Gamma-Ray Assay of Plutonium-238 in Waste Drums

Plutonium-238 in contaminated trash and equipment contained in steel drums was determined by measuring, with a Nal(Tl) detector, the intensity of the 765-keV gamma ray from the decay of ²³⁸Pu. By gamma-ray stripping, the contributions of higher energy gamma rays to the 76S-keV gamma-ray photoelectric peak could be subtracted, despite the large amount of scattering material present in the drum. The transmission of the 765-keV gamma ray inside the drum was determined by measuring the transmission through the drum of the 765-keV gamma ray from external ²³⁸Pu standard sources. The uncertainty in the measurements is ±28% at the 95% confidence level when the drum contains ≥0.180 g of ²³⁸Pu.