Laser-induced breakdown spectroscopy (LIBS) is useful for measuring hydrogen isotopes. It requires no preparation of samples and a simple experimental setup can rapidly capture data. However, it is challenging to quantify the concentration of hydrogen with this technique.
Recently, researchers at the Pacific Northwest National Laboratory (PNNL) have developed an optimized approach for the use of LIBS for analyzing hydrogen isotopes. This new research could enable rapid identification and measurement of hydrogen and other light isotopes that are important in analysis of nuclear reactor materials and other applications.
The PNNL team published the results of their work in the journal Optics Express. The researchers found that the combination of an ultrafast laser generating ultrafast pulses with certain environmental conditions helps improve the LIBS measurements of hydrogen isotopes in alloys that have important industrial uses. This new optimization technique could permit more rapid analysis of materials that have been irradiated in the cores of nuclear reactors.
PNNL research team leader Sivanandan S. Harilal said, “Improved chemical imaging of hydrogen isotopes, like what we performed in this work, can be used to monitor the behavior of materials in nuclear reactors that provide us with electricity. It can also be very valuable for the development of next generation materials for hydrogen storage that can enable new energy technologies and for analyzing material corrosion when exposed to water.”
In the new PNNL research, the team worked to find the best conditions for the measurement of hydrogen isotopes in Zircaloy-4. Zirconium alloys are widely used in nuclear technology. One application is the cladding for nuclear fuel rods in pressurized water reactors. In order to understand the performance of the alloys, it is important to measure how much hydrogen is absorbed by the alloys during the operation of nuclear reactors.
In order to perform LIBS, a pulsed laser is employed to generate a plasma on the surface of the sample. The plasma produced by the laser pulse emits light that characteristic of the different elements and isotopes in the plasma plume including ions, atoms, electrons and nanoparticles.
Using LIBS for identifying specific isotopes requires measuring extremely narrow emission spectra of atoms. This is very difficult for isotopes of lighter atoms such as hydrogen because the extreme temperatures of ten thousand degrees Kelvin or higher of laser-produced plasmas broadens the spectral lines.
In the PNNL research project, the team carried out LIBS with different plasma generation conditions by using a variety of lasers to generate the plasma and testing different analysis environments. They collected the emitted light at different times after the generation of the plasma and at difference distances from the sample using spatially and temporally resolved spectral imaging or 2-D spectral imaging.
Harilal said, “2-D spectral imaging let us track where and when emission from hydrogen isotopes was the strongest. 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.”
The results of the PNNL research showed that plasmas produced by ultrafast lasers were better for the analysis of hydrogen isotopes than traditional nanosecond laser-produced plasmas. Generating plasmas in a helium gas environment under moderate pressures provided the best analytical conditions.
Harilal 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. Our results show that ultrafast LIBS is capable of differentiating hydrogen impurities from solute hydrogen.”
The PNNL team plans to perform additional research to further optimize the use of ultrafast lasers for hydrogen isotopic analysis with LIBS.