How to Calibrate a Neutron Correlation Counter: Let Us Count the Ways

Passive neutron correlation counting is a commonly applied technique for the nondestructive assay of plutonium-bearing items. In some cases, it may be applied to assay uranium as well. Active neutron correlation counting can be used for uranium, plutonium, and mixtures of both. The dynamic range of samples extends from milligrams to multiple kilograms for both current passive and active assay techniques; these techniques also remain attractive when severe gamma attenuation is present. Coupled with its comparative simplicity, robustness, and stability, neutron correlation counting is well established and heavily relied on for nondestructive assay. For best results, calibration using representative items of known nuclear material content is preferred, but is not always possible. However, to meet a given data quality objective, it is not always necessary. An acceptable calibration can often be achieved using the best practical means and physics arguments with a fraction of the resources of a traditional mass calibration. Field measurements, waste assay, and special investigation of a small number of nonstandard items are prime examples for this strategy. Accepting this philosophy, it is evident that there is not just a single calibration approach, but many. In this paper, we outline several calibration methods for neutron correlation counters that we consider to be technically defensible, depending on the measurement scenario and local availability of materials and expertise. To our knowledge, this is the first attempt to initiate a dialog on this topic.