The Assay of Pu by Neutron Multiplicity Counting Using Periodic and Signal Trigger Methods

For the assay of Plutonium neutron multiplicity counting is used with both periodic and signal triggered multiple observation intervals. The applied triple correlation method has the advantage of providing three independent equations for each method and interval. In addition to the spontaneous fission rate (Fs), one can solve for two other unknowns from the multiplication (M), the detection probability (e) and the (cc,n) reaction rate (Sa). In most practical cases of bulk material and waste one or more of these parameters are well defined and the method constitutes an absolute measurement technique. Measurement results have indicated particular advantages for each observation interval trigger method. Bulk material is easily measured with the signal trigger method. In contrast, the periodic trigger method is consistently better for material with a small Pu mass and a high alpha ratio. This can be understood by comparing the three equations of the factorial moments of the measured neutron multiplicity distributions from the two interval trigger methods. Averaging the assay results over several observation intervals reduces the error of the result for both trigger methods compared with a single time channel multiplicity counter. Consistency between the counts of different observation intervals improves confidence in the measurement result. These conclusions are illustrated with experimental data.