Year
2002
Abstract
The assay of uranium is possible only with an active interrogation method because it has a negligible passive neutron emission rate. The active interrogation method uses external neutron sources to induce fission events in a uranium sample to determine its mass. Careful calibration of the mass versus the doubles count rate with standards that are representative of the items to be assayed is required for this technique to work well, otherwise large biases are possible. To overcome some of these biases another assay technique, active multiplicity counting, is being developed to reduce the assay bias when nonrepresentative items are assayed. Active multiplicity counting uses the measured doubles and triples count rates to determine both the neutron multiplication (M) and the product of the source-sample coupling (C) and the 235U mass (m). Since the 235U mass appears as the product of Cm, the coupling needs to be determined before the mass can be known. A technique has been developed that relates the coupling to the neutron multiplication. Known standards must be used to determine a scaling constant present in this relationship. Evaluation of nonrepresentative uranium metal and oxide configurations revealed small assay biases using the active multiplicity algorithms. These algorithms will be tested on other forms of uranium to evaluate the types of materials for which this analysis method is valid.