Determination of plutonium mass using gamma-ray spectrometry

Plutonium sources with well-known isotopic ratios are routinely used in the calibration and testing of radioactive waste assay systems. The laboratories that supply these isotopic standards often do not quantify the plutonium mass. Knowledge of total plutonium mass in the sources is important for verifying the performance of passive neutron counters. The independent determination of total plutonium mass by gamma-ray techniques is rendered difficult by the lack of adequate information regarding the sample matrix and containment of the sources. Therefore, a technique that did not require knowledge of the sample matrix or containment was used to determine the plutonium masses. The technique was based on empirically extrapolating the apparent mass as a function of photon energy to conditions approximating zero photon attenuation. It was tested using measurements performed with three certified reference materials supplied by the New Brunswick Laboratory (NBL). Results were compared to those obtained from a high-efficiency neutron counter calibrated using ²⁵²Cf. The weighted average of the ratio of gamma ray to HENC plutonium masses was determined to be 0.942±0.029.