X-ray scans for trace elements.

Locating trace elements within minerals and other materials plays an important role in studies of the origin of rocks, the transport of contaminants in groundwater, and other environmental and industrial processes. Obtaining such data more precisely may now be possible with the use of X-ray standing waves generated inside a crystal. Researchers from the Argonne National Laboratory in Argonne, Ill., and Northwestern University in Evanston, Ill., describe the technique in the Sept. 9 SCIENCE.

Michael J. Bedzyk and his coworkers aim a powerful X-ray beam at a tilted crystal surface. This beam interacts with the X rays diffracted by the crystal's rows of atoms to create a standing wave. By manipulating the beam and the crystal, the researchers obtain data they can use to pinpoint the locations of trace elements within the crystal and on its surface.

In one test of the scheme, the researchers located manganese atoms within a calcite crystal and lead atoms that had adhered to its surface after the crystal was dipped in a dilute solution containing lead. In another test, reported in the Sept. 12 PHYSICAL REVIEW LETTERS, Bedzyk and his colleagues used the technique to make the first quantitative, high-resolution measurements of the bond length, orientation, and location of pairs of gallium atoms adhering to a silicon surface.