A gamma monitoring technique for estimating plutonium contamination around nuclear weapons facilities

Abstract

The objectives of this study were to establish a ratio for241Am to239Pu in soil at the Rocky Flats Plant and to compare241Am concentrations obtained using in-situ and laboratory gamma spectroscopy measurements to concentrations determined with radiochemical analysis and alpha spectroscopy. Soil samples were collected for radiochemical and laboratory gamma spectroscopy analysis from vertical profiles in 3 cm layers to a depth of 21 cm at predetermined locations along transects oriented in the direction of prevailing winds. The origin for the transects was the center of the 903 Pad at the Rocky Flats Plant, which is believed to be the source for most of the241Am and239Pu contamination. A 100 minute in-situ gamma spectroscopy measurement was made at each soil sample location with a portable HPGe detector. Soil samples were dried, passed through a 2 mm sieve, mixed, and split in two fractions. One fraction was analyzed radiochemically for241Am and239Pu and the second was analyzed using laboratory gamma spectroscopy. The median ratio of241Am to239Pu activities, which appears to be independent of soil depth and distance from suspected sources, was 0.17. There is a strong correlation between241Am and239Pu concentrations determined using radiochemical analysis with alpha spectroscopy and concentrations determined with laboratory gamma spectroscopy. Results from in-situ gamma spectroscopy measurements were also correlated with the radiochemical analyses but exhibited greater variability than laboratory measurements. This on-going investigation has demonstrated that it is possible to indirectly measure239Pu concentrations in soil if the ratio of241Am to239Pu can be established. The results indicate that judicious use of a combination of radiochemical analyses with laboratory and in-situ gamma spectroscopy measurements may provide a cost-effective approach for characterization of large sites where241Am and239Pu contamination occur.