By Saren Johnston
Like many of us, David Baldwin knows the old phrase "good things come in small packages." Now you may agree or disagree with that philosophy, depending on your frame of reference. But Baldwin, an Ames Lab associate chemist, has taken the familiar adage to heart and to the workbench to develop a compact optical spectrometer that has the resolving power of spectrometer systems several times its size.
Baldwin was looking for a way to facilitate on-site analysis and identification of metals and isotopes of actinides, radioactive elements such as uranium and plutonium, which can be present in DOE waste sites. This characterization process is almost always large-scale, lengthy and costly. So Baldwin decided to tackle this huge problem by shrinking the instrumentation required to do the job. What he came up with was a high-resolution optical spectrometer that is portable and sturdy enough to operate in the field.
An isotope is one of two or more atoms having the same atomic number but a different mass number, that is the same number of protons in the nucleus but different numbers of neutrons. This condition can make it difficult to tell some actinide isotopes apart. Even the stalwart Sergeant Joe Friday or the relentless Lieutenant Columbo would be hard pressed to find a clue that would allow them to distinguish between apparently similar isotopes of different actinide elements. So scientists must be more deft and, yes, even more persistent than either the faithful Friday of the clever Columbo in their search for necessary but elusive evidence. They must look for either differences in spectral lines, the wavelengths isotopes emit in the visible spectrum, or in the masses of ionized atoms. Spectroscopic inductively coupled plasma (ICP) analysis, in which an electrically generated high-temperature plasma is used to excite the atoms of the sample being analyzed, is the method of choice and has the sensitivity required for these applications.
Traditionally, ICP analysis has not been suited for field applications because the optical and mass spectrometers used to analyze the excited atoms and ions are far too large for on-site analysis. Commercially available ICP mass spectrometry (ICPMS) systems are able to resolve, or discriminate, isotopes of a single element, but are not able to resolve isobars, isotopes of different elements having the same atomic mass number. And although ICP atomic emission spectrometry (ICPAES) systems can resolve isotopes and isobars, there are no commercial systems with spectrometers large enough to do the job, which requires a high-resolution, grating-based optical spectrometer with a focal length of at least 1.5 meters.
A well-established analytical technique developed at Ames Laboratory, ICPAES makes possible simultaneous multielement analysis. The powerful optical spectrometers used in this technique are capable of distinguishing the fine structure of the numerous spectral lines that would otherwise resemble a smear of color, much as objects at a distance appear to blend together for people with uncorrected nearsighted vision. The ability to see the spectral lines in detail is crucial to more accurate, faster and safer analysis and remediation of contaminated sites.
But there's a down side. Because of their huge size, these ICPAES systems, to put it bluntly, are fairly useless outside of a laboratory environment. Unlike Baldwin's new, compact optical spectrometer, these beasts are expensive and cumbersome instruments, far too large for on-site field analysis.
Baldwin knew that to track down isotopes of actinide elements in the field would require finding some way to get the resolving power of a large optical spectrometer into a much smaller package. The solution came in the form of two devices developed primarily for the communications industry. "I started getting interested in solid-state acousto-optic tunable filters (AOTF)," says Baldwin. "An AOTF is a very small solid-state crystal with a radio-frequency driver attached to it so that when you put the right frequency into the acoustic driver, it sets up a little grating, or a little wave, inside the crystal. If you shine a light through the crystal, a certain color will be diffracted out of the beam, or sent to the side. The nice thing about the AOTF is that you can switch radio frequencies very quickly, and that allows you to jump all over the spectrum. People are starting to realize that this can be very useful," he adds.
The AOTF's capability as a prefilter to rapidly select a narrow band of radiation from a polychromatic beam was a plus in Baldwin's effort to develop a field-operable analytical instrument. But although he says he and Art D'Silva, a Lab associate and coworker, thought the AOTFs were "pretty neat devices," they knew their resolution was not sufficient to do a lot of emission work, especially in soils containing a complex mix of elements. "The spectral lines are too close together to resolve them with just the AOTF," Baldwin explains. "So we were thinking, thinking, thinking, 'How can we use this device and still be able to do what we want to do, which is resolve lines?'"
The idea Baldwin came up with was to replace the long-pathlength grating spectrometer. As an alternate, they would introduce the output, or the selected wavelength region, of the AOTF into a fiber-optic Fabry-Perot (FFP) interferometer. "Fabry-Perots have been around since before they knew light was particles," says Baldwin. "These are tunable, inherently high-resolution optical devices, so we said, 'Well, let's use the AOTF with an FFP.'"
Baldwin notes that during the patent application process, it was discovered that the combination of an AOTF and a Fabry-Perot (FP) interferometer for high-resolution optical filtering had been patented over 20 years ago, but the device saw only limited use and was not reported in the analytical literature. "We can't have a patent on it, but the technology is still usable," he says. He also notes that few applications have been reported even though the AOTF-FFP system is compact and thermally and vibrationally stable, ideal characteristics for his effort to move the power of high-resolution optical spectrometry out of the lab and into the field. In addition, the system can be used in wavelength ranges from the infrared to the ultraviolet.
A Fabry-Perot interferometer operates on the principle of constructive and destructive interference of light within a linear cavity defined by mirrored coatings on the surfaces of air-spaced optical flats. When the spacing of these surfaces is an integral, or a whole number, of wavelengths, the FP transmits light. However, if the spacing of the cavity surfaces is nonintegral, or some fraction of a wavelength, light is reflected back toward the source. Electronically driven components called piezoelectric crystals can change the spacing between the mirrored surfaces of the cavity according to the voltage applied and so tune the wavelength of the interferometer. By scanning the cavity spacing, scientists can obtain a spectrum from which they can discern isotopic composition.
