Laser-induced Breakdown Spectroscopy with Improved Detection Sensitivity and Isotopic Discrimination

• Main Author: Hartig, Kyle C
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2016
• Subjects: Nuclear engineering
• ISBN: 0438770382; 9780438770386

Laser-induced breakdown spectroscopy (LIBS) could be used to perform rapid, real-time, elemental, and isotopic analysis of materials relevant to nuclear forensics, safeguards, and counterproliferation. The detection sensitivity of LIBS is element dependent and the limits of detection (LODs) are higher than competing techniques used in nuclear forensics like secondary ionization mass-spectroscopy or gamma spectroscopy. LIBS non-invasiveness, small sample size requirements, and a need for minimal or no sample preparation, are the main attributes and moderately responsible for the challenges encountered when identifying and quantifying elements with this technique. Secondly, interference of the individual spectral lines in the plasma emission spectrum limit the LIBS detection selectivity and reliability. Finally, the matrix effects also limit the performance of LIBS since the emission intensity from the target element depends on not only the concentration, but also the chemical and physical properties of the matrix. In this dissertation, optimization of the characteristics of the laser pulse have been pursued and new analysis techniques were introduced to improve LIBS detection sensitivity and isotope selectivity. Optimization of the laser pulse shape through simple and more complex pulse shaping techniques was shown to influence the overall LIBS emission intensity, signal-to-background, and the ionization state of the plasma. Similarly, 2.05 μm femtosecond (fs) laser pulses were observed to produce a plasma of lower temperature and density compared to 800 nm fs laser pulses, resulting in an increased signal-to-background ratio (SBR). Ab initio modeling was applied to show that the reduced plasma temperature and density for longer wavelength laser pulses leads to the increased SBR. As an extension of LIBS, laser ablation molecular isotopic spectrometry (LAMIS) was adapted for use with fs-laser pulses. Expansion of the LAMIS technique to isotopic analysis of uranium using fs-laser pulses and the capability to perform this analysis at remote distances through the combination of fs-filamentation LAMIS (F2–LAMIS) was demonstrated. The molecular isotope shift for the UO emission band at 593.57 nm was measured to be ∼ 0:05 ± 0:007 nm, which is twice as large as the largest known atomic/ionic isotope shift of 0.025 nm for the 424.43 nm U II emission line. Spatio-temporally resolved spectral and shadowgraphic measurements showed that the UO species expanded at a slower rate compared to the uranium atomic species. The bulk matrix composition of the sample was observed to affect the measured LIBS and LAMIS signal, and resulted in a reduced analytic capability of the technique for both elemental and isotopic measurement. In specific examples, the measured LIBS emission intensity for an analyte of interest was observed to increase with the presence of a second element of greater concentration, which is the inverse of what is expected for many other analytical techniques. The increase in the analyte emission intensity was studied via temporally resolved plasma and emission diagnostics and through the use of ab initio modeling. The results of this work provide new insights that will aid the development of LIBS/LAMIS as a technique capable of not just elemental analysis, but also isotopic analysis at standoff distances in real time, which is important to remote sensing applications in nuclear forensics, safeguards, and counterproliferation.