A method for the direct measurement of relative capture rates in 232Th, 238U containing lattices

A method has been developed for directly measuring the neutron capture rate for 232Th relative to that for 238U, in an experimental reactor lattice containing both these isotopes. The basis for the method is the simultaneous use of 237Np as the long-lived, α-emitting parent of 233Pa (decay of which in an irradiated thorium foil characterizes the 232Th capture rate), and 243Am as the long-lived parent of 239No (which characterizes captures in 238U). Through relative α-counting of the two reference sources and through relative γ-counting of the sources as well as lattice-irradiated thorium, uranium foils, the ratio of the effective capture cross-sections is shown to be directly obtainable. The experimental feasibility of the method was established by determination of the capture cross-section ratio for 232Th, 238U in a thermal-column flux. A mean value of 2.725 +-0.025 (0.5% systematic error, 0.8% statistical) was obtained, in good agreement with the value of 2.71 +- 0.05 deduced from the known, individual thermal cross-sections. (author)