Assay of small fissile masses in waste by the active neutron correlation technique

The Joint Research Centre recently obtained a license to operate a new experimental device intended for research in the field of nuclear safeguards. One of the research projects currently being planned for the new device concerns the mass determination of small amounts of fissile materials by means of the active neutron correlation technique. The device incorporates a commercial pulsed neutron generator and a large graphite mantle surrounding the sample cavity. In this configuration a relatively high thermal neutron flux with a long lifetime is achieved inside the sample cavity. By pulsing the neutron generator, a sample may be interrogated by a pure thermal neutron flux during repeated time periods. The neutrons emitted from the fission events in the sample are partly detected in time intervals in a fashion similar to the standard passive neutron correlation technique (multiplicity counting). The frequency distributions of detected neutrons, or rather the moments hereof, can be expressed as function of the fissile mass, and sample and instrument parameters in analytical equations. The present paper describes the new experimental device, the data acquisition systems and the analysis method currently being investigated for the assay of small fissile quantities. (author)