MATERIALS PROCESSING: Tabletop laser enriches isotopes

Materials made from isotopically pure elements have medical, industrial, and other uses. The production of bulk quantities of isotopes has traditionally been left to government facilities, where large pieces of sometimes—antiquated equipment perform the task slowly and expensively.

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Materials made from isotopically pure elements have medical, industrial, and other uses. The production of bulk quantities of isotopes has traditionally been left to government facilities, where large pieces of sometimes—antiquated equipment perform the task slowly and expensively. Now, researchers at the University of Michigan (Ann Arbor, MI) have enriched isotopes of gallium, boron, and other elements using an ultrafast tabletop Ti:sapphire laser. Although the amount of isotopic matter produced thus far is small, so is the size of the equipment needed to produce it.

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The discovery of the technique was accidental, according to research scientist Peter Pronko. Aided by an instrument called a sector—field electrostatic analyzer, the group was studying the ion plumes given off when a femtosecond laser ablates material in a vacuum. The analyzer produces a time—of—flight spectrum containing components separated by ionic charge and isotopic mass. "The isotope ratios were way off," says Pronko.

PHOTO: View through the window of a vacuum chamber shows isotope-enrichment apparatus based on ultrafast-laser ablation. A boron nitride substrate is held by a cylindrical assembly (center, hanging from bracket); boron ions produced by femtosecond-laser pulses striking the substrate form a blue-violet plume. Ions are deposited on a silicon target (at right edge of plume), with lighter isotopes preferentially deposited at the center of the target.

After checking the apparatus over carefully, the researchers concluded that the effect—whatever it was—was real. They theorized that toroidal and axial magnetic fields produced by the laser pulse were retained by the plasma plume as it left the ablation site, creating the proper conditions for a "plasma centrifuge," in which ions of varying mass accelerated by the fields take different paths. Ongoing study is indeed showing this to be the probable explanation, says Pronko.

By placing a silicon disk in the path of the plume, the researchers collect the isotopically separated material as a film on the disk, with the lightest isotope landing at the center and heavier isotopes depositing toward the edge (see figure). In one example, pulses of 780—nm wavelength, 200—fs duration, and peak intensity of 2.5 x 1014 W/cm2 strike boron (B) at a 45° angle, producing a plume of ions with charge state ranging from +1 to +5. The center of the resulting deposit exhibits a ratio of B10 to B11 of 0.66—a factor of 2.6 higher than the ratio of 0.25 occurring naturally in boron.

In another example, gallium (Ga) is enriched, with the ratio of Ga69 to Ga71 rising to 2.8 compared to its naturally occurring ratio of 1.5. The researchers have enriched isotopes of other elements as well, including those of copper and zinc, although the effectiveness of the technique disappears for elements of higher atomic number than tin.

Practical applications of isotopes abound. For example, silicon (Si), when made in isotopically pure Si28 form, has 60% higher thermal conductivity than natural silicon—a property that may someday be crucial for the production of faster, more—reliable integrated—circuit chips. Materials science could benefit from smaller neutron detectors made from isotopic elements. Isotopes are used medically as tracers that, when injected or swallowed, outline a portion of the body, making it visible for various forms of detection. Tabletop production could make such isotopes more widely available; it may even make possible the simultaneous enrichment of an isotope and deposition of it directly onto a device in final, working form.

John Wallace

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