Year
2000
Abstract
In the safeguards, arms control, and nonproliferation regimes measurements are required which give the quantity of fissile material in an accounting item, e.g., a standard container of plutonium or uranium oxide. Because of the complexity of modeling the absorption of gamma rays in high-Z materials, gamma-ray spectrometry is not customarily used for this purpose. Gamma-ray measurements can be used to determine the fissile mass when two conditions are met: 1. The material is in a standard container, and 2. The material is finely divided, or a solid item with a reproducible shape. The methodology consists of: A. Measurement of the emitted gamma rays, and B. Measurement of the transmission through the item of the high-energy gamma rays of Co-60 and Th-228. We have demonstrated that items containing nuclear materials possess a characteristic \"fingerprint\" of gamma rays which depends not only on the nuclear properties, but also on the mass, density, shape, etc.. The material's spectrum confirms its integrity, homogeneity, and volume as well. While there is attenuation of radiation from the interior, the residual radiation confirms the homogeneity of the material throughout the volume. Transmission measurements, where the attenuation depends almost entirely on Compton scattering, determine the material mass. With well-characterized standards, this methodology can provide an accurate measure of the contained fissile material.