Sep. 21, 1999 ANN ARBOR --- Gaseous diffusion, a dirty, expensive process which provides relatively pure forms of elements for microelectronics, medical tracers and nuclear fuel, may have met its match.
Using only a tabletop laser and a 1-inch disk of target material, researchers at the University of Michigan College of Engineering have found they can produce relatively pure amounts of materials, sorted by atomic weight, across the entire spectrum of elements. And unlike gaseous diffusion, an industrial-scale process in use since the Manhattan Project, the separation doesn't require huge electro-magnets or leave behind a lot of cross-contaminated byproducts.
Every chemical element, be it oxygen or uranium, naturally occurs with a few slightly different masses, called isotopes. Carbon, for example, is most abundant as an atom with six protons and six neutrons, giving it an atomic weight of 12. But about 1 percent of Carbon has an extra two neutrons and a molecular weight of 14. These special forms of elements, called isotopes, are often very useful for things like medical tracers, specialty materials and nuclear fuel. But sorting something as small as atoms by weight has always been a tremendous technical problem. The U-M breakthrough holds the promise of eliminating most of that difficulty and expense.
The heart of this new system is the terawatt laser, a beam of pulsed laser energy that delivers incredible energies in femtoseconds, mere quadrillionths of a second. "It's like a karate chop," said Gérard Mourou, director of the U-M's Center for Ultrafast Optical Science (CUOS), a National Science Foundation Science and Technology Center. When this burst of concentrated energy strikes a target material, a donut-shaped magnetic field is created as a natural byproduct. As highly excited ions of the target material are blown off (the target), the magnetic field, linear and ring-shaped components, exerts a force that sorts them out by molecular weight. The lightest isotopes of the material end up deposited at the center of a silicon disk about two inches away from the target, while the heavier ones separate out toward the edges, allowing for a relatively simple purification process. [See schematic diagram].
The U-M team happened on this separation effect by accident while studying the plasmas produced by a laser ablating, or scouring away, of material. When they noticed a distinct pattern to the weights of the ions in the plasma, "we thought the instrument wasn't working correctly," said Peter Pronko, a research scientist in CUOS. In a paper to be published in the Sept. 27 edition of Physical Review Letters, the team describes this happy accident as "an unusually efficient isotope enrichment process."
Until now, sorting out the various weights of isotopes in a gas or plasma has always required carefully tuned magnetic fields and centrifuges. That's no longer necessarily the case, Pronko points out. "You don't have to build an apparatus to do the separation; the separation is part of the process." And you don't have to re-tune the fields to sort a different material; you merely change the target.
"The thing I like about this process is that everything can be reduced by a factor of a thousand or ten thousand," Mourou said, noting how much less energy and target material is required to yield a significant quantity of isotopes.
Graduate student Paul VanRompay, whose dissertation relies on the isotope separation experiment, offers a tour of the somewhat cramped room where the experiment occurred. The beam of a titanium sapphire laser travels a convoluted path along a 20-foot table arrayed with all sorts of devices to compress it and boost its power, then it is bounced off a mirror to zip through a tube that leads through the wall and into the next room. At the end of the tube there is an 18-inch vacuum chamber which holds four of the 1-inch plates of the target material on rotating arms. Losing only a 500-angstrom pit with each blast, the target is carefully turned to be ablated over and over in a spiral pattern as the laser delivers ten blasts per second.
The new process, which has a patent pending, might be applied to any material, not just the heavy radioactive elements used for weapons and power plants which constitute most of the material processed by gaseous diffusion plants. The research team has produced enriched boron, gallium, titanium, zinc and copper. Economically important isotopes such as Cesium 137 and Iodine 131, which are essential to medical diagnosis and treatment, may also be cheaply produced by the process. So too can important isotopically enriched thin-films like diamond and silicon which are essential to microelectronics.
Purity is essential to thin-film technology, which is emerging as an exciting new approach to creating better microelectronics, wear-resistant coatings, optical coatings and smart materials. Thin films, which are only a few atoms thick, work best when all the spherical atoms in the film are the same size, and are laid down in a perfectly uniform, regular pattern, like a cookie sheet full of ping-pong balls. But the odd heavy isotope disrupts the pattern because it's larger. A baseball on that cookie sheet full of ping-pong balls, for example, would disrupt the straight, regular lines of the balls.
In the case of diamond thin-films, a promising material for absorbing the excess heat from a semiconductor chip, the 1 percent of carbon in the diamond that weighs more than 12 mass units robs the film of most of its thermal efficiency. But so far, purifying isotopes for thin films has been so expensive that it has only been done for the most costly and mission-critical components of military satellites.
The inexpensive ultrafast laser is capable of spraying pure thin-films of isotopes directly onto microelectronic devices, Pronko said.
Traditional gaseous diffusion isotope separation, in addition to being costly, time consuming, and dirty, has also become politically controversial. Workers at two giant Department of Energy gaseous diffusion facilities in Ohio and Kentucky recently learned of their long-term exposures to plutonium, uranium and other radioactive and hazardous wastes while producing materials for nuclear weapons and fuel.
Earlier this year, the Piketown, Ohio, gaseous diffusion plant operated by U.S. Enrichment Corp., abandoned plans to use a proposed laser-based separation process. So-called "Atomic Vapor Laser Isotope Separation" used precisely tuned lasers to excite select isotopes in a hot vapor, but then still required large and expensive magnetic fields to sort the isotopes out by size. This approach was too costly to justify its use, the company concluded after a brief trial. The U-M process requires no such magnets for its operation and uses solid materials instead of vapors.
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