Introduction
As a result of increased regulations, DOE orders, and the Laboratory's own commitment to safeguarding the environment, scientists are increasingly being tasked with developing and modifying their techniques and procedures in order to reduce the amount of waste they generate. We are developing an analytical technique, based on optical emission spectroscopy, that promotes this goal of waste minimization and can be used in situ to provide a rapid determination of the elements in actinide-containing materials.
Atomic emission spectroscopy of laser-induced plasmas can be used to detect elements analytically, at ppm concentrations, in harsh or radioactive environments. With this technique a laser beam is focused onto the surface of a sample. A tiny portion of the sample is vaporized, and a high-temperature plasma is formed. Spectral analysis of the plasma light shows the elemental content of the original sample. Because the constituents of the laser-created plasma are not constant in time, time-resolved spectral analysis, obtaining spectra of the plasma only during a specified time period, allows one to discern atomic emissions from ionic emissions. We can further characterize the laser-generated plasma by studying its spatial evolution. The study is accomplished using a gated, two-dimensional detector, which allows the time-resolved spectroscopic image of the plasma plume to be captured. Because the f-elements, such as actinides, have unique physical properties, we expect that the temporal and spatial evolution of f-element plasmas will differ significantly from the temporal and spatial evolution of other elements, such as the transition metals. The goal is to use this technique for the analysis of trace amounts of actinides in a variety of actinide-containing materials.
This particular method of actinide and hazardous material analysis has significant advantages over other, well-established analytical techniques. It is a fairly straight-forward method that requires little or no chemical preparation of the sample, and only a very small amount of material, on the order of nanogram or microgram quantities, is required for analysis. Thus, this technique approaches the ideal from the perspective of waste minimization. Measurements can be carried out rapidly and on-line, thereby avoiding the long lag times associated with some other analytical techniques. In fact, elements can be identified in minutes once a reference database has been established and calibrations have been made. Because the laser beam is focused to a small spot (1mm-50 mm), the technique provides spatial resolution of the sample composition for the evaluation of inhomogeneities. This method can also be adapted for remote analysis, which is particularly useful in radioactive environments.
Although laser-induced plasma spectroscopy has been in use for decades, recent improvements in the instrumentation supporting this technique have been substantial. The pulse-to-pulse stability of the output of current lasers is much better than the output of lasers made only a few years ago, resulting in greater accuracy and reproducibility of analytical results. Detector advancements are also contributing to the improvements in this technique. Data collection can be accomplished using a personal computer, and mathematical software is available for rapid analysis of the data and manipulation of the spectra. Finally, very compact lasers now produce the pulse energy required for this technique. Thus, it is feasible to develop portable instrumentation to detect and analyze actinides and hazardous materials on-site.
Results to Date
Experiments were conducted to study the time-resolved emission spectra and images of laser-generated plasma plumes from a target of gadolinium (Gd). We chose Gd for our experiments because many of its physical properties (e.g., ionization potential, vapor pressure, heat of vaporization, etc.) that are important to the processes of laser ablation and plasma formation and evolution are similar to those of plutonium (Pu). The objective of this work was to determine the initial conditions to be used for future experiments with Pu by optimizing signal detection and element identification by varying laser energy, buffer gas, and buffer gas pressure. The ultimate goal of our research is to optimize this method to detect trace amounts of actinide elements and determine relative actinide concentrations.
Figure 5. A schematic diagram of the experimental setup used for laser-induced plasma spectroscopy. Atomic emission spectroscopy of laser-induced plasmas can be used to analyze small samples on-line for elements at ppm concentrations in harsh or radioactive environments, even remotely.
A schematic of the experimental setup is given in Figure 5. A 0.25-in. Gd rod containing known quantities of trace elements (200 ppm Tb, 50 ppm Zr, 20 ppm Cu, 20 ppm Cr, 10 ppm Ca and 1 ppm-3 ppm Al, Fe, Si, Mn and Mg) was housed in a sample chamber, which was evacuated and backfilled with a flowing buffer gas. The 1.064-mm, 5-ns output of a Nd:YAG laser, having 15 mJ/pulse was focused onto the Gd target by a single lens. The plume, formed at a right angle to the target, was imaged onto the front entrance slit of the spectrometer. The results of single-shot spectra and images taken of the plasma plume show that both temporal and spatial evolution are highly dependent on buffer gas and buffer gas pressure. Ar rather than He enhances atomic emissions in the red wavelength range. Three different buffer gases (Ar, He, and N) at three different pressures (100, 300, and 500 torr) were compared. The intensity of the atomic Gd emissions in He was over a factor of 30 less than the intensity observed in Ar. Although the intensity of the Gd emissions in Ar is greater than the intensity in N at all three pressures, at 500 torr the intensity in both gases is essentially the same. Because air is composed primarily of nitrogen and because 500 torr is approaching atmospheric pressure (at Los Alamos), it is reasonable to suggest that these experiments can be performed successfully in air with no special control of the pressure.
We also wanted to determine whether any of the trace elements present in the Gd rod could be detected. Figure 6 is a spectrum obtained using a 5 µs integration time 4 µs after the laser hit the target. All of the known Gd lines published in the National Bureau of Standards (now National Institute of Standards and Technology) wavelength tables in the wavelength range of 792 nm to 812 nm were observed, as indicated in the figure. Unknown peaks above 2000 most likely represent Gd emissions. Both strong Cu I emissions that occur in this wavelength range were observed, suggesting detection limits of Cu of at least 20 ppm.
Figure 6. A spectrum obtained using a 5-µs integration time 4 µs after the laser hit the target. All of the known Gd lines were observed. The ultimate goal of the research is to optimize this method to detect trace amounts of actinide elements and determine relative actinide concentrations.
Future Directions
We are currently in the process of setting up our laser spectroscopy system inside the Plutonium Facility of Building PF-4. We will begin characterizing actinide materials by their atomic emissions from laser-induced plasmas in the near future, and emphasis will be on detecting trace amounts of actinide elements. In addition, we are developing a technique based on laser-induced fluorescence of laser-created plasmas that uses a second, tunable solid-state laser to determine individual isotopes.
Pamela K. Benicewicz of NMT-6 is the principal investigator of this project. Additional contributors to this work include Thomas K. Gamble of CST-1 and Vicente D. Sandoval, James T. McFarlan, and D. Kirk Veirs of NMT-6.
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