Laser Simplifies Isotope Enrichment

Dr. James P. Smith

Researchers at the University of Michigan have developed a simple and efficient means to produce films of enriched isotopes. Using a tabletop terawatt laser, the method offers a compact alternative to the traditional gaseous diffusion technique.

As detailed in the Sept. 27, 1999, issue of Physical Review Letters, a 780-nm Ti:sapphire chirped-pulse amplified laser ablates a solid target with ultrashort, high-energy pulses. The expanding plasma plume keeps the lighter isotopes in its center, while the heavier isotopes spiral to its outer regions. Because the pulses are so short (150 to 200 fs), the intensities can be as high as 1 PW/cm².

An ultrafast laser pulse focused on a target produces magnetic fields that deflect atoms, separating the heavier from the lighter atoms. A substrate captures the separated atoms. Courtesy of the University of Michigan.

The researchers have experimented with targets of boron nitride, gallium nitride, zinc, titanium and copper. In all cases, the lighter isotope was enriched in the center of the direction of the ablation. The laser pulse creates a plasma at the surface of the material; the plume expands normal to the surface, and it contains a high concentration of charged ion species and hot electrons, explained Peter Pronko, a research physicist at the university's Center for Ultrafast Optical Science. The charged particles accelerate in the electromagnetic field created by the expanding plasma. "This is essentially a plasma centrifuge in its own dynamic magnetic field," he said.

Heat dissipation

The semiconductor industry could be a beneficiary of this development, Pronko said, because isotopically pure films are particularly good at dissipating heat. "For example, diamond made of pure carbon 12 can have an order of magnitude greater thermal conductivity than natural diamond, which contains 99 percent carbon 12," he said.

"The good thing about this method is that it is very simple and direct -- all you need is the laser and a solid target," Pronko noted. "The process is self-operative, and you need no external magnetic fields or lengthy alignment procedure. It is all contained in a very small laser plasma."