Researchers at Lawrence Livermore National Laboratory (LLNL) are experts in nuclear forensics: the art and science of extracting information about the provenance and history of nuclear materials. Now, they have a new technique to add to their toolkit.
In a study published in the Journal of Nuclear Materials, LLNL and Lawrence Berkeley National Laboratory scientists described how synchrotron-based scanning transmission X-ray microscopy (STXM) can identify chemical states and material impurities at the scale of individual particles - a resolution never before achieved.
"STXM allows us to see details in nuclear materials that traditional methods simply could not detect," said lead author and LLNL scientist Rachel Lim. "This ability to pinpoint chemical states and impurities of individual particles marks a major advance for nuclear forensics capabilities."
The method uses an X-ray beam - focused down to a pinprick that is only tens of nanometers wide - from a synchrotron to scan across a uranium sample. The characteristics of this beam, generated at the Advanced Light Source, allow the team to achieve that superior resolution.
As the X-rays pass through the sample, detectors measure how many X-rays are absorbed at each point in the material for multiple X-ray energies.
"Because each element has its own unique absorption profile - like a fingerprint - STXM can create detailed images and identify the specific elements and their chemical states in very small regions of the sample," said Lim.
With this technique, the authors identified and quantified the most common uranium oxides.
In a companion paper, published in the Journal of Vacuum Science & Technology A, they extended the approach to plutonium oxides formed in high humidity. In that case, they found a wide variety among individual particles, including phases with iron.
STXM can be used to analyze minute amounts of nuclear materials quickly and safely without damaging the sample. But reference datasets will be required to connect STXM signatures to a material's provenance and history.
"The chemical state and impurity profile of a material act as forensic signatures linking it to its origin, proce.ssing and environmental exposure, but meaningful interpretation requires high-quality reference data," said Lim. "As more reference data becomes available, this approach could become a standard tool for tracing the history and origins of nuclear materials, making it easier to monitor and protect them."
