Absolute measurements of the fast neutron capture cross -section of uranium-238.

Abstract

The radiative capture cross sections of $\sp{238}$U for antimony-beryllium and sodium-beryllium photoneutrons have been absolutely measured. Since the corrections to the cross section values were only few percent, this work is considered as a direct measurement independent of any other measured cross sections. The reaction rates were determined by $\gamma$-ray counting of the $\sp{239}$Np decay activity using a germanium detector. The efficiency of the $\gamma$-ray detector was not needed to be explicitly determined because an $\sp{243}$Am standard source was employed in a relative-measurement scheme. The absolute $\alpha$ activity of the americium source was measured independently using a low-geometry $\alpha$-spectrometer. The photoneutron source emission rates were determined by intercomparison with a californium spontaneous fission neutron source in The University of Michigan Manganese Bath. The Californium source, as a secondary standard source, was itself calibrated against the national neutron standard NBS-I. Pure uranium metal sheets, wrapped into cylindrical shape, were used as the targets. The averaged neutron flux was calculated from the geometry of the source-target arrangement. Chemical separation was developed in this work in order to isolate the trace amount of the reaction product $\sp{239}$Np from the massive uranium target. The separation yield of $\sp{239}$Np was measured by isotopic tracing with $\sp{238}$Np. Several correction factors were applied to the measurements. The correction for the room-returned neutrons was made relative to the measurements of gold foil activations at various distances. A Monte Carlo computer program was specifically written for the correction of neutron scattering with the components of irradiation structure. The experimental cross section results were normalized to the primary centroid neutron energy. The absolute cross sections obtained for the $\sp{238}$U(n,$\gamma$)$\sp{239}$U reaction were 494 $\pm$ 11 and 138 $\pm$ 5 millibarns at 23 and 967 keV, respectively.