LIBS, 2D Spectral Imaging Combine to Improve Isotopic Measurements

Pacific Northwest National Laboratory researchers have improved laser-induced breakdown spectroscopy (LIBS) measurements of hydrogen isotopes in what they characterized as “industrial important” alloys. In optimizing an ultrafast (ultrashort pulse) laser with certain, controlled environmental conditions, the analytical technique revealed in testing where and when emissions from hydrogen isotopes were the strongest.

Among other uses, the technique could enable a faster analysis of materials that have undergone irradiation in nuclear reactor cores, and aid in the development of next-generation materials for hydrogen storage — supporting new energy technologies and the analysis of corrosion when a material is exposed to water.

LIBS involves a pulsed laser generating a plasma on a sample — in this case, hydrogen isotopes. The laser-produced plasma emits light that is characteristic of the different species in the plasma plume. These include nanoparticles, as well as ions, atoms, and electrons.

Using LIBS to detect specific isotopes requires performing measurements of extremely narrow emission spectra of atoms. Because extreme temperatures of laser-produced plasmas (10,000 K or higher) broadens the spectral lines, this is difficult for isotopes of lighter elements, such as hydrogen.

The researchers in the study performed LIBS with different plasma generation conditions, using different lasers to generate plasmas and testing different analysis environments. They collected emitted light at different times after the lasers generated the unique plasmas, and at different distances from the imaged sample. The researchers used spatially and temporally resolved spectral imaging, or 2D spectral imaging.

“2D spectral imaging let us track where and when emission from hydrogen isotopes was the strongest,” said research team leader Sivanandan Harilal. “Because of the multiple species present in a plasma plume and its transient nature, it is critical to analyze plasmas in a spatially and temporally resolved manner.”

Researchers have developed an optimized approach to using laser-induced breakdown spectroscopy (LIBS) for analyzing hydrogen isotopes. They used 2D spectral imaging to track where and when emission from hydrogen isotopes was the strongest. This image shows an example 2D spectral image and changes in emission intensity with different distances from the target. Courtesy of OSA via Sivanandan Harilal, Pacific Northwest National Laboratory.

The team’s work featured zirconium alloys — specifically, Zircaloy-4 — to determine optimal conditions for performing hydrogen isotope measurements. Alloys of zirconium are used throughout nuclear technology, including as cladding for nuclear fuel rods in pressurized water reactors. Collecting measurements of how much hydrogen the material picks up in reactor operation helps determine material performance.

Results ultimately showed that plasmas produced by ultrafast lasers were better for hydrogen isotopic analysis than traditional nanosecond laser-produced plasmas. Further, generating the plasmas in a helium gas environment with moderate pressure provided the best conditions in which to perform their analyses, the researchers said.

“Hydrogen is present in all environments, making it challenging to distinguish the hydrogen that needs to be measured from that in the environment using any analytical technique,” Harilal said. “Our results show that ultrafast LIBS is capable of differentiating hydrogen impurities from solute hydrogen.”

The researchers plan to perform additional studies to further optimize the use of ultrafast lasers for hydrogen isotopic analysis with LIBS.