Using an ultrapowerful laser system that sits on a tabletop, University of Michigan researchers have demonstrated a new way to separate different forms of the same chemical element. Resting on a 20-foot-long table, this laser system can deliver up to 1000 times more power than the entire electrical generating capability of the United States for an extremely brief instant--between 150 and 200 quadrillionths of a second. To be reported in the 27 September issue of the journal Physical Review Letters, this technique provides a futuristic alternative to the bulky Manhattan Project-era methods of separating chemical isotopes--and opens new possibilities for preparing important medical compounds and making ultrapure thin films which can be used as electronic devices.
Every element on the Periodic Table comes in several different forms, known as isotopes. Each isotope of an element has a slightly different mass. Different isotopes of the same element can have radically different properties. For example, the relatively abundant uranium-238 is nowhere near as suitable for nuclear fission as the slightly lighter and much rarer uranium-235.
In natural deposits of uranium, these two different isotopes are mixed together. One of the main challenges in the race to build the atomic bomb was to develop a device that could separate the two isotopes in order to accumulate a concentrated amount of U-235. Scientists working on the Manhattan Project in the 1940s solved the problem by inventing the gaseous diffusion chamber--a technique that is still used today for producing nuclear fuel. In the gaseous diffusion method, a sample is suspended in a gas, often made of a toxic solvent, and passed through pores in which lighter isotopes become separated from heavier ones. It is a bulky system with numerous disadvantages and risks, some which are only becoming more apparent today. According a statement from the Department of Energy, "The operation of the Paducah Gaseous Diffusion Plant has produced a number of contaminated areas, both at the site and beyond its boundaries."
The new isotope-separation process at the University of Michigan is a lot cleaner and potentially safer. The technique uses a laser to extract the desired isotopes from a solid disk of material--all in a vacuum chamber isolated from the environment. While it's an open question if it will be feasible on the large scales required at nuclear facilities, the researchers envision it to be useful in other applications for separating isotopes.
The researchers are currently focusing their efforts on producing ultrathin films with superior electronic properties. To make high-quality thin-film electronic devices, one often wants to use pure ingredients--atoms containing just one particular isotope. The presence of other isotopes in the devices can distort the crystalline structure of such films--and result in less desirable properties.
In their demonstration, Peter Pronko and his colleagues at the University of Michigan produced laser pulses that deliver between 10 trillion and 1 quadrillion watts of power per square centimeter for an extremely short instant--between 150 and 200 quadrillionths of a second. Aimed at a target containing the isotopes of interest, the pulse vaporized some of the isotopes, which escaped in the form of ions (charged atoms). Intense magnetic fields associated with the pulses exerted forces on the ions which deposited them at different locations on a nearby silicon disk depending on the isotope's weight. With their technique, the researchers separated boron-10 from boron-11 and gallium-69 from gallium-71. They envision using their technique to deposit pure thin films of isotopes directly onto microelectronic devices.
Source: Isotope Enrichment in Laser-Ablation Plumes and Commensurately Deposited Thin Films, P.P. Pronko, P.A. VanRompay, Z. Zhang, and J.A. Nees, in Physical Review Letters, v. 83, p. 2596, 27 September 1999. (Journalists can contact Ben Stein, bstein@aip.org, 301-209-3091, for a copy of the article.)
"Making Industrial Isotopes Cheaper and With Less Pollution," University of Michigan News Release, http://www.umich.edu/~newsinfo/Releases/1999/Sep99/r091699b.html
Laser-Based Isotope Separation, in AIP Physics News Graphics, http://www.aip.org/physnews/graphics/html/umisoto.htm
Experts: Peter Pronko University of Michigan 734-763-6008
Gerard Morou University of Michigan 734-763-4877
Journal
Physical Review Letters