There are many suppliers of fiber-optic-coupled Fabry-Perots, but Baldwin and D'Silva happened to see a magazine ad for an FFP interferometer produced by Micron Optics, which caught their attention. "It advertised resolution capabilities that were much superior to other Fabry-Perots," says Baldwin. The Micron Optics FFP consists of a pair of single-mode fibers (fibers in which light can travel in either direction but on only one path). The cavity mirrors of the FFP are coated directly on the fiber faces. The single-mode optical fiber does not allow incoming light to form other modes in the cavity, which enables the Micron Optics FFP to produce a spectrum with much higher resolution than can be achieved with conventional FPs.
So Baldwin had his high-resolution FFP, but the problem was that Micron Optics had developed it for diode-laser wavelengths -- at 850, 1350 and 1500 nanometers. "They had made their FFP for multiplexing communications, meaning that the finer the resolution you can make on wavelength selection, the more conversations you can fit into a narrow bandwidth of light frequency," he explains. "We wanted it to work where there was strong emission in an ICP from uranium and plutonium, which is around 400 to 420 nanometers, in the blue region of the electromagnetic spectrum."
Micron Optics agreed to work on developing an FFP that would meet Baldwin's needs, but it took several attempts before they achieved success. "Blue emission isn't as interesting to some scientists as to others," says Baldwin. "And we were working with communications people who cared more about the red and not so much about the blue. It turned out there were very few test sources in the blue for these optics manufacturers, so we did a lot of 'Send it to us, and we'll figure out whether it works or not.'"
Eventually, Baldwin and Dan Zamzow, assistant chemist and coworker, finished a bench-scale prototype of the AOTF-FFP. "It was on a breadboard in a big black box, much bigger than it needed to be," says Baldwin with a smile. "But everything worked. It worked well. We now have our next-level engineering prototype sitting beside the first one, and you can see the difference in scale. The sensitivity and resolution are comparable to the 1.5-meter spectrometer, but they come in a much smaller, much more versatile package."
With the success of the bench scale and engineering prototypes came the need for a shorter, catchier and more marketable name. The AOTF-FFP has become the HiRIS, for high-resolution interferometric spectrometer, and it's ready to take on a case.
Baldwin has close ties with Kevin Carney who works in the engineering-division program development area at Argonne National Lab (ANL) West in Idaho Falls, ID. Carney has been collaborating on projects related to mixed waste with the Diagnostics and Instrumentation Analysis Laboratory (DIAL) at Mississippi State University in Starkville. About a year ago, he funded Baldwin to do some work for the project. "We worked with Kevin on some air plasma ICP for emission monitoring," says Baldwin. "In addition, DIAL has one of the many Laser-Induced Breakdown Spectroscopy (LIBS) programs." Baldwin explains that in LIBS a laser spark is used to generate emission from a sample, rather than transferring the sample to an ICP. "Kevin is interested in having us replace their grating-based spectrometer system for LIBS with the HiRIS system for analyzing uranium and plutonium in a thermal treatment system for mixed waste. They have a thermal treatment testbed at Mississippi State," Baldwin adds.
But Carney's interest in the HiRIS is not limited to the DIAL project. He also wants to bring the new system to ANL-West. "They're actually using a 1.5- or 3-meter monochromator to look at uranium and plutonium in a hot cell," says Baldwin, "and Kevin wants us to come out there and show him how well the HiRIS works." A hot cell is a heavily shielded compartment.
Another application is to measure the efficiency with with new nuclear fuels "burn up." Argonne West is involved in the development of advanced metallic nuclear fuels that "burn" more efficiently than the fuels in commercial nuclear reactors. The efficiency of fuel burn-up can be determined by monitoring levels of fission products, such as neodymium. "Argonne West is currently utilizing a high-resolution grating spectrometer for the burn-up determinations of irradiated metallic fuel from our experimental breeder reactor," says Carney. "High-resolution ICP spectrometry allows one to measure neodymium concentrations in dissolver fuel solutions," Carney continues. "The 1.5-meter spectrometer has a fairly large footprint, and space is an expensive commodity at ANL-West. In addition, optical alignment using the larger spectrometers is cumbersome and tedious. The high-resolution FFP will downsize and simplify the detection aspect of these measurements. If this system performs as expected, it will simplify our analysis tremendously."
"The real beauty of HiRIS," Baldwin summarizes, "is making use of all of the capabilities of the AOTF for rapid selection of wavelength, solid-state tuning and stability -- making use of all of those and then correcting for the fact that it just doesn't have the resolution to do a lot of things by adding the interferometer. When you add the FFP, you gain the resolution needed to make it a much better and more useful device."
In spite of the promise the HiRIS offers in terms of on-site analysis for isotopes of actinide elements, Baldwin knows it will take a lot of time, hard work and a few good breaks to make the device a commercial success. "It's difficult to be successful because the HiRIS is designed for very specialized problems that aren't going to sell that many units," he says. "And without the continuous DOE funding to push something you've developed into the field, you need customers to pull you out."
So Baldwin, like a good detective, plays a waiting game -- waiting for renewed DOE funding, for more opportunities to demonstrate his novel device, and for anyone who may need the HiRIS to see isotopes in their "true colors."
For more information:
David Baldwin, (515) 294-2069, dbaldwin@ameslab.gov
Current research funded by:
Environmental Management Office, DOE
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Last revision: 4/17/98 sd
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