Improving Neutron Assays of Dilute Surplus Plutonium Using Gamma-Ray Measurements

Neutron coincidence counting has been chosen as the method for assay of diluted surplus plutonium in Criticality Control Overpacks (CCOs) produced at the Savannah River Site. However, some portion of the material consists of scrap oxide containing low-Z elemental impurities. These impurities interact with Pu and Am alpha particles to create neutrons. In large quantities, these (?,n) neutrons can cause erroneous values with standard neutron analysis techniques and, in fact, can even prevent the analysis from succeeding at all. These large uncertainties and biases can result in measurements falling outside the required bounds for both Material Control & Accountability and Waste Acceptance Criteria. Some of the relevant (?,n) reactions provide an associated gamma ray and, if not, sometimes there is a similar reaction (for example, (?,p)) that produces gamma rays that can be detected. A methodology is under development to allow for the estimation of the quantity and impact of (?,n) neutrons on a Pu neutron assay. This makes it possible to calculate alpha (the ratio of (?,n) to Pu fission neutrons) and utilizing a known-alpha analysis, greatly reduces the associated uncertainty and bias of the measurement. Preliminary implementation has shown positive results and elucidated avenues for further development.