The State-of-the-Art of Thermal Neutron Multiplicity Counting

Neutron multiplicity counting is a maturing technology. It has been implemented at many facilities to address the increasing need to rapidly measure impure plutonium bearing materials. At Hanford Site and Rocky Flats Environmental Technology Site, multiplicity counting has also been used with excellent results by the International Atomic Energy Agency to verify excess plutonium inventories now under their safeguards. Neutron multiplicity counting as currently implemented, however, will not address all forms of impure plutonium. Materials containing large concentrations of matrix elements like fluorine and beryllium cannot be assayed successfully without extremely long count times. Assays of compact plutonium metals and oxides having a large uranium concentration relative to their plutonium content tend to bias low because of to a breakdown in the theoretical model now used to translate the measured multiplicity distributions to plutonium mass. In this paper, we will discuss the most recent efforts to extend the range of materials that can be measured successfully with thermal neutron multiplicity counting and a use of multiplicity counting to detect sample changes during long-term storage.