Charging ahead on antimatter microscope.

Charging ahead on antimatter microscopes

Materials scientists and biologists may get a new charge out of their microscopes with the development of two different positron reemission microscopes by separate research teams. These new antimatter microscopes should give scientists a much different view of subtle semiconductor flaws and delicate biological molecules, the developers say.

A high-resolution microscope that shoots not electrons but high-energy positrons--the electron's antimatter twin--through a sample has long been theorized but only recently developed. Since the January unveiling of such a transmission positron microscope, developed by two scientists at the University of Michigan in Ann Arbor, the Michigan team and a second group from Brandeis University in Waltham, Mass., and AT&T Bell Laboratories in Murray Hill, N.J., have sped along separate but parallel tracks to develop a far more useful microscope: the positron reemission microscope (SN: 2/20/88, p.124).

Positrons from a reemission microscope move fast when they leave their radiocative cobalt-68 source, but slow when they enter the sample, then bounce around at random until exiting the material. Besides causing much less damage to the sample than the speeding electrons in an electron microscope, the slow-moving positrons also reveal some of the subtle molecular and chemical structures around them. "Positrons are extremely sensitive to defects [in a crystal]," says James Van House of the University of Michigan. "They are far more sensitive than anything I know of."

Positron microscopes are so sensitive they can see where a single atom has been knocked out of position in a crystal, says Brandeis researcher Karl Canter. "A single missing atom will leave a hole in the material like a pothole, and it will trap the positron and annihilate it," resulting in fewer positrons reemitted from that position, Canter says. A head-on collision of a positron and electron converts both to pure energy.

The Brandeis and AT&T group has come up with a transmitting positron reemission microscope, in which positions filter through the sample and come out the other side, while the Michigan team has developed a microscope that measures positrons that penetrate a sample's first 10 or 20 molecular layers and then are reflected back out the same side they entered. Both groups announce their findings in the Aug. 1 PHYSICAL REVIEW LETTERS.

The reflecting reemission microscope has the advantage of being able to scan thicker samples rather than the ultra-thin sample-wafers the transmitting microscope must use. But because of inherent design limitations, the reflecting microscope cannot get as high a resolution as the transmitting miscrocope, says Van House. The University of Michigan's reflecting microscope now has a resolution of less than 1 micron, while the Brandeis-AT&T microscope has a resolution about three times better. It eventually may be possible to push the transmitting reemission microscope's resolution into the atomic range by using a type of positron holography, Canter says.

Van House speculates that the positron's positive charge may allow chemists to spot the similarly charged hydrogen ions in chemical reactions. And because positrons interact with the electron "holes" that are central to the workings of semiconductors, positron reemission microscopes may also give computer engineers the opportunity to look into an operating integrated chip, he says.