Measurements and calculations of the Doppler effect on the reactions 238U(n, γ), 235U(n, f) and 239Pu(n, f) with neutrons in the energy range 0–25 keV

Abstract

Spherical samples, 2 cm in diameter, of ²³⁸U, ²³⁵U and ²³⁹Pu were irradiated in turn at various temperatures ranging from 170 to 770°K in a central cavity of a spherically symmetrical SbBe photoneutron source. The ²⁹⁸U(n, γ) reaction rate was measured by counting the ²³⁹U activity produced and the (n, f) reactions were monitored by counting the fission neutrons emitted. The spherical symmetry of the apparatus was chosen to minimize any effects of thermal expansion and to facilitate the comparison of the results obtained with those from a computer calculation based on a programme developed by Brissenden and Durston. In this programme a statistical distribution of resonances based on resolved resonance data is tabulated over a fine energy mesh. This tabulation was used in a 4-region slowing down calculation resulting in a region-dependent neutron spectrum from which the particular reaction rates at various temperatures were obtained.