Characterization of Emissions from Metalized Energetic Formulations Using Laser-Induced Breakdown Spectroscopy
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2017 by Dr. Waruna Kulatilaka Laser induced breakdown spectroscopy (LIBS) is an analytical technique that uses intense, laser pulses for detecting primarily the elemental composition of a medium. In general, nanosecond-duration laser pulses on the order of 10-500 mJ, are used to generate a weak plasma, typically less than 10% electrons to other species ratio. Light emitted from this plasma is then collected, sent through a spectrometer, and imaged on to a CCD array. Based on the spectral lines detected and their relative intensities, the elemental composition of the material can be determined and quantified. LIBS is a versatile technique because it can be used on materials in all phases: solid, liquid, and gas. LIBS experiments are relatively easy to set up, robust, and can be used in the field with minimal or no sample preparation. Because of these advantages of LIBS compared to other analytical techniques, it has been widely employed in the defense sector, specifically in the detection of chemical, biological, radiological, nuclear, and explosive (CBRNE) materials. The objective of this work is to investigate LIBS for characterization of emissions from explosive materials, in particular to detect the release of metal additives of aluminum, copper, mercury and lead into the surrounding air. A 10-Hz Nd:YAG laser is used as the source to generate the plasma. Solid propellant strands cured with predetermined quantities of trace metals are used as a source for the flame, and the plasma is generated using a tightly focused laser pulse. Initial analysis of plasma decay was conducted using solid test pieces of each of the elements or their compounds to be able to better understand the decay times of broadband background emissions compared to the specific spectral features corresponding to the species of interest. A laser energy dependence study was also conducted to determine the optimal energy to generate the plasma in order to achieve the best signal-to-noise ratio. Aluminum has been detected in the plume of reacting propellant strands doped with known concentrations, and a general correlation between concentration of metal and LIBS signal has also been found.
