Effect of induced fission on plutonium assay with a neutron coincidence well counter

This report describes an investigationof the neutron- multiplication effects in the Brookhaven neutron well coincidence counter designed for plutoniurn assay.The study was initiated when anomalous results were obtained after calibrating the counter with samples of plutonium having various Pu contents.The data with the cadmium-lined 240 counter indicate that multiplication contributes to the coincident count in three ways: (1) Source size variation from 2.5 to 40 g of 240Pu in 10% 240Pu causes 10-25% of the coincident neutrons tobe from induced fission.(2) Induced fission is strongly influenced by the relative position of samples in the counter well.(3) Neutron moderation in the source pro- duces a 10% increase in gross count and a 4% increase in net coincident count.In extreme cases, if these multiplication effects are not taken into account, the apparent (i.e., measured) pluto- nium content can exceed the gross weight of the sample by more than a factor of two.