An experimental measurement of the effective delayed neutron fraction (β-) has been made for a clean critical assembly by determining the asymptotic period associated with introduction of a known amount of reactivity, The "known amount" of reactivity was obtained by replacing, uniformly throughout the reactor, a small quantity of U-235 with an alloy of B10 and Hf designed to match the absorption properties of U-235. The replacement was thus equivalent to a uniform reduction in v，the number of neutrons emitted per fission from the fuel. Such a reduction introduces a reactivity change equal exactly to δv/v0.nTwo analyses of the experiment were made using different high energy cross sections in conjunction with four group, two dimensional diffusion theory. The measured value of β- lay between the results of these computations, the error spread (an average root-means-square error of ± 5.2％) being too great to permit any conclusion regarding the significance of the comparison.
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