Elements Sifted in the Blink of a Laser

Ever since Manhattan Project scientists raced to separate uranium-235 from its slightly heavier cousin U-238 to create the first atomic bomb, isotope separation has remained an industrial-scale operation. Now, a tabletop laser could change all that: Its ultrashort, powerful pulses separate isotopes of elements ranging from boron to zinc. If the technique can be scaled up, it could provide a cheap new isotope source for research, industry, and medicine.

Isotopes are elemental kin: They share the same number of protons in their core but have different numbers of neutrons. The resulting mass differential allowed scientists in the 1940s to separate uranium isotopes through a process called gas diffusion. It's also behind the new strategy, which exploits an unusual laser: one that delivers up to 1 quadrillion watts of power per square centimeter in pulses lasting mere quadrillionths of a second.

After training the laser on a solid block of boron nitride, a team of physicists at the University of Michigan, Ann Arbor, led by Peter Pronko and John Nees learned that the beam separates two boron isotopes by generating a miniature electrical and magnetic storm. When the light hits the boron target, it kicks out electrons from the surface atoms. As the electrons fly away from the target's surface, they pull the now positively charged borons and nitrogens after them.

At the same time, the energy burst at the surface creates a powerful magnetic field, projecting from the surface as a series of magnetic field lines. These lines tug on the ions as they travel, causing them to spiral around the field lines. Key to separating the isotopes, the less massive ions fly in a tighter spiral, while the more massive ones take a wider trajectory, which moves them farther out on the target. As a result, the outer region of the disc had about twice the amount of the heavy boron isotope as the inner region, the researchers report in this week's Physical Review Letters. They have since shown that the technique works with a wide variety of elements, including copper, gallium, and zinc.

The new method is a "potentially big deal," says Todd Ditmire, a short-pulsed laser physicist at Lawrence Livermore National Laboratory in California. In addition to making better semiconductors for computer chips, the technique might be harnessed to separate medical isotopes such as yttrium-90, which is used to treat non-Hodgkin's lymphoma. Although this isn't the first technique that uses lasers to separate isotopes, says Ditmire, it doesn't require complex and expensive set-ups, making it potentially far easier and cheaper.