Neutron Coincidence Counters for Nuclear Safeguards Applications

Reliable, accurate and rapid nondestructive assay instrumentation is playing an increasingly important role in the practical implementation of nuclear material safeguards, inspection and surveillance as well as in quality control and operational safety in all types of nuclear processing facilities. The neutron coincidence counting technique has been successfully demonstrated for nondestructive assay of samples containing Pu or U. The technique involves detecting neutrons from the spontaneous fission of 240Pu and 238U with polyethylene moderated neutron coincidence counters containing 3He- or BF3-filled detectors surrounding the sample to be assayed. The coincidence counting technique is necessary for reliable assay since total neutron count rate is dependent not only on the amount of Pu in the sample but also on the chemical composition. For example, if the sample contains PuO2 the total neutron production rate would be ~100 n/sec-g; however, if the sample contained PuF4 the total rate would be ~300 times larger than this. The spontaneous fission rate of both samples would be ~25 fiss/sec-g (for 5% 240Pu isotopic composition), independent of chemical composition. By using the coincidence counting technique it is possible to distinguish spontaneous fission neutrons from the randomly produced (¿,n) neutrons. Thus coincidence counting provides a powerful method for rapid and accurate assay of Pu-bearing samples. Recently, the use of fast plastic scintillation detectors has significantly extended the range of usefulness of the coincidence counting technique to materials with extremely high (¿,n) activities.