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Synchrotron beam sees record tiny crystal.

Scientists have pushed the limits of their technologies to make supertiny wires and superintense X-rays. And by combining these two advances, they have demonstrated that crystallographers can now discern the structure of submicrometer-sized samples.

Earl F. Skelton and his colleagues spent several days working around the clock at the Synchrontron Light Source at Brookhaven National Laboratory in Upton, N.Y., trying to detect radiation bouncing off atoms in an ultrathin bismuth filament. The filament measured 0.22 micrometer in diameter--less than 1 percent of the thickness of a typical human hair.

Finally, on the third morning, they succeeded. Their computer detected scattered X-rays that provided key information for determining the material's structure. The scientists discovered that their filament actually consisted of a single crystal, not numerous tiny one as they had thought. And that crystal, they say, is the smallest ever observed.

"We're of the opinion that we have set a new record in terms of diffraction of small crystals," says Skelton, a physicist with the Naval Research Laboratory (NRL) in Washington, D.C. Using a measurement called scattering power, which takes into account the number of electrons per unit volume as well as the absolute size of the sample, the researches calculated that their samples were several orders of magnigtude smaller than the tiniest crystal previously observed, he says.

To probe the structure of such small samples, Skelton's group used a "wiggles beam," which intensifies the radiation by making the synchrotron's electrons zigzag several times through a magnetic field. The electrons emit bursts of energy with each wiggle, creating extra-bright radiation.

In addition, the team used a sophisticated technique, developed by crystallographer Larry W. Finger and his colleagues at the Carnegie Institution of Washington (D.C.), that detects any diffracted radiation.

"You get very fine beams of radiation coming out [of the sample], so it's like a spider web. You have to look at it just right [to see it]," Skelton says.

Such capability can help crystallographers overcome the frustrating inability to produce large samples of key molecules, says Nobel laureate Jerome Karle, a crystallographer at the NRL. "The possibility of looking at much smaller crystals than was possible before is a worthwhile development," he told SCIENCE NEWS.

NRL metallurgist Jack D. Ayers produced the filaments for the synchrotron study by first putting a tiny piece of bismuth into a glass capillary tube. He heated the materials until the glass softened and the metal melted, then drew the glass into a longer, thinner tube. As it cooled, the metal expanded and was pulled lengtwise by the glass. Ayers then put the glass-metal filament into another capillary, repeating the procedure to make ever-thinner wires.

Theorists have predicted that such small dimensions would alter a metal's character--perhaps leading to superconductors. The filament-drawing technique thus opens up new avenues in research, says Skelton.

Skelton and his collaborators describe their study in the Sept. 6 SCIENCE. The team has since used the wiggler beam to examine samples as tiny as 0.042 micrometer across. They observed that the ultrathin bismuth underwent a phase transition, most likely due to the stress imposed by the glass surrounding it, Skelton Told SCIENCE NEWS. The atoms in the filament arranged themselves as a cubic crystal instead of the hexagonal array that typifies bismuth crystals, he says.
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Title Annotation:Synchrotron Light Source used to detect radiation bouncing off ultrathin bismuth filament
Author:Pennisi, Elizabeth
Publication:Science News
Date:Sep 14, 1991
Words:549
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