[en] Use of a uranium double spike in analysis of environmental samples showed that a ²³⁵U enrichment of 1% (²³⁵U/²³⁸U = 0.00732) can be distinguished from natural (²³⁵U/²³⁸U = 0.00725). Experiments performed jointly at Los Alamos National Laboratory (LANL) and Oak Ridge National Laboratory (ORNL) used a carefully calibrated double spike of ²³³U and ²³⁶U to obtain much better precision than is possible using conventional analytical techniques. A variety of different sampling media (vegetation and swipes) showed that, provided sufficient care is exercised in choice of sample type, relative standard deviations of less than ± 0.5% can be routinely obtained. This ability, unavailable without use of the double spike, has enormous potential significance in the detection of undeclared nuclear facilities