Laser Detection of Actinides and Other Elements

New technique measures uranium, thorium, and palladium with efficiencies up to 500 times greater than current standard.

Image courtesy of Yuan Liu, ORNL
Three Ti:Sapphire laser system used for three-step resonance ionization of Uranium (U), Thorium (Th), and Paladium (Pd). This system can provide resonance ionization of many atomic species in the Periodic Table.

The Science

Resonance laser ionization has become the tool of choice for a broad range of applications including generation of isotopically pure radioactive ion beams for experimental nuclear research, studies of rare isotopes by laser spectroscopy, and detection of trace impurities and isotopes. Researchers achieved ionization efficiencies up to 500 times higher than previously attained for Uranium, Thorium, and Palladium—actinides which have critical applications in nuclear energy and security—using a combination of three tunable Ti:Sapphire lasers.

The Impact

The dramatic increases in ionization efficiencies of these elements, which were made possible with RILIS, enable a variety of new studies relevant to nuclear fuels cycles, neutrino detection, isotope production, and other topics in nuclear science.


Nuclear Physics experiments with high-quality beams of radioactive nuclei can advance our understanding of the structure of and reactions between nuclei, the creation of nuclei in cosmic explosions, and the scientific foundations for innovative applications of nuclear science in medicine, environmental protection, energy, and national security. This research involves beams of unstable nuclei, which are often produced with low currents and include overwhelming contaminants. Purification of these beams is thus absolutely critical. Resonance laser ionization has become an essential tool for beam purification. The ionization is highly selective because the sequential multi-step process that results in the ionization of the target requires a unique combination of laser wavelengths for each chemical element. At Oak Ridge National Laboratory (ORNL), a resonance ionization laser ion source (RILIS) based on multi-step resonance ionization has been developed. The RILIS uses three tunable Ti:Sapphire lasers and a hot-cavity ion source. In the ion source, atomic species are selectively ionized by laser radiation via stepwise atomic resonant excitations. This ionization process requires a unique ionization scheme for each individual atomic species and is thus highly selective. Since there are very limited spectroscopic data on atomic excitations and ionizations for RILIS applications, developing efficient excitation and ionization schemes for each element of interest is crucial. Extensive spectroscopic studies have been conducted at ORNL, in collaboration with the Laser Resonance Ionization Spectroscopy for Selective trace Analysis (LARISSA) group at University of Mainz, for the development of the highest possible ionization efficiencies for a wide range of elements of interest. Recently, we studied resonance ionization of U, Th, and Pa and determined efficient three-step ionization schemes for these elements yielding overall ionization efficiencies of 18%, 40%, and 60% for U, Th, and Pd, respectively. These results are significantly higher than the prior best values of 0.04% for U and 0.6% for Th. Development of new ionization schemes for other important actinides is underway.

This new technique will enable a variety of new research, including: improving composition estimates of nuclear fuel rods in the U-Pu cycle, significantly enhancing the detection limits of U and Th isotopes in post-explosion debris for nuclear forensics,  refining searches for U and Th impurities in materials used for ultra-low background neutrino detectors, enabling the production of intense beams of unstable actinides for forefront nuclear science studies, increasing the sensitivity of laser spectroscopic identification of the predicted lowest-energy exotic nuclear isomer 229mTh, and improving the precision of rare neutron-rich Pd isotope mass measurements to gain new insight into the creation of the heaviest elements in the universe.


Yuan Liu
Physics Division, Oak Ridge National Laboratory


This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, the Laboratory Directed Research and Development Program at Oak Ridge National Laboratory, and the German Bundesministerium für Bildung und Forschung under Grant 06MZ215.


Y. Liu, et al., “On-Line Commissioning of HRIBF Resonance Ionization Laser Ion Source.” Nucl. Instrum. Methods Phys. Res. B 298 (2013) 5-12. [DOI:10.1016/j.nimb.2012.12.041]

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Program: NP

Performer: DOE Laboratory

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