Transmission Measurement Correction for Self-Attenuation in Gamma-Ray Assays of Special Nuclear Materials

During the past several years, nondestructive assay (NDA) methods have become increasingly important in the measurement of special nuclear materials (SNM). Both active and passive methods have been vigorously exploited with much success in diverse applications, but among the various methods, passive gamma-ray assay is probably the most used at this time. Many applications of passive gamma-ray assay have been made in quality and process control, portal monitoring, inventory verification, waste management, and environmental monitoring. Frequently in such applications the self-attenuation of the naturally emitted gamma rays by the sample itself is significant and cannot be ignored. In fact, the central (and most difficult) problem in the NDA of bulk samples by passive gamma-ray spectroscopy is the correction of the sample self-attenuation. The difficulty arises from the rather low gamma-ray energies (typically 100 keV to 400 keV), the high mass attenuation coefficients of high-Z elements, the often unknown chemical composition of the sample, and sample volumes extending up to many liters. Quite often, in fact, samples are encountered for which passive gamma-ray assay for plutonium and uranium is impossible because the very large self-attenuation corrections required cannot be accurately made. Nevertheless, there are many cases and classes of material for which a careful passive gamma-ray assay is the most precise and accurate and the cheapest method for SNM determinations.