NUCLEAR DETECTIONSimultaneous Detection of Uranium Isotopes, Fluorine Advances Nuclear Nonproliferation Monitoring
Fluorine is essential for converting uranium into a form suitable for enrichment, so spotting both elements together may help inspectors determine the intended use of a nuclear material.
Combining two techniques, analytical chemists at the Department of Energy’s Oak Ridge National Laboratory became the first to detect fluorine and different isotopes of uranium in a single particle at the same time. Because fluorine is essential for converting uranium into a form suitable for enrichment, spotting both elements together may help inspectors of the International Atomic Energy Agency, or IAEA, determine the intended use of a nuclear material.
The findings, published in the Journal of the American Chemical Society, push the limit of how fast single particles can be characterized in terms of their chemical, elemental and isotopic compositions. Critical for understanding chemical processes and dating materials, isotopes are different forms of a chemical element having the same number of protons but a different number of neutrons.
“Determining isotopic ratios on single particles takes a lot of time,” said ORNL’s Benjamin Manard, who led the study. “Rapid particle analysis for fluorine and uranium isotopic determination is what we’ve enabled.” His team combined two techniques to analyze 40 particles — each about the size of a red blood cell — in less than five minutes.
The first technique is laser-induced breakdown spectroscopy, or LIBS. It quickly spots the element fluorine with great sensitivity. “LIBS vaporizes a sample, like a uranyl fluoride particle, breaking it down and forming a plasma, or cloud of excited ions. As the plasma cools, light is emitted,” said ORNL’s Hunter Andrews, the study’s LIBS task lead. Spectroscopy then measures the light to characterize elements in the plasma. “It’s like fireworks,” Manard said. “Different elements emit different colors, or wavelengths.”
Simultaneously, helium gas sweeps atoms of the plasma into a mass spectrometer, where isotopes of uranium are characterized via the second technique, called laser ablation multicollector inductively coupled plasma mass spectrometry, or ICP-MS. Inductively coupled means radio-frequency energy heats the plasma. It reaches 8,000 kelvin — hotter than the surface of the sun.
“LIBS tells us if and how much fluorine is in the particle, while ICP-MS tells us all of the uranium isotopes present,” Manard said. “This integrated equipment is a one-stop shop to measure fluorine as well as uranium isotopes at the same time.”