Backscatter Diffraction Uncovers Removed Serial Numbers

Feb. 25, 2015

Crooks trying to hide their crimes or identities by filing the telltale serial numbers and markings from firearms, cars, or bullet casings could soon have a new forensic technique to worry about: backscatter diffraction. Engineers at the National Institute of Standards and Technology (NIST) have found a way to use this technique to read the imprint of letters and markings that were stamped into metal, but then polished or ground off.

The process relies on the fact that any symbols imprinted in the metal alter that metal’s crystal pattern and that alteration goes beyond the surface layer.

The process uses a scanning electron microscope to send a beam of electrons bouncing off the piece of metal of interest. The metal’s surface atoms are arranged in a regular pattern if the surface has not been altered, so the back-scattered electrons interact and form a pattern that is picked up by the scanning microscope. Software analyzes the returns to reveal areas of crystal damage. These areas were damaged by the imprinting process, so researchers can determine what was imprinted in the metal. In tests, the team determined that imprinted numbers affect the crystal structure as deep as 760 microns beneath the surface.

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NIST researchers hand-stamped XX into a small piece of stainless steel to simulate a VIN or a serial number on a gun (left). Then they polished the letter off the steel (center). Researchers recovered the imprints by combining pattern-quality maps created by software, which revealed crystal damage and deformation in the steel (right).

The backscatter technique still needs to be streamlined to make it viable for law-enforcement organizations to use. It would take a technician three full days to reconstruct an 8-digit number. Researchers hope to shrink this time down to an hour.