Identify a Bomb from 1 Kilometer Away

This article was originally published on RealClearScience.

One of the problems with bomb detection is that bombs tend to explode. It would be advantageous for a bomb squad to be able to positively identify the contents of a purported bomb before attempting to dismantle it. Existing techniques, however, are inefficient or of limited utility.

In a new PNAS paper, American researchers propose using a variation of Raman spectroscopy to detect bombs from a safe distance. This technique relies on detecting slight changes in a molecule’s vibrational energy. Most molecules vibrate in some sort of fashion; atoms move back and forth as if the bonds holding them together are actually springs. Firing photons from a laser at molecules can either increase the vibration of the molecule (thus, decreasing the energy of the photon), or decrease the vibration of the molecule (thus, increasing the energy of the photon). Such photons are said to be “scattered,” and the resulting patterns serve as fingerprints to identify particular molecules.

Traditional Raman spectroscopy, however, produces a very weak signal that is difficult to detect. So, the authors used a powerful laser, but unfocused it slightly, to create a larger spot on the target. The large spot trapped the Raman scattered light inside of the target, causing it to glow even brighter. This allowed them to detect explosives, such as potassium nitrate, from 400 meters away. Various tweaks suggested that they could actually detect explosives from 1 km away.

In the figure above, the authors demonstrate the possibility of using aircraft to detect explosives. The airplane would carry a spectrometer that fires a laser (green arrow) at an unknown and possibly explosive substance. This would induce Raman scattering, and the photons would bounce back toward the aircraft (red arrow). The energy of these photons would reveal the identity of the substance.

The authors claim this technology may also help detect the sorts of molecules that would indicate the presence of extraterrestrial life. Indeed, that would be the bomb.

Source: Brett H. Hokr, Joel N. Bixler, Gary D. Noojin, Robert J. Thomas, Benjamin A. Rockwell, Vladislav V. Yakovlev, and Marlan O. Scully. “Single-shot stand-off chemical identification of powders using random Raman lasing.” PNAS Early Edition. DOI: 10.1073/pnas.1412535111