You would not want to drink radioactively contaminated water, nor would you want your children to play on radioactively contaminated soil, but it is quite possible that some uranium cannot be detected using current techniques.

For 100 years, scientists have known that uranium emits green-yellow fluorescence upon exposure to UV radiation, enabling its detection. However, water and sediment typically contain other heavy metals and have a neutral pH, which obfuscate the signal because of the formation of colorless uranyl-carbonate complexes. Zheming Wang and colleagues at the Pacific Northwest National Laboratory in Richland, Wash., used a technique that involved freezing the soil and water samples prior to focusing the UV laser beam, revealing uranium that was not previously detected.

At the American Chemical Society’s 232nd annual meeting in San Francisco in September, the researchers reported that they cooled their soil samples to –267 °C, which is near the temperature of liquid helium. They analyzed the water and soil using high-power pulsed lasers and intensified, gated imaging CCD detectors to perform time-resolved laser-induced fluorescence spectroscopy and imaging spectromicroscopy. They found that, at the low temperature, uranium in soil and water samples manifested enhanced spectral resolution and intensity.