ± 6, α = 0.172 ± 0.007, ν = 2.434 ± 0.019, and η = 2.077 ± 0.010. It is recommended that a least squares adjustment similar to that made for U 233 by Evans and Fluharty should be made for the U 233 and U 235 parameters as a single set. For Pu 239 the lack of precise determinations of 1 + α preclude the development of a set of parameters in the same manner as done for U 233 and U 235 . The Pu 239 parameters of σ nf , v, and η which can be derived from ratio measurements with U 235 are shown to have an internal inconsistency of 4.3% when combined with the most probable value of σ nx . On the assumption that the measurements of η are most likely discrepant, the most probable values of the 2200-meter/sec parameters for Pu 239 are taken to be: σ nT = 1018 ± 5, σ nn = 10 ± 3, σ nx = 1008 ± 6, σ nγ = 254 ± 11, σ nf = 754 ± 9, α = 0.337 ± 0.017, ν = 2.89 ± 0.05, and η = 2.16 ± 0.05.